CH-6 Organic Evolution – Integrated Principles of Zoology : Chapter Notes

This document contains Chapters 6 to 7 CHAPTER 6 ORGANIC EVOLUTION CHAPTER OUTLINE 6.1 A Legacy of Change A. The natural world appears to be unchanged; however fossil records indicate life has historically changed or evolved. B. Scientists observe and measure these changes. C. Biologists consider organic evolution as the keystone of all biological knowledge. 6.2. Origins of Darwinian Evolutionary Theory (Figure 6.1) A. Pre-Darwinian Evolutionary Ideas 1. Before the 18th century, speculation on origin of species was not scientific. 2. Creation myths portrayed a constant world after a creation event. 3. Early Greek philosophers considered some ideas of evolutionary change. a. Xenophanes, Empedocles and Aristotle developed early ideas about evolution. b. Fossils were recognized as former life destroyed by natural catastrophe. c. Lacking a full evolutionary concept, the idea faded before the rise of Christianity. 4. Biblical account of creation became a tenet of faith. a. Evolutionary views were heretical. b. Archbishop Ussher calculated 4004 BC as date of life’s creation. 5. French naturalist Georges Luis Buffon suggested that environment modified animal types; set age of earth at 70,000 years. French biologist Jean Baptiste de Lamarck offered first complete explanation in 1809. (Figure 6.2) a. He convincingly argued that fossils were remains of extinct animals. b. Lamarck’s mechanism was inheritance of acquired characteristics. c. He explained long necks of giraffes to stretching efforts of ancestral giraffes. d. Lamarck’s concept is transformational; individuals transform their own traits by the use or disuse of body parts to evolve. e. In contrast, Darwin’s theory is variational or due to differential survival among offspring. 7. Geologist Sir Charles Lyell established the principle of uniformitarianism. (Figure 6.3) a. Uniformitarianism consists of two important principles: 1) Laws of physics and chemistry remain the same throughout earth’s history. 2) Past geological events occurred by natural processes similar to those observed today. b. Natural forces acting over long periods could explain formation of fossil-bearing rocks. c. Earth’s age must be measured in millions of years. d. Geological changes are natural and without direction; both concepts underpinned Darwin. B. Darwin’s Great Voyage of Discovery (1831–1836) 1. In 1831, Charles Darwin (almost 23) sailed aboard the small survey ship HMS Beagle. 2. Darwin made extensive observations in the five-year voyage. (Figures 6.4, 6.5) a. Darwin collected the fauna and flora of South America and adjacent regions. b. He unearthed long extinct fossils and associated fossils of South and North America. c. He saw fossil seashells embedded in the Andes rocks at 13,000 feet altitude. d. Observing earthquakes and severe erosion confirmed his views of geological ages. 3. The Galápagos Islands provided unique observations. a. These volcanic islands are on the equator 600 miles west of Ecuador. (Figure 6.6) b. Galápagos means “tortoise”; the giant reptiles were exploited for food. c. Each island varied in tortoises, iguanas, mockingbirds and ground finches. d. He later wrote that these unique animals and plants were the “origin of all my views.” e. The islands had similar climate but varied vegetation. f. Island species therefore originated from South America and were modified under the varying conditions of different islands. 4. Darwin conducted most of his work at home in England. (Figure 6.7) a. His collections and notebooks had been sent back before his return in October 2, 1836. b. His popular travel journal, The Voyage of the Beagle, was published three years later. c. In 1838, Darwin read an essay on population by Thomas R. Malthus. d. Having studied artificial selection, a “struggle for existence” because of overpopulation gave him a mechanism for evolution of wild species by natural selection. e. He presented his ideas in a paper in 1844 and began work on a larger volume in 1856. f. In 1858, he received a manuscript from a young naturalist, Alfred Russel Wallace, summarizing the main points of natural selection. g. Geologist Lyell and botanist Hooker persuaded Darwin to publish a paper jointly with Wallace’s paper. h. Darwin then rushed to publish a shorter “abstract” version in 1859: On the Origin of Species by Means of Natural Selection. i. All 1250 copies of the first printing sold in one day. j. Darwin wrote a series of important books in the next 23 years. 6.3. Darwinian Evolutionary Theory: The Evidence A. Perpetual Change 1. The living world is constantly changing in form and diversity. 2. Change in animal life is directly seen in the 600–700 million-year animal fossil history. 3. A fossil is a remnant of past life. (Figure 6.8) a. Insects in amber and frozen mammoths are actual remains. b. Teeth and bones can petrify or become infiltrated with silica and other minerals. c. Molds, casts, impressions and fossil excrement are also fossils. 4. Most organisms leave no fossils; the record is always incomplete and requires interpretation. B. Interpreting the Fossil Record The fossil record is biased because preservation is selective. Vertebrate skeletons and invertebrates with shells provide more records. Soft-bodied animals leave fossils only in exceptional conditions such as the Burgess Shale. (Figure 6.9) Fine fossil sites also include South Australia, Rancho La Brea, and dinosaur beds in Alberta, Canada and Utah. (Figure 6.10) Fossils form in stratified layers; new deposits are on top of older material. “Index” or “guide” fossils are “indicators” of specific geological periods. Layers often tilt and crack, and can erode or become covered with new deposits. Under heat and pressure, rock becomes metamorphic and fossils are destroyed. Stratigraphy for two major groups of African antelopes shows the relationship between extinct and living species by using comparisons of homologous structural features such as horns. (Figure 6.11) C. Geological Time 1. Sedimentary Rock Layers a. The law of stratigraphy dates oldest layers at the bottom and youngest at the top. b. Time is divided into eons, eras, periods and epochs. (See inside back cover.) 2. Radiometric Dating a. In the late 1940s, this dating method was developed to determine the age of rocks. b. Radioactive decay of naturally occurring elements is independent of heat and pressure. c. Potassium-Argon Dating 1) Potassium-40 (40K) decays to argon-40 (40Ar) and Calcium-40 (40Ca). 2) Half-life of potassium-40 is 1.3 billion years; half of remainder will be gone at end of next 1.3 billion years, etc. 3) Calculating the ratio of remaining potassium-40 to amount originally there provides mathematically close estimate of age of deposit. d. Rate of decay of uranium into lead can date age of earth itself; error is less than 1% over 2 billion years. 3. Fossil Record of Macroscopic Organisms a. The Cambrian period of Paleozoic era began about 600 million years ago. b. Previous Precambrian era occupies 85% of geological time on earth. c. Little research occurred on Precambrian rocks because they have few oil deposits. d. Precambrian contains well-preserved fossils of bacteria and algae, casts of lower invertebrates and many microscopic fossils. D. Evolutionary Trends 1. Fossil record allows observation of evolutionary change over broad periods of time. 2. Animals species arise and become repeatedly extinct. 3. Animal species typically survive 1–10 million years; there is much variability. 4. Trends are directional changes in features and diversity of organisms. 5. Horse Evolution Shows Clear Trend (Figure 6.12) a. From Eocene to Recent periods, genera and species of horses were replaced. b. Earlier horses had smaller sized and fewer grinding teeth, and more toes. c. Reduction in toes and increase in size and numbers of grinding teeth correlate with environmental changes. d. Change occurred in both features of horses and numbers of species. 6. Trends in fossil diversity are due to different rates of species formation and extinction. (Figure 6.13) 7. Lineages vary in producing new species or suffering extinction; Darwin provided explanations for this. E. Common Descent 1. Darwin proposed that all plants and animals descended from a common ancestor. 2. A history of life forms a branching tree called a phylogeny. 3. This theory allows us to trace backward to determine converging lineages. 4. All forms of life, including extinct branches, connect to this tree somewhere. 5. Phylogenetic research is successful at reconstructing this history of life. F. Homology and Phylogenetic Reconstruction 1. Darwin saw homology as major evidence for common descent. 2. Richard Owen described homology as “the same organ in different organisms under every variety of form and function.” 3. Vertebrate limbs show the same basic structures modified for different functions. (Figure 6.14) 4. Darwin’s central idea that apes and humans have a common ancestor was explained by anatomical homologies; however, this idea was not met well with Victorians. (Figure 6.15) 5. Ground-dwelling birds illustrate homologies. (Figure 6.16) a. A new skeletal homology arises on each lineage shown. b. Different groups located at tips of branches contain homologies that reflect ancestry. c. Branches of the tree combine species into nested hierarchies of groups within groups. d. Analysis of the living species alone can reconstruct the branching pattern. e. The pattern of nested hierarchies forms the basis for classification of all forms of life. f. Structural, molecular, and chromosomal homologies are all combined to reconstruct evolutionary trees. g. Older theories that life arose many times forming unbranched lineages fails to predict the nested hierarchies of lineages; creationism (or intelligent design) fails to provide testable predictions; all fail as scientific hypotheses. G. Ontogeny, Phylogeny and Recapitulation 1. Ontogeny is the history of development of an organism throughout its lifetime. 2. Evolutionary alteration of developmental timing generates new traits allowing divergence among lineages. 3. German zoologist Ernst Haeckel stated stages of development represented adult forms from evolutionary history. 4. “Ontogeny recapitulates phylogeny” also became known as recapitulation or the biogenetic law. 5. Haeckel, Darwin’s contemporary, thought that this change was caused by adding new features onto the end of ancestral ontogeny; but this idea is Lamarckian. 6. Embryologist K.E. von Baer showed early developmental features were simply more widely shared among different animal groups. (Figure 6.17) 7. However, early development can undergo divergence among lineages too. 8. Evolutionary change in timing of development is called heterochrony. 9. Characteristics can be added late in development and features are then moved to an earlier stage. 10. Ontogeny can be shortened in evolution; terminal stages may be deleted causing adults of descendants to resemble youthful ancestors. 11. Paedomorphosis is the retention of ancestral juvenile characteristics in descendent adults. (Figure 6.18) 12. Organisms are a mosaic of both; ontogeny rarely completely recapitulates phylogeny. H. Developmental Modularity and Evolvability 1. Heterotopy describes a change in the physical location of a developmental process in an organism’s body. 2. In order for this change to occur, the development must be compartmentalized into independent modules whose expression can be activated in a new location. a. Gecko have setae on the ventral surface of their toepads that permit climbing. b. An unusual gecko species has developed these toepads on the ventral surface of their tail as well. 3. Modularity is important in explaining some evolutionary changes. a. Tetrapod limbs evolved from finlike limbs by the activation of homeobox genes at the site of limb formation. b. Homeobox genes evolved initially as a module for forming part of the vertebral column. c. The genes that express the development of forelimbs, hindlimbs, and the vertebral column share a pattern. d. This helps to explain the genetic and developmental mechanics of this type of module. 4. Evolvability denotes the evolutionary opportunities created by semi-autonomous modules whose expression can be moved from one part of the body to another. I. Multiplication of Species 1. Evolution as a Branching Process a. A branch point occurs where an ancestral species splits into two different species. b. Darwin’s theory is based on genetic variation. c. Total number of species increases in time; most species eventually become extinct without leaving descendants. d. Much evolutionary research centers on mechanisms causing branching. 2. Definition of species varies and may include several criteria. a. Members descend from a common ancestral population forming a lineage. b. Interbreeding occurs within a species but not among different species. c. Genotype and phenotype within a species is similar; abrupt differences occur between species. 3. Reproductive Barriers a. Reproductive barriers are central to forming new species. b. If diverging populations reunite, before they are isolated, interbreeding maintains one species. c. Evolution of diverging populations requires they be kept physically separate a long time. d. Geographical isolation with gradual divergence provides chance for reproductive barriers to form. 4. Allopatric Speciation (Figure 6.19) a. Allopatric populations occupy separate geographical areas. b. They cannot interbreed because they are separated, but could do so if barriers were removed. c. Separated populations evolve independently and adapt to respective environments. d. Even if the respective environments remain similar, separated populations genetically diverge because their genetic variation arises independently. d. Eventually they are distinct enough they cannot interbreed when reunited. e. Allopatric speciation occurs in two ways: 1) Vicariant speciation occurs when climate or geology causes populations to fragment; this may affect many populations at one time but does not itself induce genetic change. 2) Founder effect occurs when a small number of individuals disperse to a distant place; this has occurred with fruit flies in Hawaii. f. Hybridization is mating between divergent populations; offspring are hybrids. (Figure 6.20) g. Premating barriers impair fertilization. 1) Members may not recognize each other. 2) Male and female genitalia may not be compatible. 3) Behavior may be inappropriate to elicit reproduction. 4) Sibling species are indistinguishable in appearance but cannot mate. h. Postmating barriers impair growth and development or survival. 5. Nonallopatric Speciation (Figure 6.21) a. When there is no evidence of physical barriers, it is difficult to explain diversity of close species by allopatric speciation. b. The huge variety of cichlid fishes in African lakes are found nowhere else; yet lakes are evolutionarily young and without barriers. c. Sympatric speciation is the term for the hypothesis that individuals can speciate while living in different components of the environment. d. African cichlid fishes are very different in feeding specialization. e. Parasites may evolve with their host species. f. It is difficult to observe formation of distinct evolutionary lineages in allopatric speciation. g. From one-third to one-half of plant species show sympatric evolution using polyploidy; in animals polyploidy is rare. 6. Parapatric speciation a. This type of speciation is in intermediate between allopatric and nonallopatric speciation. b. It results when two species are geographically isolated (allopatric) but share a border and come in contact with each other yet neither have successfully crossed that border (nonallopatric). c. Populations are not isolated by a physical barrier but maintain genetic interactions along border between the habitat types. d. In most cases, parapatrically distributed species shared past allopatry. e. This model is controversial. f. Parapatric speciation predicts that these types of populations differ in adaptive features observed from differences in environment but show homogeneity for other genetic variation. 7. Adaptive Radiation (Figures 6.22, 6.23) a. Adaptive radiation produces diverse species from common ancestral stock. b. New lakes and islands provide new opportunities for organisms to evolve. c. Founders who were under heavy competition are now free to colonize the new habitat. d. The Galápagos Islands provided excellent isolation from mainland and each other. e. Darwin’s finches are example of adaptive radiation from ancestral finch; finches varied to assume characteristics of missing warblers, woodpeckers, etc. J. Gradualism 1. Darwin’s theory of gradualism is based on accumulation of small changes over time. 2. He agreed with Lyell; past changes do not depend on catastrophes not seen today. 3. We observe small, continuous changes; major differences therefore require thousands of years. 4. Accumulation of quantitative changes leads to qualitative change. 5. Ernst Mayr distinguishes between populational gradualism and phenotypic gradualism. 6. Populational gradualism occurs when a new trait becomes more common; this is well established. 7. Phenotypic Gradualism (Figure 6.24) a. This theory states that strikingly different traits are produced in a series of small steps. b. It remains controversial ever since Darwin proposed it. c. Mutations that cause substantial phenotypic change are called “sports.” d. Animal breeding has used sports to produce short-legged sheep, etc. Opponents of phenotypic gradualism contend such mutations would be selected against. Large effect mutations may be responsible for an adaptive polymorphism in an African finch. Recent work in evolutionary developmental genetics illustrates the continuing controversy surrounding phenotypical gradualism. 8. Punctuated Equilibrium (Figures 6.25, 6.26) a. Phyletic gradualism predicts that fossils would show a long series of intermediate forms. b. Fossil record does not show the predicted continuous series of fossils. c. Some Darwinists contend that fossilization is haphazard and slow compared to speciation. d. Niles Eldridge and Stephen Jay Gould proposed punctuated equilibrium. e. This theory contends phenotypic evolution is concentrated in brief events of speciation followed by long intervals of evolutionary stasis. f. Speciation is episodic with a duration of 10,000 to 100,000 years. g. Species survive for 5–10 million years; speciation may be less than 1% of species life span. h. A small fraction of evolutionary history contributes most morphological evolutionary change. i. Allopatric speciation provides a possible explanation. 1) A small founder population has little chance of leaving fossils that will ever be found. 2) After a new genetic equilibrium forms and stabilizes, the larger but different population increases the chance that it will be preserved. 3) However, punctuated equilibrium occurs in groups where founder events are unlikely. j. Peter Williamson’s Freshwater Snails 1) Fossil beds in Lake Turkana had a history of earthquakes, eruptions and climate changes. 2) Thirteen lineages of snails show long periods of stability, and brief periods of rapid change when populations were fragmented by receding waters. 3) Transitions occurred within 5,000 to 50,000 years matching punctuated equilibrium. K. Natural Selection 1. Natural selection gives a natural explanation for origins of adaptation. 2. It applies to developmental, behavioral, anatomical and physiological traits. 3. Color patterns concealing moths from predators and beaks suited for different modes of feeding in finches show natural selection leading to adaptation. 4. Darwin’s theory of natural selection consists of five observations and three inferences. a. Organisms have great potential fertility. 1) If all individuals produced would survive, populations would explode exponentially. 2) Darwin calculated that a single pair of elephants could produce 19 million offspring in 750 years. b. Natural populations normally remain constant in size with minor fluctuations. 1) Natural populations fluctuate in size across generations, sometimes going extinct. 2) No natural populations can sustain exponential growth. c. Natural resources are limited. 1) Inference: struggle for food, shelter, and space becomes increasingly severe with overpopulation. 2) Survivors represent only a small part of those produced each generation. d. All organisms show variation. e. Some variation is heritable. 1) Darwin only noted the resemblance of parents and offspring. 2) Gregor Mendel’s mechanisms of heredity were applied to evolution many years later. 3) Inference: There is differential survival and reproduction among varying organisms in a population. 4) Inference: Over many generations, natural selection generates new adaptations and new species. 5. Natural selection can be viewed as a two-part process: random and non-random. a. Production of variation among organisms is random; mutation does not generate traits preferentially. b. The nonrandom component is the survival of different traits. 1) Differential survival and reproduction is called sorting; random processes may sort. 2) Natural selection is sorting that occurs because certain traits give their possessors advantages relative to others. c. Orthogenesis was the hypothesis that directed that non-random variation propels evolution. 1) Supposedly, variation has a momentum that forces a lineage to evolve in a direction. The Irish elk supposedly was driven to larger antlers until they became cumbersome and became extinct. (Figure 6.27) The disappearance of the Irish elk is not extraordinary and probably not related to the size of its antlers. 4) This was an explanation for apparently nonadaptive evolutionary trends. 5) Modern genetic research has rejected the genetic predictions of orthogenesis. d. Some critics contend natural selection cannot generate new structures, only modify old ones. 1) Many structures could not perform their function in early evolutionary stages. 2) However, many structures evolved initially for purposes different from the present. 3) Early feathers functioned in thermoregulation; they later became useful for flight. 4) Exaptation denotes the utility of a structure for a biological role that was not part of the structure’s evolutionary origin. 6.4. Revisions of Darwin’s Theory A. Neo-Darwinism 1. Darwin did not know the mechanism of inheritance. a. Darwin saw inheritance as a blending of parental traits. b. He also considered an organism could alter its heredity through use and disuse of parts. 2. August Weismann’s experiments showed an organism could not modify its heredity. 3. Neo-Darwinism is Darwin’s theory as revised by Weismann. 4. Mendel’s work provided linkage through inheritance that Darwin’s theory required. 5. Ironically, early geneticists thought mutations could cause speciation in a single large step; selection was merely an eliminator. B. Emergence of Modern Darwinism: The Synthetic Theory 1. In 1930s, a synthesis occurred that tied together population genetics, paleontology, biogeography, embryology, systematics and animal behavior. 2. Population genetics studies evolution as change in gene frequencies in populations. 3. Microevolution is change of gene frequency over a short time. 4. Macroevolution is evolution on a grand scale, originating new structures and designs, trends, mass extinctions, etc. 5. The synthesis combines micro- and macroevolution and expands Darwinian theory. 6.5. Microevolution: Genetic Variation and Change Within Species A. The Gene Pool 1. Different allelic forms of a gene constitute polymorphism. 2. All alleles of all genes that exist in a population are collectively the gene pool. 3. Allelic frequency is the frequency of a particular allelic form in a population. a. Blood types are coded at codominant alleles IA (type A) and IB (type B) and recessive type ii (O). b. Since each person carries two alleles; the total numbers of alleles is twice the population size. c. Blood type frequencies for: (Figure 6.28) 1) France are IA = .46, IB = .14 and i = .40. 2) Russia are IA = .38, IB = .28 and i = .34. d. Dominance describes the phenotypic effect of an allele only, not its relative abundance. B. Genetic Equilibrium (Figure 6.29) 1. Whether a gene is dominant or recessive does not affect its frequency; dominant genes do not supplant recessive genes. 2. In large two-parent populations, genotypic ratios remain in balance unless disturbed. 3. This is called the Hardy-Weinberg equilibrium. 4. It accounts for the persistence of rare traits such as albinism and cystic fibrosis caused by recessive alleles. 5. Genotype frequency can be calculated by expanding the binomial (p - q)2 where p and q are allele frequencies. 6. For example, an albino is homozygous recessive and the trait is represented by q2 in the formula: p2 + 2pq + q2 = 1. 7. Albinos (homozygous recessive) occur in one in 20,000; therefore q2 = 1/20,000 and q = 1/141. 8. Non-albino (p) is 1 - q = 140/141. 9. Carriers would be 2pq or 2 x 140/141 x 1/141 = 1/70; one person in 70 is a carrier. 10. Eliminating a “disadvantageous” recessive allele is nearly impossible. 11. Selection can only act when it is expressed; it will continue through heterozygous carriers. C. How Genetic Equilibrium is Upset 1. In natural populations, Hardy-Weinberg equilibrium is disturbed by one or more of five factors. 2. Genetic Drift (Figure 6.30) a. A small population does not contain much genetic variation. b. Each individual contains at most two alleles at a single locus; a mating pair has a maximum of four alleles to contribute for a trait. c. By chance alone, one or two of the alleles may not be passed on. d. Chance fluctuation from generation to generation, including loss of alleles, is genetic drift. e. There is no force causing perfect constancy in allelic frequencies. f. The smaller the population, the greater the effect of drift. g. If a population is small for a long time, alleles are lost and response to change is restricted. h. Large reductions in population size leading to increased significance of genetic drift are known as bottlenecks. Bottlenecks associated with the formation of a new geographic population are referred to as founder effects and may lead to speciation. 3. Nonrandom Mating a. If two alleles are equally frequent, one half of the population will be heterozygous and one quarter will be homozygous for each allele. b. In positive assortative mating, individuals mate with others of the same genotype. 1) This increases homozygous and decreases heterozygous genotypes. 2) It does not change allelic frequencies. c. Inbreeding is preferential mating among close relatives. 1) Inbreeding increases homozygosity. 2) While positive assortative mating affects one or a few traits, inbreeding affects all variable traits. 3) Inbreeding increases the chance that recessive alleles will become homozygous and express. 4) Inbreeding cannot change gene frequencies; genetic drift does and both are common in small populations. 4. Migration a. Migration prevents different populations from diverging. b. Continued migration between Russia and France keeps the ABO allele frequencies from becoming completely distinct. 5. Natural Selection a. Natural selection changes both allelic frequencies and genotypic frequencies. b. An organism that possesses a superior combination of traits has a higher relative fitness. c. Relative fitness values can be measured using the genetical theory. 1) W = the expected average fitness of a genotype in a population. 2) Highest fitness genotype = 1; all other genotypes indicated as fractions. 3) Example using sickle cell anemia: AS = 1 (highest fitness); AA = 0.9 (slightly decreased fitness); and SS = .02 (lowest fitness) 4) Using these values, the average effect can be calculated. d. Some traits are advantageous for certain aspects of survival or reproduction and disadvantageous for others. e. Sexual selection is selection for traits that obtain a mate but may be harmful for survival. (Figure 6.31) f. Changes in environment alter selective value of traits making fitness a complex problem. 6. Interactions of Selection, Drift and Migration a. Subdivision of a species into small populations that exchange migrants promotes rapid evolution. b. Genetic drift and selection allow many combinations of many genes to be tested. c. Migration allows favorable new combinations to spread. d. Interactions of all factors produce change different from what would result from one alone. e. Geneticist Sewall Wright called this interaction shifting balance. f. Perpetual stability almost never occurs across any significant amount of evolutionary time. D. Measuring Genetic Variation within Populations 1. Protein Polymorphism (Figure 6.32; Table 6.1) a. Dominance, interactions between alleles and environmental effects make it difficult to measure genetic variation from phenotype. b. Different allelic forms encode proteins with different amino acid sequences; this is protein polymorphism. 2. Over the last 45 years, geneticists have discovered unexpected variation. 3. Electrophoresis does not detect protein polymorphisms if there are no change differences. 4. Since there is more than one codon for most amino acids, the codons possess more variation. E. Quantitative Variation (Figure 6.33) 1. Quantitative traits show continuous variation with no Mendelian segregation pattern. a. Such traits are influenced by variation at many genes. b. Such traits show a bell-shaped frequency distribution. 2. Stabilizing selection favors the average and trims the extreme. 3. Directional selection favors an extreme value to one side. 4. Disruptive selection favors the extremes to both sides and disfavors the average. 6.6. Macroevolution: Major Evolutionary Events (Figure 6.34) A. Speciation links macroevolution to microevolution. 1. The timescale of population genetics processes is from tens to thousands of years. 2. Rates of speciation and extinction are measured in millions of years. 3. Periodic mass extinctions occur in tens to hundreds of millions of years. a. Five mass extinctions have been dramatic. b. Study of long-term changes in animal diversity focuses on this longest timescale. B. Speciation and Extinction Through Geological Time 1. A species has two possible fates: become extinct or give rise to new species. 2. Rates of speciation and extinction vary among species. 3. Lineages with high speciation and low extinction produce the greatest diversity. 4. Lineages whose characteristics increase probability of speciation and confer resistance to extinction should come to dominate. 5. Species selection is differential survival and multiplication of species based on variation among lineages. 6. Species-level properties include mating rituals, social structuring, migration patterns, geographic distribution, etc. 7. Some mammalian lineages have a “harem” system, others do not. 8. Effect macroevolution is similar but differential speciation and extinction is caused by variation in organismal-level properties rather than species-level properties. a. Food specialists would therefore be more likely to be geographically isolated. b. A lineage of specialized grazers and browsers has high speciation and extinction rates. c. A lineage of generalist grazers and browsers shows neither branching speciation nor extinction during the same time. d. Interestingly, the two lineages have similar numbers of individual animals alive today. C. Mass Extinction (Figure 6.35) 1. Periodic events where huge numbers of taxa go extinct simultaneously are mass extinctions. 2. The Permian extinction occurred 225 million years ago; half of the families of shallow water invertebrates and 90% of marine invertebrates disappeared. 3. The Cretaceous extinction occurred 65 million years ago and marked the end of the dinosaurs and many other taxa. 4. Mass extinctions appear to occur at intervals of 26 million years. a. Some consider them artifacts of statistical or taxonomic analysis. b. Walter Alvarez proposed that asteroids occasionally bombard the earth. (Figure 6.35) Catastrophic species selection would result from selection by these events; for instance, mammals were able to use resources due to dinosaur extinction. Paleontologist Elisabeth Vrba uses the term Effect Macroevolution to describe differential speciation and extinction rates among lineages caused by organismal-level properties. Lecture Enrichment 1. Clarify the fact that Lamarck was the first to try to develop a possible mechanism for evolution. He should be considered among the great biologists of history, rather than just the “one who got evolution wrong.” 2. Speculate how Darwin would have reacted to Mendel’s work if he had read it; Darwin had a copy of the publication of Mendel’s paper, but the pages had never been cut so that it could be read. 3. Note that both Darwin and Wallace had extensive travel experiences, where they saw a wide range of organisms, before they formulated their theory of evolution by natural selection. Biology is so complex and laden with emergent properties that it is not possible to discover one underlying law, as in physics and chemistry, and then extrapolate to all cases. The need for a wide range of observational experiences in biology is therefore a difference in philosophy of science between the life and physical sciences that many students will not perceive. 4. In a modern age of public relations, few students will comprehend why Darwin waited over twenty years from his seminal trip on the HMS Beagle to publish his theory. One of the topics to consider is Darwin’s feelings for his wife, who was a very religious woman, and the impact that the theory of natural selection and evolution had on those who considered the theory of special creation to be absolute truth, which students should understand. 5. Students do not immediately internalize the meaning of terms such as “allopatric” and “sympatric.” Since you will not be using these terms across the course, the concept is most efficiently taught using examples and referring to the cases rather than generalizing. 6. Students who have already completed a course in botany will have an understanding of the ability of plants to speciate sympatrically via polyploidy. Other students will be puzzled why animals and plants vary so greatly on this issue. Commentary/Lesson Plan Background: The concepts of multiple alleles and recombination are not explained in many secondary biology texts. Algebra skills are necessary for students to understand the Hardy-Weinberg formula and high school math requirements vary by state. Some African-American students will be very aware of the sickle-cell gene. The comet and meteorite impact theories have gone from speculative to fairly well accepted in the last decade and the instructor will probably face students who were taught the previous uncertainties. Breaking news on recent major fossil finds in China and Africa make evolution a timely topic to cover. And new discoveries requiring reinterpretation of our phylogeny will cause this chapter to present new or different concepts from what students may have learned in high school biology. Visuals of fossils will help students see the distinctions described in this chapter. Some students will have some concerns with this chapter based on religious background but an honest and clear explanation of the current status of our science knowledge should help. Misconceptions: Most students view mutations as rare and the only source of genetic variation. Dinosaurs are a hot topic now covered heavily in K–12 science curricula; however, new developments make many textbooks obsolete; therefore, the college instructor will have to deal with many misconceptions. Some students and even molecular biologists will not understand why classifications change–why can’t systematists just settle on one set of higher taxa and stick with it? While ongoing changes may seem casual and arbitrary, there is a complex rationale for the evolutionary schema that students can understand. Schedule: An instructor may vary considerably in how much background history, philosophical setting, etc. to provide before introducing Darwin’s voyage, etc. Speed of coverage of population genetics concepts will also vary greatly depending on students’ previous coursework. HOUR 1 6.1. Legacy of Change 6.2 Origins of Darwinian Evolutionary Theory A. Pre-Darwinian Evolutionary Ideas B. Darwin’s Great Voyage of Discovery 6.3. Darwinian Evolutionary Theory: The Evidence A. Perpetual Change B. Interpreting Fossil Record C. Geological Time D. Evolutionary Trends E. Common Descent F. Homology and Phylogenetic Reconstruction G. Ontogeny, Phylogeny and Recapitulation HOUR 2 H. Developmental Modularity and Evolvability I. Multiplication of Species J. Gradualism K. Natural Selection 6.4. Revisions of Darwin’s Theory A. Neo-Darwinism B. Emergence of Modern Darwinism: The Synthetic Theory HOUR 3 6.5. Microevolution: Genetic Variation and Change Within Species A. The Gene Pool B. Genetic Equilibrium C. How Genetic Equilibrium is Upset D. Measuring Genetic Variation Within Populations E. Quantitative Variation 6.6. Macroevolution: Major Evolutionary Events A. Speciation links macroevolution to microevolution. B. Speciation and Extinction Through Geological Time C. Mass Extinction ADVANCED CLASS QUESTIONS: 1. Will evolution occur if there is no variation in a population, if the only variation is acquired and not inherited, or if all progeny survive and equally reproduce? Answer: No, evolution will not occur under these conditions. Evolution depends on genetic variation within a population, which arises from inherited genetic differences. If there is no genetic variation or if the only variation is acquired and not inherited, there will be no genetic material for natural selection to act upon. Additionally, if all progeny survive and equally reproduce, there will be no differential reproductive success, which is a fundamental requirement for natural selection to drive evolutionary change. Therefore, in the absence of genetic variation or differential reproductive success, evolution will not occur. 2. Could bacteria and humans possibly share a common ancestor; and what evidence do we have? Answer: Yes, bacteria and humans could share a common ancestor, and there is substantial evidence supporting this idea. Evidence for Common Ancestry: 1. Genetic Similarities: Comparative genomics has revealed significant similarities in the genetic material (DNA) of bacteria and humans. Both bacteria and humans share many genes and genetic sequences, indicating a common ancestry. 2. Universal Genetic Code: All living organisms, including bacteria and humans, use the same genetic code to translate DNA sequences into proteins. This universal genetic code suggests a common evolutionary origin. 3. Shared Cellular Structures: Bacteria and humans share many fundamental cellular structures and processes, such as ribosomes, DNA replication machinery, and cellular membranes. 4. Fossil Evidence: Although bacteria do not leave behind fossils in the same way that multicellular organisms do, molecular clock analyses and the discovery of ancient microbial mats provide evidence of early bacterial life on Earth. 5. Phylogenetic Analysis: Phylogenetic studies, which analyze the evolutionary relationships among different species based on genetic similarities, support the idea of a common ancestry for all life on Earth. In summary, the evidence from genetics, biochemistry, and comparative genomics strongly supports the idea that bacteria and humans share a common ancestor. 3. Why are an insect wing and a bird wing not considered evidence of relatedness? Answer: While both insect wings and bird wings serve a similar function (flight), they are not considered evidence of relatedness due to their different developmental origins and structures. Differences between Insect Wings and Bird Wings: 1. Developmental Origin: • Insect wings develop from the body wall of the insect as outgrowths of the cuticle, while bird wings develop from the forelimbs of birds. 2. Structural Differences: • Insect wings are thin, membranous structures supported by veins, while bird wings are composed of feathers supported by a skeletal structure of bones. 3. Evolutionary History: • The evolutionary pathways that led to the development of insect wings and bird wings are distinct, and there is no evidence to suggest a common ancestry for these structures. 4. The presence of vestigial structures suggests that the process of adaptation is not necessarily purposeful. Why? Answer: Vestigial structures are remnants of features that were functional in the ancestors of an organism but are no longer functional in the organism itself. The presence of vestigial structures suggests that the process of adaptation is not necessarily purposeful because: 1. Loss of Function: Vestigial structures have lost their original function over evolutionary time. 2. Evidence of Evolutionary History: Vestigial structures provide evidence of an organism's evolutionary history and its ancestors' adaptations to different environments. 3. Natural Selection: The persistence of vestigial structures indicates that they do not impose a significant fitness cost on the organism. Natural selection does not always eliminate non-functional structures if they do not negatively impact an organism's survival or reproduction. 5. How can the scientific method be used to test the concept of evolutionary descent if no one was present in the past to witness it? Answer: The scientific method can be used to test the concept of evolutionary descent through several lines of evidence and inference: 1. Fossil Record: Paleontologists study fossils to reconstruct the history of life on Earth. Fossil evidence provides a record of past life forms and their evolutionary relationships. 2. Comparative Anatomy: Comparative anatomy examines the similarities and differences in the anatomy of different species. Homologous structures, vestigial structures, and embryological development provide evidence of common ancestry. 3. Molecular Biology: Molecular biology techniques, such as DNA sequencing and molecular phylogenetics, allow scientists to compare the genetic material of different organisms and infer their evolutionary relationships. 4. Biogeography: Biogeography studies the distribution of species and how it has changed over time. The distribution of species in different geographic regions provides evidence of evolutionary descent. By combining these lines of evidence and using the scientific method to formulate and test hypotheses, scientists can infer the patterns and processes of evolutionary descent, even though no one was present in the past to witness it directly. 6. Why is adaptive radiation more prevalent when there is less competition, as in the case of the finches on the Galápagos or the mammals following the dinosaur extinction? Answer: Adaptive radiation is more prevalent when there is less competition because it allows organisms to exploit unoccupied ecological niches and diversify into new forms. In situations where competition is reduced, such as after a mass extinction event, organisms can undergo rapid evolutionary diversification for several reasons: 1. Availability of Ecological Niches: With fewer competing species, there are more available ecological niches for organisms to exploit. This allows for the diversification of species into different habitats and lifestyles. 2. Reduced Competitive Pressure: With fewer competitors, organisms face reduced competitive pressure, allowing them to explore new evolutionary pathways without being outcompeted. 3. Opportunity for Innovation: Reduced competition provides an opportunity for evolutionary innovation and experimentation, leading to the development of new traits and adaptations. 4. Release from Predation: Mass extinction events can also lead to a release from predation pressure, allowing organisms to evolve new defensive strategies or occupy new habitats. These factors create conditions favorable for adaptive radiation, where a single ancestral species diversifies rapidly into a variety of different forms to exploit available resources and habitats. CHAPTER 7 THE REPRODUCTIVE PROCESS CHAPTER OUTLINE 7.1 “Omne vivum ex ovo” A. In 1651, an English physiologist, William Harvey proposed that all life developed from the egg and this development was influenced by semen. B. Reproduction is a property of life. 7.2. Nature of the Reproductive Process A. Mechanisms (Figure 7.1) 1. Asexual reproduction involves only one parent. a. There are no special reproductive organs or cells involved. b. Genetically identical offspring are produced. 2. Sexual reproduction generally involves two parents. a. Special germ cells (gametes) unite to form a zygote. b. By receiving genetic material from both parents, the offspring are unique. c. Sexual reproduction recombines parental characters and makes possible evolution of more diverse forms. B. Asexual Reproduction: Reproduction Without Gametes 1. Neither eggs nor sperm are involved. 2. Unless mutations occur, all offspring have the same genotype and are clones of the parent. 3. Asexual reproduction is widespread in archaea and eubacteria, unicellular eukaryotes and many invertebrate phyla. 4. Asexual reproduction ensures rapid increase in numbers. 5. Binary fission is common among bacteria and protozoa. a. The parent divides by mitosis into two parts; each grows into an individual similar to the parent. b. Binary fission can be lengthwise or transverse. c. In multiple fission or schizogony, the nucleus divides repeatedly; cytoplasmic division produces many daughter cells. d. Sporogony is spore formation, a form of multiple fission in parasitic protozoa. 6. Budding is unequal division of an organism. a. The bud is an outgrowth of the parent; it develops organs and then detaches. b. Budding occurs in cnidarians and some other animal phyla. 7. Gemmulation is formation of a new individual from an aggregation of cells from the parent individual surrounded by a resistant capsule (gemmule). (Figure 12.11) a. Freshwater sponges survive winter in the dried or frozen body of the parent. b. In good conditions, the enclosed cells become active, emerge and grow a new sponge. 8. Fragmentation involves a multicellular animal breaking into many fragments that become a new animal. This is seen in many anemones and hydroids. C. Sexual Reproduction: Reproduction With Gametes 1. Bisexual Reproduction a. Also called biparental, bisexual reproduction produces offspring from union of gametes from two genetically different parents. b. Offspring therefore have a genotype different from either parent. (Figure 7.2) c. Generally, individuals are male or female and produce spermatozoa or ova, respectively. 1) The female produces the ovum; it is large with stored yolk and is nonmotile. 2) The spermatozoon is produced by the male; it is small, motile and much more numerous. d. Most vertebrates and many invertebrates have separate sexes; they are dioecious. e. Some animals have both male and female organs; they are monoecious or hermaphrodites. f. Meiosis (duplication and two divisions) produces four haploid cells. g. In fertilization, 2 haploid cells combine to restore the diploid chromosome number in zygote. h. A zygote divides by mitosis for all somatic (body) cells. i. Many unicellular organisms can reproduce both sexually and asexually. j. When sexual parents merely join together to exchange nuclear material (conjugation), distinct sexes do not occur. k. Organs that produce germ cells are gonads; testes produce sperm and ovaries produce eggs. l. Gonads are primary sex organs; some animals lack any other “accessory” sex organs. m. Additional accessory sex organs include penis, vagina, oviducts and uterus. 2. Hermaphroditism (Figure 7.3) a. Hermaphrodites have both male and female organs in the same individual. b. Many sessile, burrowing and/or endoparasitic invertebrate animals and a few fish are hermaphroditic. c. Most avoid self-fertilization and exchange germ cells with another member of the same species. d. Each individual is reproductive, in contrast to dioecious species where about half is male. e. In sequential hermaphroditism, a fish starts life as one sex and is genetically programmed to change to the other sex later. 3. Parthenogenesis a. Parthenogenesis is the development of an embryo from an unfertilized egg. b. The male and female nuclei may fail to unite after fertilization. c. In ameiotic parthenogenesis, no meiosis occurs and the egg forms by mitotic division. d. In meiotic parthenogenesis, the haploid ovum is formed by meiosis and develops without fusion with the male nuclei. 1) Sperm may be absent or they may only activate development. 2) In some species, the haploid egg returns to a diploid condition by chromosomal duplication. e. Haplodiploidy occurs in bees, wasps and ants. 1) The queen controls whether the eggs are laid fertilized or unfertilized. 2) Fertilized eggs become female workers or queens; the unfertilized eggs become drones. f. Some desert lizards have modified meiosis so offspring are clones of the female parent. g. Parthenogenesis avoids the energy and dangers of bringing two sexes together; but it narrows the diversity available for adaptation to new conditions. 4. Why do so many animals reproduce sexually rather than asexually? (Figure 7.4) a. Sexual reproduction is more common among animals. b. The costs of sexual reproduction are greater. 1) It is more complicated, requires more time and uses more energy than asexual. 2) The cost of meiosis to the female is passage of only half of her genes to offspring. 3) Production of males reduces resources for females that could produce eggs. c. Sexual organisms produce more novel genotypes to survive in times of environmental change. d. Asexual organisms can have more offspring in a short time to colonize new environments. e. In crowded habitats, selection is intense and diversity prevents extinction. f. Sexual recombination provides a means for the spread of beneficial gene mutations without holding back a population by deleterious ones. g. On a geological time scale, asexual lineages with less variation are prone to extinction. h. Many invertebrates with both sexual and asexual modes enjoy the advantages of both. 7.3. Origin and Maturation of Germ Cells A. Germ Cells 1. Somatic cells are non-reproductive body cells; they differentiate, function and die before or with the animal. 2. Germ cells form gametes; the germ cell line provides a continuous line between generations. 3. Somatic cells support, protect and nourish the germ cell line. 4. The germ cell lineage may be traceable; in some invertebrates, the germ cells develop from somatic cells. B. Migration of Germ Cells (Figure 7.5) 1. Vertebrate gonads arise from a pair of genital ridges that grows into the coelom from the dorsal coelomic lining on each side of the hindgut near the anterior end of the kidney. 2. Primordial germ cells themselves arise from yolk-sac endoderm, not the developing gonad. 3. Germ plasm from the vegetal pole of the uncleaved egg mass moves to gut endoderm and migrates by ameboid movement to genital ridges. 4. Germ cells divide first by mitosis, increasing from a few dozen to several thousand. C. Sex Determination 1. Originally gonads are sexually indifferent. 2. In mammalian males, SRY (sex determining region Y) on the Y chromosome organizes the gonad into a testis. 3. Formed as a testis, it secretes testosterone which, with dihydrotestosterone (DHT), masculinizes the fetus, causing development of a penis, scrotum and male glands. 4. In the brain, testosterone is enzymatically converted to estrogen, which determines brain organization for male-typical behavior. 5. Recent molecular evidence indicates that the X chromosome expresses ovary-determining genes, such as WNT4 and DAX1. 6. Despite levels of estrogen, the female brain does not become masculinized perhaps due to low estrogen receptors. 7. Genetics of sex determination vary: XX-XY, XX-XO, haplodiploid, temperature, etc. [see Ch. 5]. (Figure 7.6) D. Gametogenesis 1. Gametogenesis is the series of transformations that result in gametes. 2. Testes carry out spermatogenesis; ovaries carry out oogenesis. 3. Spermatogenesis (Figures 7.7, 7.8) a. The wall of seminiferous tubules contains germ cells five to eight cells deep. b. Sertoli (sustentacular) cells extend from the periphery to nourish germ cells. c. The outermost layers are spermatogonia, diploid cells that have increased by mitosis. d. A spermatogonium increases in size to become a primary spermatocyte. e. A primary spermatocyte undergoes the first meiotic division to become two secondary spermatocytes. f. Without resting, each secondary spermatocyte enters the second meiotic division to produce four haploid spermatids. g. Spermatids transform into mature spermatozoa (sperm). 1) Most cytoplasm is lost. 2) The haploid nucleus condenses into a head. 3) A midpiece forms containing mitochondria. 4) The whiplike flagellar tail provides locomotion. h. The sperm head contains an acrosome (except for some fishes and invertebrates). 1) Often the acrosome contains lysins to clear an entrance through layers surrounding the egg. 2) In mammals, one lysin is hyaluronidase; it allows sperm to penetrate follicular cells around the egg. 3) In many invertebrate sperm, an acrosome filament extends suddenly upon contact with surface of the egg. i. Fusion of egg and sperm plasma membranes is the initial event for fertilization. j. Size of sperm varies from 50 µm to 2 mm in length; most are very small. (Figure 7.9) k. Sperm greatly outnumber eggs. 4. Oogenesis a. Oogonia are early germ cells in the ovary; they are diploid and increase by mitosis. b. They cease to grow in number and increase in size as primary oocytes. (Figure 7.10) c. Chromosomes pair in the first meiotic division, similar to spermatogenesis. d. In this first division, the cytoplasm is divided unequally. e. A larger daughter cell or secondary oocyte receives most of the cytoplasm; the rest goes to the first polar body. f. In the second meiotic division, the secondary oocyte forms a large ootid and a small polar body. g. Since the first polar body also divides, this produces three polar bodies that disintegrate. h. The ootid forms a functional ovum with all the cytoplasmic components necessary for development. i. Unlike spermatogenesis that forms four gametes, oogenesis forms one haploid ovum. j. Most vertebrate and some invertebrate eggs wait for fertilization to complete the last meiotic divisions. 1) Development is arrested in prophase I in the primary oocyte phase; meiosis resumes at ovulation or after fertilization. 2) Human ova begin the first meiotic division at the thirteenth week of fetal development. 3) Human ova arrest development in prophase I until puberty. 4) After puberty, some oocytes develop into secondary oocytes; meiosis II is completed only after penetration by a spermatozoon. k. Yolk 1) Egg maturation involves deposition of yolk. 2) Yolk is stored as granules of lipid, protein or both. 3) Yolk may be synthesized internally or supplied from follicle cells. 4) Accumulation of yolk granules and nutrients cause eggs to grow massively beyond normal cell size. 5. The size of an egg violates surface-area-to-volume ratios; it therefore slows metabolism. 7.4. Reproductive Patterns A. Live-birth Versus Egg-bearing 1. Oviparous animals lay eggs outside the body for development. a. Fertilization may be internal (before eggs are laid) or external (after laid). b. Some animals abandon eggs; others provide extensive care. 2. Ovoviviparous animals retain eggs in their body. a. Essentially all nourishment is derived from the yolk. b. This is common in some invertebrate groups and certain fishes and reptiles. c. Fertilization is internal. 3. Viviparous animals give live birth. a. Eggs develop in an oviduct or uterus. b. Embryos continuously derive nourishment from the mother. c. Fertilization is internal. d. This occurs in mammals and some fishes, lizards and snakes; it provides more protection to offspring. e. Some physiologists consider ovoviviparity as a special kind of vivaparity (lecithotroph vivparity). 7.5. Structure of Reproductive Systems A. Components 1. Primary organs are the gonads that produce sperm, eggs and sex hormones. 2. Accessory organs assist gonads in formation and delivery of gametes and may support embryos. B. Invertebrate Reproductive Systems (Figures 7.11 and 20.6) 1. Invertebrates that transfer sperm for internal fertilization require complex organs. 2. Invertebrates that release sperm into water for external fertilization may be simple. a. Polychaete annelids have no permanent reproductive organs; gametes are cells from the body cavity. b. Mature gametes may be released through ducts or exit through ruptures. 3. Insects have separate sexes and accomplish internal fertilization using complex systems. a. Sperm from testes are stored in seminal vesicles before ejaculated. b. Female insects have ovaries in a series of egg tubes. c. Mature ova pass to a common genital chamber and short vagina. d. Sperm inserted by male are stored in a seminal receptacle in female. e. One mating may provide enough sperm to last the reproductive life of a female insect. C. Vertebrate Reproductive Systems 1. Urogenital system of vertebrates shows close connections of reproductive and excretory systems. 2. The opisthonephric duct drains the kidney and carries sperm in male fishes and amphibians. 3. The mesonephric duct is composed of the vas deferens and a separate ureter develops in male reptiles, birds and mammals. 4. The cloaca is the common chamber for intestinal, reproductive and excretory canals, except in mammals. 5. The uterine duct of the oviduct has an independent duct opening into cloaca when present. D. Male Reproductive System 1. Paired testes are sites of sperm production. 2. Testes contain numerous seminiferous tubules where sperm develop. (Figure 7.12) 3. Sperm are surrounded by Sertoli cells that nourish developing sperm. 4. Between tubules are interstitial cells (leydig cells) that produce testosterone. 5. A sac-like scrotum suspends testes outside the warm body cavity; the lower temperature of scrotum is vital to normal sperm production. 6. Sperm pass from the testes to vasa efferentia and to coiled epididymis for maturation. 7. The vas deferens carries sperm from the epididymis to the urethra, where it exits the penis. 8. The penis is a copulatory organ used to introduce spermatozoa into the female vagina. 9. Seminal vesicles, prostate gland and bulbourethral glands form seminal fluid. a. Seminal vesicles secrete a thick fluid containing nutrients for use by sperm. b. The prostate gland secretes a milky, slightly alkaline solution that counters acidity. c. Bulbourethral glands release mucus secretions that provide lubrication. E. Female Reproductive System 1. Ovaries in female vertebrates produce ova and the female sex hormones, estrogen and progesterone. 2. In jawed vertebrates, mature ova from ovaries enter funnel-like oviducts (fallopian tube or uterine tube). 3. The terminal end of uterine tube is specialized in cartilaginous fishes, reptiles and birds to produce shelled eggs; special regions produce albumin and shell. 4. The terminal portion of amniote uterine tube expands into a muscular uterus. a. Shelled eggs may be retained here before laying. b. Embryos may complete their development here. c. Placental mammals use the walls of the uterus to intermingle vascular tissue as a placenta. 5. Ovaries are paired and slightly smaller than male testes. (Figure 7.13) a. Oocytes develop within a follicle that enlarges to release a secondary oocyte. b. Unless fertilization occurs, women release about 13 oocytes per year, 300–400 per a 30-year reproductive lifetime. c. 300–400 primary oocytes, of ca. 400,000 in ovaries at birth, reach maturity while the rest degenerate and are absorbed. 6. Uterine tubes or oviducts are lined with cilia that propel the egg. 7. The oviducts enter the upper corners of the uterus. 8. Uterus a. The uterus is specialized to house the embryo for nine months. b. The uterus has thick muscular walls and is stretchable. c. The endometrium is the specialized lining rich in blood vessels. d. Ancestrally, the uterus was paired but is fused in eutherian mammals. 9. The vagina is a muscular tube that receives the male’s penis and serves as the birth canal. 10. The cervix is the end of the uterus that extends into the vagina. 11. The vulva is external genitalia in human females. a. Labia majora and labia minora enclose urethral and vaginal openings. b. The clitoris is a small erectile organ equivalent to the glans penis of male. 7.6. Endocrine Events that Orchestrate Reproduction A. Hormonal Control of Timing of Reproductive Cycles 1. Vertebrate reproduction is seasonal or cyclic to align with food supply and survival of young. 2. Sexual cycles are controlled by hormones that respond to food intake, photoperiod, rainfall, temperature or social cues. 3. The hypothalamus region of the forebrain regulates the release of anterior pituitary gland hormones, which stimulate tissues of the gonads. 4. Estrous Cycles a. Females are receptive to males only during brief periods of estrus or “heat.” b. The estrous cycle ends with uterine lining reverting to original state; there is no menstruation. 5. Menstrual Cycles a. This cycle occurs in monkeys, apes and humans. b. Females are receptive to males throughout the cycle. c. At the end of the menstrual cycle the endometrium (uterine lining) is discharged. B. Gonadal Steroids and Their Control (Figure 7.14) 1. Ovaries produce estrogens and progesterone. 2. The three estrogens include estradiol, estrone and estriol. 3. Estrogen Functions a. Estrogens develop female accessory sex structures: oviducts, uterus and vagina. b. Estrogens stimulate female reproductive activity. c. Secondary non-reproductive characteristics include 1) skin or feather coloration, 2) bone development, 3) body size, and 4) initial development of mammary glands in mammals. 4. Both estrogen and progesterone prepare the uterus to receive an embryo. a. The hypothalamus produces gonadotropin releasing hormone (GnRH). b. GnRH governs pituitary release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). c. Light, nutrition, stress, etc. can influence this complex feedback system. 5. Testosterone a. Interstitial cells in testes manufacture testosterone. b. Testosterone and its metabolite dihydrotestosterone (DHT) are required for growth of the penis, sperm ducts, and glands, and secondary sexual traits. c. Secondary non-reproductive characteristics include 1) male plumage and pelage coloration, 2) bone and muscle growth, 3) antlers in deer, and 4) vocal cord growth in humans. d. Testosterone and DHT feedback to hypothalamus and anterior pituitary to keep secretion of GnRH, FSH and LH in check. e. Sertoli cells of testes secrete inhibin; it regulates FSH of anterior pituitary by negative feedback. C. The Menstrual Cycle (Figure 7.15) 1. The ovary has two phases: follicular and luteal. 2. The uterus has three phases: menstrual, proliferative and secretory. 3. Menstruation, shedding of the uterine lining, signals the menstrual phase. 4. The follicular phase of ovary is also occurring. a. By day three of the menstrual cycle, blood levels of FSH and LH rise slowly, prompting some ovarian follicles to grow and secrete estrogen. b. As estrogen increases, the uterine endometrium heals and begins to thicken. c. Uterine glands within the endometrium enlarge in the proliferative phase of uterus. d. By day 10, most ovarian follicles degenerate (become atretic) leaving one, two or three to continue ripening. e. Final mature follicle is the Graafian follicle; it secretes more estrogen and also inhibin. f. At day 13 or 14, high levels of estrogen from the Graafian follicle stimulate a surge in GnRH from hypothalamus. g. This stimulates a surge of LH and some FSH from anterior pituitary. h. The LH surge causes the largest follicle to rupture and release an oocyte (ovulation). 5. The luteal phase of the ovary is named for the corpus luteum, which is the remainder of the ruptured follicle. a. The corpus luteum responds to LH and secretes progesterone. b. Progesterone stimulates the uterus to undergo maturation and prepare for gestation. c. If an embryo implants, the uterus enters the secretory phase. d. If fertilization does not occur, the corpus luteum degenerates and hormones are no longer secreted. e. The uterus depends on progesterone and estrogen to maintain uterine lining; declining levels start endometrium degeneration and lead to menstrual discharge. 6. Negative feedback among the hypothalamus, anterior pituitary and ovary control the cycle. 7. Recently a possible gonadotropin-inhibiting hormone has been discovered in the hypothalamus of birds and mammals. 8. Ovulation is due to high levels of estrogen causing a surge in GnRH, LH and FSH; such positive feedback is rare since it moves events away from stable set points. D. Hormones of Human Pregnancy and Birth (Figure 7.16) 1. Fertilization normally occurs in the outer third of the uterine tube (ampulla). 2. As the zygote travels to uterus, it divides by mitosis to form a blastocyst. 3. In about six days, on contact with the uterine lining, it embeds in the endometrium (implantation). 4. The spherically-shaped trophoblast contains three layers: amnion, chorion and embryo proper. 5. The chorion is a source of human chorionic gonadotropin (hCG); it stimulates the corpus luteum to produce estrogen and progesterone. 6. The placenta is formed between the trophoblast and uterus. a. The placenta is an endocrine gland, secreting hCG, estriol, and progesterone. b. The placenta also transfers nutrients and wastes between mother and fetus. c. After a month, the corpus luteum degenerates and the placenta itself holds the lining by progesterone and estrogen. (Figure 7.17) 7. Preparation of mammary glands to secrete milk requires two additional hormones. a. Prolactin (PRL) is produced by the anterior pituitary, but is inhibited in non-pregnant women. b. During pregnancy, elevated progesterone and estrogen depress inhibition and PRL appears in the blood. c. PRL is also produced by the placenta during pregnancy. d. Human placental lactogen (hPL) aids PRL in preparing the mammary glands for secretion. e. Together with maternal growth hormone, hPL stimulates an increase in nutrients in the mother. f. The placenta also secretes -endorphin and other endogenous opioids that regulate appetite and mood during pregnancy. g. The placenta later synthesizes peptide hormone relaxin to allow expansion of pelvis by flexibility of pubic symphysis. (Figure 29.9) h. Relaxin also dilates the cervix in preparation for delivery. 8. Birth or parturition begins with rhythmic contractions of uterus called labor. a. Placental corticotropin-releasing hormone (CRH) appears to initiate the birth process. b. Estrogen secreted before birth stimulates contractions. c. Progesterone levels, which inhibit contraction, decline. d. Prostaglandin hormones increase, making the uterus more irritable. e. Uterine stretching causes neural reflexes to stimulate secretion of oxytocin from the posterior pituitary. f. Oxytocin stimulates uterine smooth muscle contractions. g. Childbirth (Figure 7.18) 1) First stage: the cervix enlarges and the amniotic sac will rupture. 2) Second stage: the baby is forced out of the uterus and through the vagina. 3) Third stage: the placenta or afterbirth is expelled. h. Milk Production 1) Milk production is triggered when infant sucks on the mother’s nipple. 2) Stimulation leads to reflex release of oxytocin from the pituitary. 3) Oxytocin causes contraction of smooth muscles lining ducts of mammary glands. 4) Suckling also stimulates release of prolactin, which continues milk production. E. Multiple Births (Figure 7.19) 1. Many mammals are multiparous, giving birth to many offspring at one time. 2. Some give birth only to one at a time; they are uniparous. 3. Exceptions occur; the armadillo gives birth to four young, all male or all female, derived from one zygote. 4. Monozygotic, or identical, twins are derived from one zygote; they have identical genomes. 5. Fraternal, dizygotic or nonidentical, twins are from two zygotes and may not resemble each other any more than other siblings. 6. Identical Twins a. They may separate early and have separate placentas. b. Two-thirds share a placenta and splitting occurred after formation of the inner cell mass, but most have individual amniotic sacs. c. A few share one amniotic sac and a single placenta; separation of the zygote occurred after day 9 of pregnancy when the amnion has formed; these twins risk becoming conjoined (Siamese twinning). Lecture Enrichment 1. Discuss the procession to internal egg fertilization as a requirement for life on land. Contrast the shelled eggs of reptiles and birds, the egg cases of insects and the internal development of the fetus in mammals. Discuss how each is adapted to prevent desiccation. 2. Have students trace the path of sperm from its development in the testes to its release during ejaculation; ask for the additions of glandular secretions along the way. 3. Research shows that when sperm enters the egg, the mitochondrial mid piece and parts of the tail also enter. However, there are far fewer sperm mitochondria and they are worn out, and the egg tags them for cell destruction. Thus, a person inherits all of his mitochondria from his/her biological mother. 4. Have students trace the path of the egg as it develops in the ovary and is released. They should describe what happens to the egg if it is fertilized or if it is not fertilized. 5. The description of the hormonal feedback of the menstrual cycle is sufficient to allow students to think through the mechanism of the birth control pill. Commentary/Lesson Plan Background: Due to American society’s preoccupation with this topic, this is generally the highest interest topic in biology. Student experiences in this subject may be varied; students from rural areas may still have experiences with animal birth, breeding, litters, etc. However, the social context may prevent using some student testimonials and experiences. Misconceptions: The term “germ” has far more recognition in meaning as a pathogen or microbe than as “germinal lineage.” It is also not easy for students to comprehend the germ line as an immortal cell lineage where we are individual temporary support systems for this cell lineage—a particularly biological viewpoint. “Womb” is a nonfunctional term since it has historically been used for ovaries as well as uterus. “Hermaphrodites” and “bisexual” have social meanings different from their biological usage here. Ambiguous human sexual development is rarely truly hermaphroditic (see John Money and Anke Erhardt’s Man and Woman, Boy and Girl), but hermaphroditism has genuine advantages for organisms isolated in soil or hosts. Woody Allen’s famous quote that bisexuality doubles your chances of a date on Friday evening clearly explains the advantages of hermaphroditism and also reveals the different social usage of “bisexual” from the scientific meaning for this term. Basic bisexual reproduction refers to the separate male and female organism system we use, but it will be difficult for some students to associate this with human heterosexuality. Schedule: Time spent on this chapter may vary greatly depending on how much an instructor wishes to elaborate on the examples and wide diversity of animal reproductive systems. HOUR 1 7.1 “Omen vivum ex ovo” 7.2. Nature of the Reproductive Process A. Mechanisms B. Asexual Reproduction: Reproduction Without Gametes C. Sexual Reproduction: Reproduction With Gametes 7.3. Origin and Maturation of Germ Cells A. Germ Cells B. Migration of Germ Cells C. Sex Determination D. Gametogenesis HOUR 2 7.4. Reproductive Patterns A. Live-birth Versus Egg-bearing 7.5. Structure of Reproductive Systems A. Components B. Invertebrate Reproductive Systems C. Vertebrate Reproductive Systems Male Reproductive System E. Female Reproductive System . HOUR 3 7.6. Endocrine Events that Orchestrate Reproduction A. Hormonal Control of Timing of Reproductive Cycles B. Gonadal Steroids and Their Control C. Menstrual Cycle D. Hormones of Human Pregnancy and Birth E. Multiple Births ADVANCED CLASS QUESTIONS: 1. About one-half of a herd of cattle is born male, but a farmer does not want all these troublesome tough-meat bulls. Therefore, most are castrated before puberty. What physiological and body changes would you expect, and would the steer resemble the cow or bull and why? Answer: Castration of male cattle, known as steers, before puberty has several physiological and body changes: 1. Hormonal Changes: Castration removes the testes, which are the primary source of testosterone production. As a result, testosterone levels decrease significantly. 2. Body Changes: • Without testosterone, secondary sexual characteristics such as muscle development, thickening of the neck, and aggression typical of bulls do not develop. • Steers tend to have a more docile temperament compared to intact bulls. 3. Physical Appearance: • Steers tend to resemble cows more closely in terms of physical appearance due to the absence of secondary sexual characteristics. • They may have a more rounded, less muscular appearance compared to intact bulls. 2. Twinning rates vary around the earth, with twins occurring in one out of 80 births in the U.S., with higher frequency in the tropics where life-span is much shorter, and with less frequency in northern regions and Asia where life-span has historically been longer. Age of first menstruation is also lower in the tropics, higher in Norway, etc. What are the physiological basis and the evolutionary implications of this? Answer: The variation in twinning rates and age of first menstruation around the world is influenced by several physiological and evolutionary factors: 1. Physiological Basis: • Higher twinning rates and earlier age of first menstruation in the tropics are influenced by factors such as nutrition, sunlight exposure (which affects vitamin D levels), and hormonal regulation. • In regions with shorter lifespans, such as the tropics, there may be evolutionary pressure to reach reproductive maturity earlier to maximize reproductive success. • Factors such as diet, stress, and environmental conditions can affect hormone levels, which in turn influence the onset of menstruation and fertility. 2. Evolutionary Implications: • Early onset of menstruation and higher twinning rates in regions with shorter lifespans may be advantageous in environments where individuals have a shorter window of opportunity to reproduce. • In regions with longer lifespans, such as northern regions and Asia, there may be less evolutionary pressure to reach reproductive maturity early, leading to later onset of menstruation and lower twinning rates. 3. If some reptiles (e.g., garter snakes) hold eggs internally until they hatch, why has this strategy not evolved in all snakes? Answer: The strategy of holding eggs internally until they hatch, known as ovoviviparity, has not evolved in all snakes due to differences in reproductive strategies and ecological niches: 1. Environmental Adaptations: • Ovoviviparity is advantageous in environments where external conditions are less favorable for egg development, such as colder climates or areas with high predation pressure. • In species that lay eggs externally (oviparity), the eggs are often deposited in protected locations such as burrows or nests, reducing the need for internal egg retention. 2. Energy Investment: • Internal egg retention requires additional energy for the mother to incubate and nourish the developing embryos. • In species that lay eggs externally, the mother can allocate energy to other activities such as foraging or evading predators. 3. Reproductive Strategies: • Snakes have evolved a variety of reproductive strategies, including oviparity, ovoviviparity, and viviparity, to adapt to different environments and ecological niches. • The diversity of reproductive strategies reflects the trade-offs between energy investment, offspring survival, and environmental conditions. 4. Why would you predict hormonal rather than nervous control of the reproductive cycle? Answer: Hormonal control of the reproductive cycle is more prevalent than nervous control due to several reasons: 1. Complexity of Reproductive Processes: • Reproduction involves a series of complex physiological processes, including the development and release of gametes, preparation of the reproductive tract for fertilization, and maintenance of pregnancy. • Hormones provide a coordinated and integrated system of control for these processes. 2. Feedback Mechanisms: • Hormonal regulation of the reproductive cycle involves intricate feedback mechanisms that allow for precise control of reproductive events. • Hormones such as estrogen, progesterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) are involved in regulating the ovarian cycle, menstrual cycle, and pregnancy. 3. Endocrine Glands: • Endocrine glands such as the hypothalamus, pituitary gland, and gonads (ovaries and testes) play key roles in the production and regulation of reproductive hormones. • These glands release hormones into the bloodstream, allowing for systemic control of reproductive processes. 5. Why would a woman who was having difficulty conceiving due to too low hormone levels be more likely to have multiple births when treated? Answer: A woman who is having difficulty conceiving due to low hormone levels may be more likely to have multiple births when treated because: 1. Hormone Therapy: • Hormone therapy, such as the administration of follicle-stimulating hormone (FSH) or human chorionic gonadotropin (hCG), is often used to stimulate ovulation and increase the chances of conception. • Higher hormone levels increase the likelihood of multiple eggs being released during ovulation, leading to the possibility of multiple fertilizations and multiple births. 2. Ovarian Stimulation: • Ovarian stimulation therapies can result in the development and release of multiple eggs during ovulation, increasing the chances of fertilization and multiple pregnancies. 3. Assisted Reproductive Technologies (ART): • In vitro fertilization (IVF) and other ART procedures often involve the stimulation of the ovaries to produce multiple eggs for fertilization. • The transfer of multiple embryos during IVF increases the likelihood of multiple pregnancies. 6. How might some mechanisms triggering contractions cause too early a delivery when a mother is carrying multiples? Answer: Mechanisms triggering contractions can cause premature delivery in mothers carrying multiples due to several factors: 1. Overdistention of the Uterus: • Carrying multiple fetuses leads to increased stretching and overdistention of the uterine muscles. • Overdistention can trigger uterine contractions prematurely, leading to preterm labor and delivery. 2. Hormonal Changes: • Hormonal changes associated with multiple pregnancies, such as increased levels of oxytocin and prostaglandins, can stimulate uterine contractions. • High levels of these hormones can lead to premature onset of labor and delivery. 3. Placental Insufficiency: • Multiple pregnancies are associated with a higher risk of placental insufficiency, where the placenta is unable to provide adequate nutrients and oxygen to the developing fetuses. • Placental insufficiency can trigger premature labor as a protective mechanism to remove the fetuses from an environment with limited resources. 7. Why would evolution not select for triggering milk production in humans based simply on a clock-like mechanism that “went off” at nine months? Answer: Evolution has not selected for triggering milk production in humans based solely on a clock-like mechanism due to several reasons: 1. Variability in Infant Needs: • Human infants have varying nutritional needs and growth rates, and a fixed "clock-like" mechanism may not adequately meet these needs. • Breast milk composition changes over time to meet the evolving nutritional requirements of the infant. 2. Environmental Factors: • Environmental factors such as maternal nutrition, infant health, and socio-cultural practices influence the timing and duration of lactation. • Flexibility in milk production allows mothers to adjust to the specific needs of their infants and environmental conditions. 3. Bonding and Social Interaction: • Breastfeeding serves functions beyond nutrition, including bonding, comfort, and social interaction between mother and infant. • A flexible mechanism for milk production allows for increased maternal-infant bonding and social development. 4. Evolutionary Adaptation: • Evolution has favored flexibility and adaptability in reproductive and parental behaviors, allowing for responses to the dynamic and unpredictable environment. • A clock-like mechanism for milk production would be less adaptive in environments with varying resource availability and infant needs. In summary, evolution has selected for flexible and adaptive mechanisms for milk production in humans to meet the varying nutritional, social, and environmental needs of infants and mothers. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214