Upon the completion of the textbook readings in Chapters 22-26 you should be able to: Chapter 22
1. Explain how the principle of gradualism and Charles Lyell's theory of uniformitarianism influenced Darwin's ideas about evolution. The basic idea of natural selection is that a population of organisms can change over the generations if individuals having certain heritable traits leave more offspring than other individuals. The result of natural selection is evolutionary adaptation, a prevalence of inherited characteristics that enhance organisms’ survival and reproduction in specific environments. -gradualism, which holds that profound change is the cumulative product of slow but continuous processes. -incorporated Hutton’s gradualism into a theory known as uniformitarianism. The term refers to Lyell’s idea that geologic processes have not changed throughout Earth’s history. Thus, for example, the forces that build mountains and erode mountains and the rates at which these forces operate are the same today as in the past. Darwin was strongly influenced by two conclusions that followed directly from the observations of Hutton and Lyell. First, if geologic change results from slow, continuous actions rather than sudden events, then Earth must be very old, certainly much older than the 6,000 years assigned by many theologians on the basis of biblical inference. Second, very slow and subtle processes persisting over a long period of time can add up to substantial change. Darwin was not the first to apply the principle of gradualism to biological evolution, however.
2. Describe Jean Baptiste Lamarck's model for how adaptations evolve. It incorporates two ideas that were popular during Lamarck’s era. The first was use and disuse, the idea that those parts of the body used extensively to cope with the environment become larger and stronger while those that are not used deteriorate. Among the examples Lamarck cited were a blacksmith developing a bigger bicep in the arm that wields the hammer and a giraffe stretching its neck to reach leaves on high branches. The second idea Lamarck adopted was called the inheritance of acquired characteristics. In this concept of heredity, the modifications an organism acquires during its lifetime can be passed along to its offspring. The long neck of the giraffe, Lamarck reasoned, evolved gradually as the cumulative product of a great many generations of ancestors stretching ever higher. 3. Describe how Charles Darwin used his observations from the voyage of the HMS Beagle to formulate and support his theory of evolution.
Darwin noted that plants and animals he studied had definite South American characteristics, very distinct from those of Europe. That in itself may not have been surprising. But Darwin also noted that the plants and animals in temperate regions of South America were more closely related to species living in tropical regions of that continent than to species in temperate regions of Europe. Furthermore, the South American fossils that Darwin found, though clearly different from modern species, were distinctly South American in their resemblance to the living plants and animals of that continent. 4. Describe how Alfred Russel Wallace influenced Charles Darwin. Wallace developed a theory of natural selection essentially identical to Darwin’s. Wallace asked Darwin to evaluate the paper and forward it to Lyell if it merited publication. Darwin complied, writing to Lyell: "Your words have come true with a vengeance ... . I never saw a more striking coincidence ... so all my originality, whatever it may amount to, will be smashed." Lyell and a colleague presented Wallace’s paper, along with extracts from Darwin’s unpublished 1844 essay, to the Linnaean Society of London on July 1, 1858. Darwin quickly finished The Origin of Species and published it the next year. Although Wallace wrote up his ideas for publication first, Darwin developed and supported the theory of natural selection so much more extensively that he is known as its main architect. And Darwin’s notebooks prove that he formulated his theory of natural selection 15 years before reading Wallace’s manuscript. 5. Describe Darwin's theory of natural selection.
Natural selection is differential success in reproduction (unequal ability of individuals to survive and reproduce). Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population . The product of natural selection is the adaptation of populations of organisms to their environment 6. Explain what is meant by ‘descent with modification.’ Descent with modification is a phrase that condensed his view of life. Darwin perceived unity in life, with all organisms related through descent from some unknown ancestor that lived in the remote past. As the descendants of that ancestral organism spilled into various habitats over millions of years, they accumulated diverse modifications, or adaptations, that fit them to specific ways of life. 7. Explain what evidence convinced Darwin that species change over time. In each generation, environmental factors filter heritable variations, favoring some over others. Differential reproduction--whereby organisms with traits favored by the environment produce more offspring than do organisms without those traits--results in the favored traits being disproportionately represented in the next generation. This increasing frequency of the favored traits in a population is evolution. 8. Explain why variation was so important to Darwin's theory. 9. Distinguish between artificial selection and natural selection. Artificial selection is the breeding of domesticated plants and animals. Humans have modified other species over many generations by selecting individuals with the desired traits as breeding stock. The plants and animals we grow for food often bear little resemblance to their wild ancestors (FIGURE 22.11). The power of selective breeding is especially apparent in our pets, which have been bred more for fancy than for utility. If so much change can be achieved by artificial selection in a relatively short period of time, Darwin reasoned, that natural selection should be capable of considerable modification of species over hundreds or thousands of generations. Even if the advantages of some heritable traits over others are slight, the advantageous variations will accumulate in the population after many generations of natural selection, eliminating less favorable variations. Natural selection occurs through interactions between individual organisms and their environment, but individuals do not evolve. Evolution can be measured only as changes in relative proportions of heritable variations in a population over a succession of generations. Another key point about natural selection is that it can amplify or diminish only heritable variations. As we have seen, an organism may become modified through its own experiences during its lifetime, and such acquired characteristics may even adapt the organism to its environment, but there is no evidence that characteristics acquired during a lifetime can be inherited. We must distinguish between adaptations an organism acquires by its own actions and inherited adaptations that evolve in a population over many generations as a result of natural selection. It must also be emphasized that the specifics of natural selection are situational; environmental factors vary from place to place and from time to time. An adaptation in one situation may be useless or even detrimental in different circumstances. Some examples will reinforce this situational quality of natural selection. 10. Distinguish between homologous and analogous structures and give example of each. Similarity in characteristics resulting from common ancestry is known as homology. * Anatomical Homologies. Descent with modification is indeed evident in anatomical similarities between species grouped in the same taxonomic category. * Embryological Homologies. Sometimes, homologies that are not obvious in adult organisms become evident when we look at embryonic development. * Molecular Homologies. Anatomical homology cannot help us link such distantly related organisms as plants and animals, which have no anatomy in common. However, plants and animals, along with all other organisms, do share certain characteristics at the molecular level: For example, all species of life use the same basic genetic machinery of DNA and RNA, and the genetic code is essentially universal (see Chapter 17). Evidently, the language of the genetic code has been passed along through all branches of the tree of life ever since the code’s inception in an early life-form. Chapter 23
11. Explain why the population is the basic unit of evolution. It is the population, not its individuals, that evolves; some characteristics become more common within the overall population, while other characteristics decline . One is the importance of populations in evolution. For now, we will define a population as a group of interbreeding individuals belonging to a particular species and sharing a common geographic area. A population is the smallest unit that can evolve. Natural selection occurs through interactions between individual organisms and their environment, but individuals do not evolve. Evolution can be measured only as changes in relative proportions of heritable variations in a population over a succession of generations. 12. Explain how microevolutionary change can affect a gene pool. Microevolution is a generation-to-generation change in a population’s allele frequencies. The two main causes of microevolution are genetic drift and natural selection
13. State the Hardy-Weinberg theorem.
The theorem states that the frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than Mendelian segregation and recombination of alleles. Put another way, the shuffling of alleles due to meiosis and random fertilization has no effect on the overall gene pool of a population. The Hardy-Weinberg theorem. The gene pool of a nonevolving population remains constant over the generations; Mendelian segregation alone will not alter the relative frequencies of alleles or genotypes. In this example (a hypothetical flower population), note that the frequencies of alleles and genotypes remain the same between one generation (a) and the next generation (b). 14. List the conditions a population must meet in order to maintain Hardy-Weinberg equilibrium. 1. Very large population size . In a population of finite size, especially if that size is small, genetic drift, which is chance fluctuation in the gene pool, can cause genotype frequencies to change over time. 2. No migration (population must be isolated) . Gene flow, the transfer of alleles between populations due to the movement of individuals or gametes, can increase the frequency of any genotype that is in high frequency among the immigrants. 3. No net mutations . By changing one allele into another, mutations alter the gene pool. 4. Random mating . If individuals pick mates with certain genotypes, then the random mixing of gametes required for Hardy-Weinberg equilibrium does not occur. 5. No natural selection . Differential survival and reproductive success of genotypes will alter their frequencies and may cause a detectable deviation from frequencies predicted by the Hardy-Weinberg equation.|
Thus, we do not really expect a natural population to be in Hardy-Weinberg equilibrium. And a deviation from the stability of a gene pool--and from Hardy-Weinberg equilibrium--usually results in evolution. 15. Write the general Hardy-Weinberg equation and use it to calculate allele and genotype frequencies.
For a gene locus where only two alleles occur in a population, population geneticists use the letter p to represent the frequency of one allele and the letter q to represent the frequency of the other allele. In our imaginary wildflower population, p = 0.8 and q = 0.2 (see FIGURE 23.3). Note that p + q = 1; the combined frequencies of all possible alleles must add to 100% for that locus in the population. If there are only two alleles and we know the frequency of one, the frequency of the other can be calculated: If p + q = 1 then p = 1 - q and q = 1 - p
Or p^2 + 2pq+ q^2=1
When gametes combine their alleles to form zygotes, the probability of generating an RR genotype is p 2 (an application of the rule of multiplication). In our wildflower population, p = 0.8, and p = 0.64, the probability of an R sperm fertilizing an R ovum to produce an RR zygote. The frequency of individuals homozygous for the other allele (rr ) is q 2, or 0.2 X 0.2 = 0.04 for the wildflower population. There are two ways in which an Rr genotype can arise, depending on which parent contributes the dominant allele. Therefore, the frequency of heterozygous individuals in the population is 2pq (2 X 0.8 X 0.2 = 0.32 in our example). If we have included all possible genotypes, the genotype frequencies add up to 1: p 2| +| 2pq | +| q 2| =1|
Frequency of RR genotype| | Frequency of Rr plus rR genotype| | Frequency of rr genotype| |
For our wildflowers, this is 0.64+0.32+0.04=1.
Population geneticists refer to this general formula as the Hardy-Weinberg equation. The equation enables us to calculate frequencies of alleles in a gene pool if we know frequencies of genotypes, and vice versa. 16. Describe the usefulness of the Hardy-Weinberg model to population geneticists. We can use the Hardy-Weinberg equation to estimate the percentage of the human population that carries the allele for a particular inherited disease The Hardy-Weinberg theorem is important conceptually and historically because it shows how Mendel’s theory of inheritance plugs a hole in Darwin’s theory of natural selection. Natural selection requires genetic variation; it cannot act in a genetically uniform population. The Hardy-Weinberg theorem explains how Mendelian inheritance preserves genetic variation from one generation to the next. Pre-Mendelian theories of inheritance were mainly "blending" theories, in which the hereditary factors in the offspring were thought to be a blend of the hereditary factors inherited from the two parents. If a red flower mates with a white one, blending theory predicts that the offspring will be a paler red and will now have hereditary factors for this paler red color. Genetic variation has been eliminated, since the two kinds of factors in the parents have been reduced to only one kind in the offspring. Such a hereditary mechanism would soon produce a uniform population. In Mendelian inheritance, however, the hereditary mechanism has no tendency by itself to reduce genetic variation. The set of alleles inherited by each generation from its parents are in turn passed on when that generation breeds. This nonblending mechanism of inheritance preserves the genetic variation upon which natural selection acts. 17. Explain how genetic drift, gene flow, mutation, nonrandom mating and natural selection can cause microevolution.
Evolution is a generation-to-generation change in a population’s frequencies of alleles. Because such change in a gene pool is evolution on the smallest scale, it is referred to more specifically as microevolution. Genetic Drift
Let’s apply coin toss logic to a population’s gene pool. If a new generation draws its alleles at random from the previous generation, then the larger the population (the sample size), the more closely the new generation will represent the gene pool of the previous generation. Thus, one requirement for a gene pool to maintain the status quo--Hardy-Weinberg equilibrium--is a large population size (actually infinitely large, an ideal never met). The gene pool of a small population may not be accurately represented in the next generation because of sampling error. It is analogous to the erratic outcome from a small sample of coin tosses. Chance causes the frequencies of the alleles for red (R ) and white (r ) flowers to change over the generations. And that change fits our definition of microevolution. This evolutionary mechanism, a change in a population’s allele frequencies due to chance, is called genetic drift. Natural Selection
Hardy-Weinberg equilibrium requires that all individuals in a population be equal in their ability to survive and produce viable, fertile offspring. This condition is probably never completely met. Populations consist of varied individuals, with some variants leaving more offspring than others. This differential success in reproduction is what Darwin meant by natural selection. Selection results in alleles being passed along to the next generation in numbers disproportionate to their relative frequencies in the present generation. One possible cause of such a disproportion might be differential survival. For example, in our imaginary wildflower population, perhaps white flowers (rr ) are more visible to herbivorous insects, so that more white flowers are eaten. Plants with red flowers (RR or Rr ) would therefore have more opportunity to produce offspring. Selection can also affect reproductive success more directly. For example, red flowers may be more effective than white ones in attracting the pollinators required for seed production. This difference would disturb Hardy-Weinberg equilibrium; the frequency of the R allele would increase in the gene pool, and the frequency of the r allele would decline. Of all the agents of microevolution that change a gene pool, only selection is likely to adapt a population to its environment. Natural selection accumulates and maintains favorable genotypes in a population. Natural selection and genetic drift cause most of the changes in allele frequencies that we observe in evolving populations. However, allele frequencies can also be changed by migration between populations or by mutation, and in some cases these factors are important. Gene Flow
A population may gain or lose alleles by gene flow, genetic exchange due to the migration of fertile individuals or gametes between populations. Gene flow tends to reduce differences between populations. If it is extensive enough, gene flow can eventually amalgamate neighboring populations into a single population with a common gene pool. Mutation
A mutation is a change in an organism’s DNA (see Chapter17). A new mutation that is transmitted in gametes can immediately change the gene pool of a population by substituting one allele for another. For any one gene locus, mutation alone does not have much quantitative effect on a large population in a single generation, because a mutation at any given gene locus is a very rare event. If some new allele produced by mutation increases its frequency by a significant amount in a population, it is not because mutation is generating the allele in abundance, but because individuals carrying the mutant allele are producing a disproportionate number of offspring as a result of natural selection or genetic drift. Although mutations at a particular gene locus are rare, the cumulative impact of mutations at all loci can be significant. This is because each individual has thousands of genes, and many populations have thousands or millions of individuals. Certainly over the long term, mutation is, in itself, very important to evolution because it is the original source of the genetic variation that serves as raw material for natural selection. 18. Explain the role of population size in genetic drift. The gene pool of a small population may not be accurately represented in the next generation because of sampling error. It is analogous to the erratic outcome from a small sample of coin tosses. 19. Distinguish between the bottleneck effect and the founder effect. The Bottleneck Effect. Disasters such as earthquakes, floods, droughts, and fires may reduce the size of a population drastically. The small surviving population may not be representative of the original population’s gene pool. By chance, certain alleles will be overrepresented among the survivors. Other alleles will be underrepresented. And some al leles may be eliminated altogether. Genetic drift may continue to change the gene pool for many generations until the population is again large enough for sampling errors to be less significant. Genetic drift due to a drastic reduction in population size is called the bottleneck effect. Bottlenecking usually reduces the overall genetic variability in a population because at least some alleles are likely to be lost from the gene pool. An important example of this concept is the potential loss of individual variation, and hence adaptability, in bottlenecked populations of endangered species The Founder Effect. Genetic drift is also likely whenever a few individuals from a larger population colonize an isolated island, lake, or some other new habitat. The smaller the sample size, the less the genetic makeup of the colonists will represent the gene pool of the larger population they left. The most extreme case would be the founding of a new population by one pregnant animal or a single plant seed. If the colony is successful, genetic drift will continue to affect the frequency of alleles in the gene pool until the population is large enough for sampling errors from generation to generation to be minimal. Genetic drift in a new colony is known as the founder effect. The founder effect probably accounts for the relatively high frequency of certain inherited disorders among human populations established by a small number of colonists. 20. Explain the difference between geographic and reproductive isolation. Reproductive Isolation
A group of genes becomes isolated from another to begin a separate evolutionary history. Once separated, the two isolated populations may begin to diverge genetically under the pressure of different selective forces in different environments. Geographic Isolation
A group of genes are separated by mountain ranges, canyons, rivers, lakes, or longitude. 21. List and describe three major causes of genetic variation Genetic variation occurs within and between populations. Mutation and sexual recombination generate genetic variation Variation Within Populations
Both quantitative and discrete characters contribute to variation within a population. Most heritable variation consists of quantitative characters that vary along a continuum within a population. For example, plant height may vary continuously in our hypothetical wildflower population, from very short individuals to very tall individuals and everything in between. Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypic character (see Chapter 14). Discrete characters , such as red versus white flowers, can be classified on an either-or basis, usually because they are determined by a single gene locus with different alleles that affect distinct phenotypes. Polymorphism. When two or more forms of a discrete character are represented in a population, the different forms are called morphs --as in the red-flowered and white-flowered morphs of our wildflower population. A population is said to be polymorphic for a character if two or more distinct morphs are each represented in high enough frequencies to be readily noticeable. Mutation
New alleles originate only by mutation, or change in the nucleotide sequence of DNA. A mutation affecting any gene locus is an accident that is rare and random. Most mutations occur in somatic cells and are lost when the individual dies. Only mutations that occur in cell lines that produce gametes can be passed along to offspring. Sexual Recombination
Members of a sexually reproducing population owe nearly all their genetic differences to the unique recombinations of existing alleles each individual receives from the gene pool. 22. Explain the concept of relative fitness and its role in adaptive evolution. Population geneticists define relative fitness as the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus. For example, consider our wildflower population, in which RR and Rr plants have red flowers and rr plants have white flowers. Let’s assume that, on average, individuals with red flowers produce more offspring than those with white flowers. The relative fitness of the most reproductively successful variants is set at 1 as a basis for comparison; so in this case, the relative fitness of an RR or Rr plant is 1. If plants with white flowers average only 80% as many offspring, their relative fitness is 0.8. Survival alone does not guarantee reproductive success. Relative fitness is zero for a sterile plant or animal, even if it is robust and outlives other members of the population. But, of course, survival is a prerequisite for reproducing, and longevity increases fitness if it results in certain individuals leaving more descendants than other individuals leave. Then again, an individual that matures quickly and becomes fertile at an early age may have a greater reproductive potential than individuals that live longer but mature late. Thus, many factors that affect both survival and fertility determine an individual’s evolutionary fitness. An organism exposes its phenotype--its physical traits, metabolism, physiology, and behavior--not its genotype, to the environment. Acting on phenotypes, selection indirectly adapts a population to its environment by increasing or maintaining favorable genotypes in the gene pool. The entity subjected to natural selection is the whole organism, which is an integrated composite of its many phenotypic features, not a collage of individual parts. Thus, the relative fitness of an allele depends on the entire genetic context in which it works. For example, alleles that enhance the growth of the trunk and limbs of a tree may be useless or even detrimental in the absence of alleles at other loci that enhance the growth of roots required to support the tree. On the other hand, alleles that contribute nothing to an organism’s success, or may even be slightly maladaptive, may be perpetuated because they are present in individuals whose overall fitness is high. The whole baseball team wins the league pennant, even the player with the worst batting average and the most errors. 23. Give examples of how an organism's phenotype may be influenced by the environment.
24. Distinguish among stabilizing selection, directional selection and diversifying selection. Directional selection is most common during periods of environmental change or when members of a population mi grate to some new habitat with different environmental conditions. Directional selection shifts the frequency curve for variations in some phenotypic character in one direction or the other by favoring what are initially relatively rare individuals that deviate from the average for that character. Diversifying selection occurs when environmental conditions are varied in a way that favors individuals on both extremes of a phenotypic range over intermediate phenotypes Stabilizing selection acts against extreme phenotypes and favors the more common intermediate variants. This mode of selection reduces variation and maintains the status quo for a particular phenotypic character. For example, stabilizing selection keeps the majority of human birth weights in the range of 3 - 4 kg. For babies much smaller or larger than this, infant mortality is greater. 25. Define sexual dimorphism and explain how it can influence evolutionary change The males and females of an animal species obviously differ in their reproductive organs. But in many species, there are also marked differences, called secondary sexual characteristics, that are not directly associated with reproduction. This distinction in appearance is called sexual dimorphism. It is often manifested in a size difference, usually one in which males are larger than females. But sexual dimorphism can also be evident in such features as colorful plumage of male birds, manes on male lions, antlers on male deer, and other adornments. In fact, males are usually the showier sex in most cases of sexual dimorphism, at least among vertebrates. Sexual dimorphism is a product of what Darwin called sexual selection. Today, biologists distinguish between intra sexual selection and inter sexual selection. Meaning selection "within the same sex,"intrasexual selection is a direct competition among individuals of one sex (usually the males in vertebrates) for mates of the opposite sex. Males may use secondary sexual equipment such as antlers to battle competitors. Chapter 24
26. Distinguish between prezygotic and postzygotic isolating mechanisms. Prezygotic barriers impede mating between species or hinder the fertilization of ova if members of different species attempt to mate. If a sperm cell from one species does fertilize an ovum of another species, then postzygotic barriers usually prevent the hybrid zygote from developing into a viable, fertile adult. 27. Discuss several prezygotic and post zygotic isolating mechanisms. Prezygotic Barriers
Habitat Isolation. Two species that live in different habitats within the same area may encounter each other rarely, if at all, even though they are not technically geographically isolated. For example, two species of garter snakes in the genus Thamnophis occur in the same areas, but one lives mainly in water and the other is primarily terrestrial. Habitat isolation also affects parasites, which are generally confined to certain plant or animal host species. Two species of parasites living on different hosts will not have a chance to mate. Behavioral Isolation. Special signals that attract mates, as well as elaborate behavior unique to a species, are probably the most important reproductive barriers among closely related animals. Male fireflies of various species signal to females of their kind by blinking their lights in particular rhythms. The females respond only to signals characteristic of their own species, flashing back and attracting the males. The eastern and western meadowlarks shown in FIGURE 24.2a are almost identical in shape, coloration, and habitat, and their ranges overlap in the central United States. Yet they remain two separate biological species, partly because of the differences in their songs, which enable them to recognize individuals of their own kind. Behavioral isolation often depends on the elaborate courtship rituals of a particular species (FIGURE 24.3). Temporal Isolation. Two species that breed during different times of the day, different seasons, or different years cannot mix their gametes. The geographic ranges of the western spotted skunk (Spilogale gracilis ) and the eastern spotted skunk (Spilogale putorius ) overlap, but these two very similar species do not interbreed because S. gracilis mates in late summer and S. putorius mates in late winter. Three species of the orchid genus Dendrobium living in the same rain forest do not hybridize because they flower on different days. Pollination of each species is limited to a single day because the flowers open in the morning and wither that evening. Mechanical Isolation. Closely related species may attempt to mate but fail to consummate the act because they are anatomically incompatible. For example, mechanical barriers contribute to reproductive isolation of flowering plants that are pollinated by insects or other animals. Floral anatomy is often adapted to certain pollinators that transfer pollen only among plants of the same species. In another example of mechanical isolation, if insects of closely related species attempt to mate, the male and female copulatory organs may not fit together and no sperm would be transferred. Gametic Isolation. Even if the gametes of different species meet, they rarely fuse to form a zygote. For animals whose eggs are fertilized within the female reproductive tract (internal fertilization), the sperm of one species may not be able to survive in the environment of the female reproductive tract of another species. Many aquatic animals release their gametes into the surrounding water, where the eggs are fertilized (external fertilization). Even when two closely related species release their gametes at the same time in the same place, cross-specific fertilization is uncommon. Gamete recognition may be based on the presence of specific molecules on the coats around the egg, which adhere only to complementary molecules on sperm cells of the same species. A similar mechanism of molecular recognition enables a flower to discriminate between pollen of the same species and pollen of different species. Postzygotic Barriers
If a sperm cell from one species does fertilize an ovum of another species, then postzygotic barriers usually prevent the hybrid zygote from developing into a viable, fertile adult. Reduced Hybrid Viability. When prezygotic barriers are crossed and hybrid zygotes are formed, genetic incompatibility between the two species may abort development of the hybrid at some embryonic stage. Of the numerous species of frogs belonging to the genus Rana , some live in the same regions and habitats, where they may occasionally hybridize. But the hybrids generally do not complete development, and those that do are frail. Reduced Hybrid Fertility. Even if two species mate and produce hybrid offspring that are vigorous, reproductive isolation is intact if the hybrids are completely or largely sterile. Since the infertile hybrid cannot backbreed with either parental species, genes cannot flow freely between the species. One cause of this barrier is a failure of meiosis to produce normal gametes in the hybrid if chromosomes of the two parent species differ in number or structure. A familiar case of a sterile hybrid is the mule, a robust cross between a horse and a donkey; horses and donkeys remain distinct species because, except very rarely, mules cannot backbreed with either species (FIGURE 24.4). Hybrid Breakdown. In some cases when species cross-mate, the first-generation hybrids are viable and fertile, but when these hybrids mate with one another or with either parent species, offspring of the next generation are feeble or sterile. For example, different cotton species can produce fertile hybrids, but breakdown occurs in the next generation when offspring of the hybrids die as seeds or grow into weak and defective plants. 28. Explain why many hybrids are sterile.
Hybrids are often sterile because the set of chromosomes from one species cannot pair during meiosis with the set of chromosomes from the other species. 29. Distinguish between allopatric and sympatric speciation. Allopatric speciation: Geographic barriers can lead to the origin of species Sympatric speciation: A new species can originate in the geographic midst of the parent species In sympatric speciation, new species arise within the range of parent populations rather than in geographically separate populations 30. Explain the allopatric speciation model and describe the role of intraspecific variation and geographical isolation.
In allopatric speciation, a new species forms while geographically isolated from its ancestor. As the isolated population’s gene pool evolves by genetic drift and natural selection, reproductive isolation from the ancestral species may evolve as a by-product of the genetic change. Such reproductive barriers prevent interbreeding with the ancestor, even if the populations come back into contact.
31. Explain the importance of the fossil record to the study of evolution and describe how fossils form. Fossils, the preserved remnants or impressions left by organisms that lived in the past, are the historical documents of biology. The fossil record is the ordered array in which fossils appear within layers, or strata, of sedimentary rocks that mark the passing of geologic time The fossils that paleontologists find in many of their digs are not the actual remnants of organisms at all, but rocks that form as replicas of the organisms. These fossils result when a dead organism captured in sediment decays and leaves an empty mold that becomes filled with minerals dissolved in water. The minerals may subsequently crystallize, forming a cast in the shape of the organism. Trace fossils consist of footprints, animal burrows, or other impressions left in sediments by the activities of animals. These rocks are in essence fossilized behavior; they tell paleontologists something about how the animals that left the trace fossils lived. If an organism happens to die in a place where bacteria and fungi cannot decompose the corpse, the entire body, including soft parts, may be preserved as a fossil. When aquatic life-forms and terrestrial organisms swept into the seas and swamps die, they settle along with the sediments. A tiny fraction of them are then preserved as fossils. 32. Distinguish between relative dating and absolute dating. Relative Dating
The trapping of dead organisms in sediments freezes fossils in time. Thus, the fossils in each stratum of sedimentary rock are a local sample of the organisms that existed at the time that sediment was deposited. Because younger sediments are superimposed upon older ones, this book of sedimentary pages tells the relative ages of fossils. The strata at one location can often be correlated with strata at another location by the presence of similar fossils, known as index fossils. The best index fossils for correlating strata that are far apart are the shells of sea animals that were widespread. At any one location where a roadcut or canyon wall reveals layered rocks, there are likely to be gaps in the sequence. That area may have been above sea level during different periods, and thus no sedimentation occurred; or some of the sedimentary layers that were deposited when the area was submerged may have been scraped away by subsequent periods of erosion. Absolute Dating
"Absolute" dating does not mean errorless dating, but only that age is given in years instead of relative terms such as before and after , early and late . Radiometric dating, the measurement of certain radioactive isotopes in fossils or rocks, is the method most often used to determine the ages of rocks and fossils on a scale of absolute time. Fossils contain isotopes of elements that accumulated in the organisms when they were alive. For example, the carbon in a living organism includes both common carbon isotope, carbon-12, and a less common radioactive isotope, carbon-14, in the same ratio as is present in the atmosphere. Once an organism dies, it stops accumulating carbon. The radioactive carbon-14 that it contained at the time of death slowly decays and becomes another element, nitrogen-14, causing the proportion of carbon-14 to total carbon content to decline. Each radioactive isotope has a fixed rate of decay. An isotope’s half-life, the number of years it takes for 50% of the original sample to decay, is unaffected by temperature, pressure, and other environmental variables. Carbon-14 has a half-life of 5,730 years, a reliable rate of decay that can be used to date relatively young fossils. Thus, by measuring either the amount of nitrogen-14 in a fossil or the amount of carbon-14 remaining, we can determine the fossil’s age. 33. Explain how isotopes can be used in absolute dating.
Paleontologists use radioactive isotopes with longer half-lives to date older fossils. By measuring either the amount of nitrogen-14 in a fossil or the amount of carbon-14 remaining, we can determine the fossil’s age. 34. Explain how continental drift may have played a role in history of life On a global scale, the drifting of continents is the major geographic factor correlated with the spatial distribution of life and with such evolutionary episodes as mass extinctions and explosive increases in biological diversity. Plate movements rearrange geography incessantly, but two chapters in the continuing saga of continental drift had an especially strong influence on life. About 250 million years ago, near the end of the Paleozoic era, plate movements brought all the landmasses together into a supercontinent that has been named Pangaea, meaning "all land" (FIGURE 25.4, p. 490). Imagine some of the possible effects on life. Species that had been evolving in isolation came together and competed. When the landmasses coalesced, the total amount of shoreline was reduced, and there is evidence that the ocean basins increased in depth, which lowered sea level and drained the shallow coastal seas. Then, as now, most marine species inhabited shallow waters, and the formation of Pangaea destroyed a considerable amount of that habitat. It was probably a long, traumatic period for terrestrial life as well. The continental interior, which has a drier and more erratic climate than coastal regions, increased in area substantially when the land came together. Changing ocean currents also would have affected land life as well as sea life. The formation of Pangaea surely had a tremendous environmental impact that reshaped biological diversity by causing extinctions and providing new opportunities for taxonomic groups that survived the crisis. The second dramatic chapter in the history of continental drift was written about 180 million years ago, during the Mesozoic era. Pangaea began to break up, causing geographic isolation of colossal proportions. As the continents drifted apart, each became a separate evolutionary arena, and the faunas and floras of the different biogeographic realms diverged. 35. Describe how cladistic analysis may be used to determine branches on phylogenetic trees. Classification based on evolutionary history is called phylogenetic systematics. So far, we have assumed that the phylogeny of a group of species is already known, and then we have applied that knowledge to a hierarchical classification of the species A phylogenetic diagram based on cladistics is called a cladogram. It is a tree constructed from a series of dichotomies, or two-way branch points. Each branch point represents the divergence of two species from a common ancestor. Each diagram of phylogeny, be it a cladogram or an evolutionary tree that incorporates a time scale, represents a hypothesis or a set of hypotheses about how the organisms in the tree are related. New evidence can compel systematists to revise their trees. And such reassessment has accelerated with the application of molecular methods for comparing species and tracing phylogeny. 36. Explain the concept of adaptive radiation.
The islands are far enough apart to permit populations to evolve in isolation, but close enough together for occasional dispersion events to occur. Such evolution of many diversely adapted species from a common ancestor is called adaptive radiation Adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage.Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits with which they can exploit a range of divergent environments. Adaptive radiation, a characteristic example of cladogenesis, can be graphically illustrated as a "bush", or clade, of coexisting species 37. Explain how mass extinctions could occur and affect evolution of surviving forms. A species may become extinct because its habitat has been destroyed or because the environment has changed in a direction unfavorable to the species. If ocean temperatures fall by even a few degrees, many species that are otherwise beautifully adapted will perish. Even if physical factors in the environment are stable, biological factors may change; the environment in which a species lives includes the other organisms that live there, and evolutionary change in one species is likely to have some impact on other species in the community. For example, the evolution by some Cambrian animals of hard body parts, such as jaws and shells, may have made some organisms lacking hard parts more vulnerable to predation and thereby more prone to extinction. Chapter 26
38. Describe the contributions that A.I. Oparin, J.B.S. Haldane, Stanley Miller and Harold Urey made towards developing a model for abiotic synthesis of organic molecules. In the 1920s, A. I. Oparin, of Russia, and J. B. S. Haldane, of Great Britain, independently postulated that conditions on the primitive Earth favored chemical reactions that synthesized organic compounds from inorganic precursors present in the early atmosphere and seas. This cannot happen in the modern world, Oparin and Haldane reasoned, because the present atmosphere is rich in oxygen produced by photosynthetic life. The oxidizing atmosphere of today is not conducive to the spontaneous synthesis of complex molecules because the oxygen attacks chemical bonds, extracting electrons. Before oxygen-producing photosynthesis, Earth had a much less oxidizing atmosphere, derived mainly from volcanic vapors. Such a reducing (electron-adding) atmosphere would have enhanced the joining of simple molecules to form more complex ones. Even with a reducing atmosphere, making organic molecules would require considerable energy, which was probably provided by lightning and the intense UV radiation that penetrated the primitive atmosphere. The modern atmosphere has a layer of ozone produced from oxygen, and this ozone shield screens out most UV radiation. There is also evidence that young suns emit more UV radiation than older suns. Oparin and Haldane envisioned an ancient world with the chemical conditions and energy resources needed for the abiotic synthesis of organic molecules. In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis by creating, in the laboratory, conditions comparable to those that scientists had postulated for the early Earth. Their apparatus produced a variety of amino acids and other organic compounds found in living organisms today. Fig 26-10. The Miller-Urey experiment. A warmed flask of water simulated the primeval sea. The "atmosphere" consisted of H2O, H2, CH4, and NH3. Sparks were discharged in the synthetic atmosphere to mimic lightning. A condenser cooled the atmosphere, raining water and any dissolved compounds back to the miniature sea. As material circulated through the apparatus, the solution in the flask changed from clear to murky brown. After one week, Miller and Urey analyzed the contents of the solution and found a variety of organic compounds, including some of the amino acids that make up the proteins of organisms.
The atmosphere in the Miller-Urey model was made up of H2O, H2, CH4 (methane), and NH3 (ammonia), the gases that researchers in the 1950s believed prevailed in the ancient world. This atmosphere was probably more strongly reducing than the actual atmosphere of early Earth. Modern volcanoes emit CO, CO2, N2, and water vapor, and it is likely that these gases were abundant in the ancient atmosphere. Hydrogen gas was probably not a major component, and traces of O2 may even have been present, formed from reactions among other gases as they baked under the powerful UV radiation. Many laboratories have repeated the Miller experiment using a variety of recipes for the atmosphere. Abiotic synthesis of organic compounds occurred in these modified models, although yields were generally smaller than in the original experiment. The Miller-Urey experiments still stimulate debate on the origin of Earth’s early stockpile of organic ingredients. 39. Provide plausible evidence to support the hypothesis that chemical evolution resulting in life's origin occurred in four stages:
a. Abiotic synthesis of organic monomers
b. Abiotic synthesis of polymers
c. Formation of protobionts
d. Origin of genetic information
40. Describe the basis for Whittaker's five-kingdom system. The five-kingdom system recognized the two fundamentally different types of cells, prokaryotic and eukaryotic, and set the prokaryotes apart from all eukaryotes by placing them in their own kingdom, Monera. Whittaker distinguished three kingdoms of multicellular eukaryotes--Plantae, Fungi, and Animalia--partly on the criterion of nutrition. Plants are autotrophic in nutritional mode, making their food by photosynthesis. Fungi are heterotrophic organisms that are absorptive in nutritional mode. Most fungi are decomposers that live embedded in their food source, secreting digestive enzymes and absorbing the small organic molecules that are the products of digestion. Most animals live by ingesting food and digesting it within specialized cavities. We are left with the kingdom Protista. In Whittaker’s five-kingdom system, Protista consisted of all eukaryotes that did not fit the definition of plants, fungi, or animals. Most protists are unicellular forms. However, the boundaries of Whittaker’s kingdom Protista were expanded to include some multicellular organisms, such as seaweeds, because of their relationships to certain unicellular protists. With such refinements, the five-kingdom system prevailed in biology for over 20 years. 41. Describe the domain alternative to the five-kingdom system and explain the rationale for this. These new data led to a three-domain system The three domains, Bacteria, Archaea, and Eukarya, are essentially superkingdoms, a taxonomic level even higher than the kingdom level. Note that the three-domain system makes the kingdom Monera obsolete, since it would have members in two different domains. In fact, many microbiologists now divide each of the two prokaryotic domains into multiple kingdoms based on cladistic analysis of molecular data The second major challenge to the five-kingdom system is being mounted by systematists who are sorting out the phylogeny of the diverse eukaryotes formerly collected in the kingdom Protista. Molecular systematics and cladistics have revealed that Protista is not a monophyletic grouping. The specialists who study these organisms now split most of them into five or more newly designated kingdoms, but have also assigned certain groups that used to be included in Protista to Plantae, Fungi, and Animalia 42. List the three domains of living things and the major characteristics of each domain. Bacteria, Archaea, and Eukarya