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Chapter 21 Biology Evolution by Natural Selection requires o 1. Phenotypic variation must exist in the population o 2. This variation must lead to differences among individuals in lifetime reproductive success o 3. Phenotypic variation among individuals must be genetically transmissible to the next generation Natural Selection occurs when there is a selecting factor that gives one phenotype an advantage o Examples in islands During droughts or dry years when the main type of seed available is large and tough the larger beaks hold the advantage During wet years small seeds are more available the smaller beaks hold the advantage Beak depth is genetic o Natural Selection can occur if something in the environment changes Example when pollution caused the trees in a forest to turn darker in color the population of black moths increased in relation to the population of “peppered” moths The theory being that light colored moths were more easy for predators to see on the darkened trees which made their coloration unfavorable Industrial Melanism: Refers to the phenomenon in which darker individuals come to predominate over lighter ones. Artificial Selection: Operates by favoring individuals with certain phenotypic traits allowing them to reproduce and pass their genes on to the next generation Domesticated breeds have arisen from artificial selection o They managed to domesticate foxes by selecting the most docile of the parent generation o Their ears, tails, and legs also changed with their temperaments. Homologous Structures o Structures with different appearances and functions that all derived from the same part of a common ancestor Most species are similar during the embryonic stage Not all features are perfectly suited for their use Vestigial Structures o Structures that have no apparent function but resemble structures their ancestors possessed Fossil Genes or Pseudogenes o Genes that have been made inactive through some mutation however still remain a part of the organisms genome Convergent Evolution o Evolutionary change that occurs parallel in similar environments Chapter 26 Biology Notes Debris from a meteor hitting earth formed the moon (4.6 BYA) Atmospheric temperature is 2000˚C Increased CO c2used Increased temperature Decreased CO 2 caused decreased Weathering increases in hot wet conditions and pulls CO f2om the atmosphere Plates o Ridged slabs of rock that forms earths crusts Plate tectonics o The movement of plates Supercontinents o Rodinia: All continents, during the Proterozoic era (early life) Broke up during 650 MYA o Gondwana: All southern hemisphere continents o Pangea: All continents Phanerozoic = Visible life o 12% of earth’s history Paleozoic era/ Cambrian period = High diversification of multicellular life Birds and mammals o 4% of earth’s history Humans o .2% of earth’s history Fossils are dated by the half-life’s of their isotopes o Carbon dating, uses carbons half-life which is 5700 years How did life originate? o Arose from early waters filled with ammonia, formaldehyde, folic acid, cyanide, methane, hydrogen sulfide, and organic hydrocarbons o Meteorites may have brought some of the organic molecules Example Tagish with 3% of its mass being organic molecules o Reducing Atmosphere (high hydrogen content) Makes forming molecules easier o Miller and Urey Experiment Step 1: Assembled a reducing atmosphere Step 2: Placed atmosphere over liquid water Step 3: Maintained at a temperature bellow 100˚C Step 4: Simulated lightning This created small organic molecules and amino acids RNA was the first nucleic acid that promoted self-replication Early membranes were made of fatty acids Craton: A rock layer of undisturbed continental crusts o Microfossils Stromatolites: Sedimentary deposits held in place by mats of micro- organisms Carbon Fixation: Changing inorganic carbon to usable organic carbon o Done by the Calvin Cycle and the reductive Krebs cycle Organic molecules are biomarkers o Example Hydrocarbons from fatty acids Climate: Temperature and water availability Atmosphere: Levels of O and2CO 2 o Volcanic Eruptions alter the atmosphere Highest temperature of earth was 2000˚C Lowest temperature of earth was -50˚C Snowball Earth: Ice or glaciers Glaciation results in massive extinctions Oxygenic photosynthesis produced O encour2ging the evolution of cellular respiration Increased O i2 the atmosphere caused O to int2ract with UV rays forming O 3 or Ozone Chapter 20 Biol Genetic Variation provides the raw material for evolution Evolution: An entity changing over time Population genetics: The study of the genetic composition of a population and how/why it changes Blending inheritance: Offspring are expected to be phenotypically intermediate relative to their parents HardyWeinberg Equilibrium: When the proportions of genotypes do not change in a population o This can only occur if No mutation takes place No genes are transferred to or from other sources (No immigration or emigration) Mating is random Large population No selection o Equation to calculate allele frequencies P= Dominant Q= Recessive P*p =p 2 P+q=1 2 2 P +2pq+q =1 o Why would a population have an excess of homozygotes vs heterozygotes? Natural selection favors the homozygotes Individuals choosing to mate with genetically similar individuals An influx of homozygous individuals from an outside population 5 agents that cause evolutionary change o Mutation o Gene Flow: Movement of alleles from one population to another o Nonrandom mating Assortative mating: When phenotypically similar individuals mate more often than different individuals produces an excess of Homozygotes Disassortative mating: When phenotypically different individuals mate, producing an excess of Heterozygotes o Genetic drift: The change in frequency of an allele by change alone Founder effect: When one or a few individuals of a population disperse and become the founders of a new isolated population can cause the allele’s that were rare in the parent population to be common in the new population Bottleneck effect: When populations are drastically reduced no matter the cause the surviving individuals will likely be a random genetic sample of the original population o Natural Selection There must be phenotypic variation in the population The variation must give some individuals a reproductive advantage The variation must be genetic Sexual selection: Selection based on mating success Frequency dependent selection: When the fitness of a phenotype depends on its frequency Negative: The rare phenotype is selected for Positive: The rare types stick out and are more vulnerable to predators Oscillating selection: One phenotype is favored then the other Heterozygote advantage: When the heterozygous condition provides an advantage over both homozygous conditions Disruptive Selection: When selection acts to eliminate intermediate types Directional Selection: When selection moves to eliminate one extreme in a population Stabilizing Selection: Selection that moves to eliminate both extremes of a population Lecture Notes Darwin argued o Species are not immutable Immutable: They do not change o Modification of the “normal form” are caused by abiotic and biotic changes Adaptations: Beneficial traits from modification o Populations over time become “fit” to local environmental conditions o Populations can become so different in form that they may become unique species Mechanics of natural selection o Observations 1. Every species on earth has an enormous reproductive potential 2. The number of organisms in a species cannot increase endlessly do to limited resources, meaning there is not such thing as unlimited growth Conclusion: Species can and do produce far more offspring than the environment can support. This leads to intraspecific competition or “struggle for existence” 3.If you look at any population individuals are not identical in form. Variation: Provides the raw material for natural selection The ability to survive and reproduce depends on form o Conclusion: Individuals with favorable traits “win” on average and survive longer and produce more offspring than those without those advantages 4. Natural selection only works with inherited characteristics If there is no genetic basis the form will not be passed on, individuals with favorable traits have more offspring that also share those traits. Over time the populations have an increase in favorable traits. Blending inheritance: An idea that offspring should be intermediate to their parents Population: A group of individuals of the same species in the same area that can interbreed Gene Pool: All alleles in a populatulation. Population genetics: The study of properties of genes in a population Convergent evolution: Similar forms having evolved in different areas due to similar selective pressures in a similar environment Phylogeny: An evolutionary tree Chapter 20 Biol Genetic Variation provides the raw material for evolution Evolution: An entity changing over time Population genetics: The study of the genetic composition of a population and how/why it changes Blending inheritance: Offspring are expected to be phenotypically intermediate relative to their parents Hardy-Weinberg Equilibrium: When the proportions of genotypes do not change in a population o This can only occur if No mutation takes place No genes are transferred to or from other sources (No immigration or emigration) Mating is random Large population No selection o Equation to calculate allele frequencies P= Dominant Q= Recessive 2 P*p =p P+q=1 P +2pq+q =1 o Why would a population have an excess of homozygotes vs heterozygotes? Natural selection favors the homozygotes Individuals choosing to mate with genetically similar individuals An influx of homozygous individuals from an outside population 5 agents that cause evolutionary change o Mutation o Gene Flow: Movement of alleles from one population to another o Nonrandom mating Assortative mating: When phenotypically similar individuals mate more often than different individuals produces an excess of Homozygotes Disassortative mating: When phenotypically different individuals mate, producing an excess of Heterozygotes o Genetic drift: The change in frequency of an allele by change alone Founder effect: When one or a few individuals of a population disperse and become the founders of a new isolated population can cause the allele’s that were rare in the parent population to be common in the new population Bottleneck effect: When populations are drastically reduced no matter the cause the surviving individuals will likely be a random genetic sample of the original population o Natural Selection There must be phenotypic variation in the population The variation must give some individuals a reproductive advantage The variation must be genetic Sexual selection: Selection based on mating success Frequency dependent selection: When the fitness of a phenotype depends on its frequency Negative: The rare phenotype is selected for Positive: The rare types stick out and are more vulnerable to predators Oscillating selection: One phenotype is favored then the other Heterozygote advantage: When the heterozygous condition provides an advantage over both homozygous conditions Disruptive Selection: When selection acts to eliminate intermediate types Directional Selection: When selection moves to eliminate one extreme in a population Stabilizing Selection: Selection that moves to eliminate both extremes of a population Lecture Notes Darwin argued o Species are not immutable Immutable: They do not change o Modification of the “normal form” are caused by abiotic and biotic changes Adaptations: Beneficial traits from modification o Populations over time become “fit” to local environmental conditions o Populations can become so different in form that they may become unique species Mechanics of natural selection o Observations 1. Every species on earth has an enormous reproductive potential 2. The number of organisms in a species cannot increase endlessly do to limited resources, meaning there is not such thing as unlimited growth Conclusion: Species can and do produce far more offspring than the environment can support. This leads to intraspecific competition or “struggle for existence” 3.If you look at any population individuals are not identical in form. Variation: Provides the raw material for natural selection The ability to survive and reproduce depends on form o Conclusion: Individuals with favorable traits “win” on average and survive longer and produce more offspring than those without those advantages 4. Natural selection only works with inherited characteristics If there is no genetic basis the form will not be passed on, individuals with favorable traits have more offspring that also share those traits. Over time the populations have an increase in favorable traits. Blending inheritance: An idea that offspring should be intermediate to their parents Population: A group of individuals of the same species in the same area that can interbreed Gene Pool: All alleles in a populatulation. Population genetics: The study of properties of genes in a population Convergent evolution: Similar forms having evolved in different areas due to similar selective pressures in a similar environment Phylogeny: An evolutionary tree 1 Chapter Contents • 20.1 Genetic Variation and Evolution 2 • 20.2 Changes in Allele Frequency • 20.3 Five Agents of Evolutionary Change • 20.4 Quantifying Natural Selection • 20.5 Natural Selection's Role in Maintaining Variation • 20.6 Selection Acting on Traits Affected by Multiple Genes • 20.7 Experimental Studies of Natural Selection • 20.8 Interactions Among Evolutionary Forces • 20.9 The Limits of Selection Introduction No other human being is exactly like you (unless you have an identical twin). Often the particular characteristics of an individual have an important bearing on its survival, on its chances to reproduce, and on the success of its offspring. Evolution is driven by such factors, as different alleles rise and fall in populations. These deceptively simple matters lie at the core of evolutionary biology, which is the topic of this chapter and chapters 21through 25 . Learning Outcomes 1. Define evolution and population genetics. 2. Explain the difference between evolution by natural selection and the inheritance of acquired characteristics. Genetic variation, that is, differences in alleles of genes found within individuals of a population, provides the raw material for natural selection, which will be described shortly. Natural populations contain a wealth of such variation. In this chapter, we explore genetic variation in natural populations and consider the evolutionary forces that cause allele frequencies in natural populations to change. The word evolution is widely used in the natural and social sciences. It refers to how an entity—be it a social system, a gas, or a planet—changes through time. Although development of the modern concept of evolution in biology can be traced to Darwin's landmark work, On the Origin of Species, the first five editions of his book never actually used the term. Rather, Darwin used the phrase “descent with modification.” Although many more complicated definitions have been proposed, Darwin's words probably best capture the essence of biological evolution: Through time, species accumulate differences; as a result, descendants differ from their ancestors. In this way, new species arise from existing ones. Page 397 Many processes can lead to evolutionary change You have already learned about the development of Darwin's ideas in chapter 1. Darwin was not the first to propose a theory of evolution. Rather, he followed a long line of earlier philosophers and naturalists who deduced that the many kinds of organisms around us were produced by a process of evolution. Unlike his predecessors, however, Darwin proposed natural selection as the mechanism of evolution. Natural selection produces evolutionary change when some individuals in a population possess certain inherited characteristics and then produce more surviving offspring than individuals lacking these characteristics. As a result, 3 the population gradually comes to include more and more individuals with the advantageous characteristics. In this way, the population evolves and becomes better adapted to its local circumstances. A rival theory, championed by the prominent biologist JeanBaptiste Lamarck, was that evolution occurred by the inheritance of acquired characteristics .According to Lamarck, changes that individuals acquired during their lives were passed on to their offspring. For example, Lamarck proposed that ancestral giraffes with short necks tended to stretch their necks to feed on tree leaves, and this extension of the neck was passed on to subsequent generations, leading to the longnecked giraffe (figure 20.1 a). In Darwin's theory, by contrast, the variation is not created by experience, but is the result of preexisting genetic differences among individuals (figure 20.1 b). Figure 20.1 Two ideas of how giraffes might have evolved long necks. One way to monitor how populations change through time is to look at changes in the frequencies of alleles of a gene from one generation to the next. Natural selection, by favoring individuals with certain alleles, can lead to change in such allele frequencies, but it is not the only process that can do so. Allele frequencies can also change when mutations occur repeatedly, changing one allele to another, and when migrants bring alleles into a population. In addition, when populations are small, the frequencies of alleles can change randomly as the result of chance events. Often, natural selection overwhelms the effects of these other processes, but as you will see later in this chapter, this is not always the case. Evolution can result from any process that causes a change in the genetic composition of a population. We cannot talk about evolution, therefore, without also considering population genetics , the study of the properties of genes in populations. Populations contain ample genetic variation Biologists have always wanted to know how much genetic variation exists in natural populations. The ability to ask this question has been limited by the techniques available to analyze variation at different levels: proteins, genes, 4 and now genomes. The story of genetic variation in natural populations is told using increasingly sophisticated tools for detecting differences. Initial approaches to understanding variation examined the most obvious differences—the morphological. At this level, natural populations usually show substantial genetic variation, as figure 20.2 illustrates. An example closer to home are the genes that influence blood groups in humans. Chemical analysis has revealed the existence of more than 30 blood group genes in humans, in addition to the ABO locus. At least one third of these genes are routinely found in several alternative allelic forms in human populations. In addition to these, more than 45 variable genes encode other proteins in human blood cells and plasma that are not considered blood groups. In short, many genetically variable genes are present in this one system alone. Figure 20.2 Polymorphic variation.This natural population of lupines,Lupinus, exhibits considerable variation in flower color. Individual differences are inherited and passed on to offspring. Page 398 The first approach to directly assay genetic variation within populations was the use of electrophoresis to separate proteins produced by alternative alleles of enzymeencoding genes (see chapter 17). As DNA analysis tools were 5 developed in the laboratory, they were adapted for use with samples collected from natural populations. This led to, in order of increasing detail, examining restriction fragment length polymorphisms (RFLPs, see chapter 17), sequencing specific genes, and, most recently, sequencing entire genomes. One of the most useful tools, both for genetic mapping and for the analysis of populationlevel variation, has been singlenucleotide polymorphisms (SNPs). These are defined as singlebase differences between individuals that exist in the population at more than 1% (see chapter 24). A largescale international effort has identified 3.1 million SNPs in the human genome, which translates to a density of about 1 per kilobase of DNA. Such variation is also being assayed in many other species of interest. The results for most species are similar to those in humans: the closer you look, the more variation you see. One reason for the appeal of SNPs is that we have been able to automate the analysis of multiple samples. This approach is being applied to many species of scientific and economic interest. The U.S. storehouse for such information, the National Center for Biotechnology Information (NCBI), now has a database with SNPs found in over 100 species, revealing the near ubiquity of genetic variation and providing tools for assessing patterns in human and natural populations. Learning Outcomes Review 20.1 Evolution can be described as descent with modification. Natural selection occurs when individuals carrying certain alleles leave more offspring than those without the alleles. Natural populations generally contain considerable amounts of genetic variation. Population genetics studies this variability through statistical analyses. Why is genetic variation in a population necessary for evolution to occur? Answer Section 20.01 Quiz 20.2 Changes in Allele Frequency Learning Outcomes 1. Explain the Hardy–Weinberg principle. 2. Describe the characteristics of a population that is in Hardy–Weinberg equilibrium. 3. Demonstrate how the operation of evolutionary processes can be detected. Genetic variation within natural populations was a puzzle to Darwin and his contemporaries in the mid1800s. The way in which meiosis produces genetic segregation among the progeny of a hybrid had not yet been discovered. And, although Mendel performed his experiments during this same time period, his work was largely unknown. Selection, scientists then thought, should always favor an optimal form, and so tend to eliminate variation. Moreover, the theory of blending inheritance—in which offspring were expected to be phenotypically intermediate 6 relative to their parents—was widely accepted. If blending inheritance were correct, then the effect of any new genetic variant would quickly be diluted to the point of disappearance in subsequent generations. The Hardy–Weinberg principle allows prediction of genotype frequencies Following the rediscovery of Mendel's research, two people in 1908 solved the puzzle of why genetic variation persists—Godfrey H. Hardy, an English mathematician, and Wilhelm Weinberg, a German physician. These workers were initially confused about why, after many generations, a population didn't come to be composed solely of individuals with the dominant phenotype. The conclusion they independently came to was that the original proportions of the genotypes in a population will remain constant from generation to generation, as long as the following assumptions are met: Page 399 1. No mutation takes place. 2. No genes are transferred to or from other sources (no immigration or emigration takes place). 3. Mating is random (individuals do not choose mates based on their phenotype or genotype). 4. The population size is very large. 5. No selection occurs. Because the genotypes' proportions do not change, they are said to be inHardy–Weinberg equilibrium . The Hardy–Weinberg equation with two alleles: A binomial expansion In algebraic terms, the Hardy–Weinberg principle is written as an equation. Consider a population of 100 cats in which 84 are black and 16 are white. The frequencies of the two phenotypes would be 0.84 (or 84%) black and 0.16 (or 16%) white. Based on these phenotypic frequencies, can we deduce the underlying frequency of genotypes? If we assume that the white cats are homozygous recessive for an allele we designate as b, and the black cats are either homozygous dominant BB or heterozygous Bb, we can calculate the allele frequencies of the two alleles in the population from the proportion of black and white individuals, assuming that the population is in Hardy– Weinberg equilibrium. Let the letter p designate the frequency of the B allele and the letter q the frequency of the alternative allele. Because there are only two alleles, p plusq must always equal 1 (that is, the total population). In addition, we know that the sum of the three genotype frequencies must also equal 1. If the frequency of the B allele is p, then the probability that an individual will have two B alleles is simply the probability that each of its alleles is a B. The probability of two events happening independently is the product of the probability of each event; in this case, the probability that the individual received a B allele from its father is p, and the probability the individual received a B allele from its mother is also p, so the probability that both happened is p p = p (figure 20.3). By the same reasoning, the probability that an individual will have two b alleles is q . 7 Figure 20.3 The Hardy–Weinberg equilibrium.In the absence of factors that alter them, the frequencies of gametes, genotypes, and phenotypes remain constant generation after generation. What about the probability that an individual will be a heterozygote? There are two ways this could happen: The individual could receive a B from its father and a b from its mother, or vice versa. The probability of the first case * * is p qand the probability of the second case is q p. Because the result in either case is that the individual is a heterozygote, the probability of that outcome is the sum of the two probabilities, or 2pq. So, to summarize, if a population is in Hardy–Weinberg equilibrium with allele frequencies of p and q, then the probability that an individual will have each of the three possible genotypes is p + 2pq+ q . You may recognize this as thebinomial expansion: Finally, we may use these probabilities to predict the distribution of genotypes in the population, again assuming that the population is in Hardy–Weinberg equilibrium. If the probability that any individual is a heterozygote is 2pq, then we would expect the proportion of heterozygous individuals in the population to be 2pq; similarly, the frequency of BB and bb homozygotes would be expected to be p and q . 2 2 Let us return to our example. Remember that 16% of the cats are white. If white is a recessive trait, then this means 2 that such individuals must have the genotype bb. If the frequency of this genotype is q = 0.16 (the frequency of white cats), then q (the frequency of the b allele) = 0.4. Because p + q = 1, therefore, p, the frequency of allele B, would be 1.0 − 0.4 = 0.6 (remember, the frequencies must add up to 1). We can now easily calculate the 2 2 expectedgenotype frequencies: homozygous dominant BB cats would make up thep group, and the value of p = (0.6) = 0.36, or 36 homozygous dominant BBindividuals in a population of 100 cats. The heterozygous cats have the Bbgenotype and would have the frequency corresponding to 2pq, or (2 0.6 0.4) = 0.48, or 48 heterozygous Bb individuals. Data analysis If all white cats died, what proportion of the kittens in the next generation would be white? Answer Page 400 Using the Hardy–Weinberg equation to predict frequencies in subsequent generations 8 The Hardy–Weinberg equation is another way of expressing the Punnett square described in chapter 12, with two alleles assigned frequencies, p andq. Figure 20.3 allows you to trace genetic reassortment during sexual reproduction and see how it affects the frequencies of the B and b alleles during the next generation. In constructing this diagram, we have assumed that the union of sperm and egg in these cats is random, so that all combinations of b and B alleles occur. The alleles are therefore mixed randomly and are represented in the next generation in proportion to their original occurrence. Each individual egg or sperm in each generation has a 0.6 chance of receiving a B allele (p = 0.6) and a 0.4 chance of receiving a b allele (q = 0.4). 2 * In the next generation, therefore, the chance of combining two B alleles is p , or 0.36 (that is, 0.6 0.6), and approximately 36% of the individuals in the population will continue to have the BB genotype. The frequency 2 * of bbindividuals is q (0.4 0.4) and so will continue to be about 16%, and the frequency of Bb individuals will be 2pq (2 0.6 0.4), or on average, 48%. Phenotypically, if the population size remains at 100 cats, we would still see approximately 84 black individuals (with either BB or Bb genotypes) and 16 white individuals (with the bb genotype). Allele, genotype, and phenotype frequencies have remained unchanged from one generation to the next, despite the reshuffling of genes that occurs during meiosis and sexual reproduction. Dominance and recessiveness of alleles can therefore be seen only to affect how an allele is expressed in an individual and not how allele frequencies will change through time. Hardy–Weinberg predictions can be applied to data to find evidence of evolutionary processes The lesson from the example of black and white cats is that if all five of the assumptions listed earlier hold true, the allele and genotype frequencies will not change from one generation to the next. But in reality, most populations in nature will not fit all five assumptions. The primary utility of this method is to determine whether some evolutionary process or processes are operating in a population and, if so, to suggest hypotheses about what they may be. Suppose, for example, that the observed frequencies of the BB, bb, and Bbgenotypes in a different population of cats were 0.6, 0.2, and 0.2, respectively. We can calculate the allele frequencies for B as follows: 60% (0.6) of the cats have two B alleles, 20% have one, and 20% have none. This means that the average number of B alleles per cat is 1.4 [(0.6 × 2) + (0.2 × 1) + (0.2 × 0) = 1.4]. Because each cat has two alleles for this gene, the frequency is 1.4/2.0 = 0.7. Similarly, you should be able to calculate that the frequency of the b allele = 0.3. If the population were in Hardy–Weinberg equilibrium, then, according to the equation earlier in this section, the 2 frequency of the BB genotype would be 0.7 = 0.49, lower than it really is. Similarly, you can calculate that there are fewer heterozygotes and more bb homozygotes than expected; then clearly, the population is not in Hardy– Weinberg equilibrium. What could cause such an excess of homozygotes and deficit of heterozygotes? A number of possibilities exist, including (1) natural selection favoring homozygotes over heterozygotes, (2) individuals choosing to mate with * * genetically similar individuals (because BB BB and bb bb matings always produce homozygous offspring, but only half of Bb Bb produce heterozygous offspring, such mating patterns would lead to an excess of homozygotes), or (3) an influx of homozygous individuals from outside populations (or conversely, emigration of heterozygotes to other populations). By detecting a lack of Hardy–Weinberg equilibrium, we can generate potential hypotheses that we can then investigate directly. 9 The operation of evolutionary processes can be detected in a second way. As discussed previously, if all of the Hardy–Weinberg assumptions are met, then allele frequencies will stay the same from one generation to the next. Changes in allele frequencies between generations would indicate that one of the assumptions is not met. Suppose, for example, that the frequency of b was 0.53 in one generation and 0.61 in the next. Again, there are a number of possible explanations: For example, (1) selection favoring individuals with b over B, (2) immigration of binto the population or emigration of B out of the population, or (3) high rates of mutation that more commonly occur from B to b than vice versa. Another possibility is that the population is a small one, and that the change represents the random fluctuations that result because, simply by chance, some individuals pass on more of their genes than others. We will discuss how each of these processes is studied in the rest of the chapter. Learning Outcomes Review 20.2 The Hardy–Weinberg principle states that in a large population with no selection and random mating, the proportion of alleles does not change through the generations. Finding that a population is not in Hardy– Weinberg equilibrium indicates that one or more evolutionary agents are operating. If you know the genotype frequencies in a population, how can you determine whether the population is in Hardy–Weinberg equilibrium? What would you conclude if you found a population not in Hardy–Weinberg equilibrium? What would be your next step? Answer Section 20.02 Quiz 20.3 Five Agents of Evolutionary Change Learning Outcomes 1. Define the five processes that can cause evolutionary change. 2. Explain how these processes can cause populations to deviate from Hardy–Weinberg Equilibrium The five assumptions of the Hardy–Weinberg principle also indicate the five agents that can lead to evolutionary change in populations. They are mutation, gene flow, nonrandom mating, genetic drift in small populations, and the pressures of natural selection. Any one of these may bring about changes in allele or genotype proportions. Evolutionary Change Mutation changes alleles Mutation from one allele to another can obviously change the proportions of particular alleles in a population. Mutation rates are generally so low that they have little effect on the Hardy–Weinberg proportions of common 10 alleles. A typical gene mutates about once per 100,000 cell divisions. Because this rate is so low, other evolutionary processes are usually more important in determining how allele frequencies change. Nonetheless, mutation is the ultimate source of genetic variation and thus makes evolution possible (figure 20.4 a). It is important to remember, however, that the likelihood of a particular mutation occurring is not affected by natural selection; that is, mutations do not occur more frequently in situations in which they would be favored by natural selection. Figure 20.4 Five agents of evolutionary change.a. Mutation, b. gene flow, c. nonrandom mating, d. genetic drift, and e. selection. Gene flow occurs when alleles move between populations Gene flow is the movement of alleles from one population to another. It can be a powerful agent of change. Sometimes gene flow is obvious, as when an animal physically moves from one place to another. If the characteristics of the newly arrived individual differ from those of the animals already there, and if the newcomer is adapted well enough to the new area to survive and mate successfully, the genetic composition of the receiving population may be altered. Other important kinds of gene flow are not as obvious. These subtler movements include the drifting of gametes or the immature stages of plants or marine animals from one place to another (figure 20.4 b). Pollen, the male gamete of flowering plants, is often carried great distances by insects and other animals that visit flowers. Seeds may also blow in the wind or be carried by animals to new populations far from their place of origin. In addition, gene flow may also result from the mating of individuals belonging to adjacent populations. Consider two populations initially different in allele frequencies: In population 1, p = 0.2 and q = 0.8; in population 2, p = 0.8 and q = 0.2. Gene flow will tend to bring the rarer allele into each population. Thus, allele frequencies will change from generation to generation, and the populations will not be in Hardy–Weinberg equilibrium. Only when allele frequencies reach 0.5 for both alleles in both populations will equilibrium be attained. This example also indicates that gene flow tends to homogenize allele frequencies among populations. Nonrandom mating shifts genotype frequencies Individuals with certain genotypes sometimes mate with one another more commonly than would be expected on a random basis, a phenomenon known as nonrandom mating (figure 20.4 c). Assortative mating , in which 11 phenotypically similar individuals mate, is a type of nonrandom mating that causes the frequencies of particular genotypes to differ greatly from those predicted by the Hardy–Weinberg principle. Page 402 Assortative mating does not change the frequency of the individual alleles, but rather increases the proportion of homozygous individuals because phenotypically similar individuals are likely to be genetically similar and thus are also more likely to produce offspring with two copies of the same allele. This is why populations of selffertilizing plants consist primarily of homozygous individuals. By contrast, disassortative mating , in which phenotypically different individuals mate, produces an excess of heterozygotes. Genetic drift may alter allele frequencies in small populations In small populations, frequencies of particular alleles may change drastically by chance alone. Such changes in allele frequencies occur randomly, as if the frequencies were drifting from their values. These changes are thus known as genetic drift (figure 20.4 d). For this reason, a population must be large to be in Hardy–Weinberg equilibrium. If the gametes of only a few individuals form the next generation, the alleles they carry may by chance not be representative of the parent population from which they were drawn, as illustrated in figure 20.5. In this example, a small number of individuals are removed from a bottle. By chance, most of the individuals removed are green, so the new population has a much higher population of green individuals than the parent generation had. Figure 20.5 Genetic drift: a bottleneck effect.The parent population contains roughly equal numbers of green and yellow individuals and a small number of red individuals. By chance, the few remaining individuals that contribute to the next generation are mostly green. The bottleneck occurs because so few individuals form the next generation, as might happen after an epidemic or a catastrophic storm. Inquiry question Why are rare alleles particularly likely to be lost in a population bottleneck? Answer 12 A set of small populations that are isolated from one another may come to differ strongly as a result of genetic drift, even if the forces of natural selection are the same for both. Because of genetic drift, sometimes harmful alleles may increase in frequency in small populations, despite selective disadvantage, and favorable alleles may be lost even though they are selectively advantageous. It is interesting to realize that humans have lived in small groups for much of the course of their evolution; consequently, genetic drift may have been a particularly important factor in the evolution of our species. Larger populations also experience the effect of genetic drift, but to a lesser extent than smaller populations—the magnitude of genetic drift is inversely related to population size. However, large populations may have been much smaller in the past, and genetic drift may have greatly altered allele frequencies at that time. Imagine a population containing only two alleles of a gene, B and b, in equal frequency (that is, p = q = 0.50). In a large Hardy–Weinberg population, the genotype frequencies are expected to be 0.25 BB,0.50 Bb, and 0.25 bb. If only a small sample of individuals produces the next generation, large deviations in these genotype frequencies can occur simply by chance. Suppose, for example, that four individuals form the next generation, and that by chance they are two Bb heterozygotes and two BB homozygotes—that is, the allele frequencies in the next generation would be p = 0.75 and q = 0.25. In fact, if you were to replicate this experiment 1000 times, each time randomly drawing four individuals from the parental population, then in about 8 of the 1000 experiments, one of the two alleles would be missing entirely. This result leads to an important conclusion: Genetic drift can lead to the loss of alleles in isolated populations. Alleles that initially are uncommon are particularly vulnerable (see figure 20.5). Although genetic drift occurs in any population, it is particularly likely in populations that were founded by a few individuals or in which the population was reduced to a very small number at some time in the past. Genetic Drift The founder effect Sometimes one or a few individuals disperse and become the founders of a new, isolated population at some distance from their place of origin. These pioneers are not likely to carry all the alleles present in the source population. Thus, some alleles may be lost from the new population, and others may change drastically in frequency. In some cases, previously rare alleles in the source population may be a significant fraction of the new population's genetic endowment. This phenomenon is called the founder effect . Founder effects are not rare in nature. Many selfpollinating plants start new populations from a single seed. Founder effects have been particularly important in the evolution of organisms on distant oceanic islands, such as the Hawaiian and Galápagos Islands. Most of the organisms in such areas probably derive from one or a few initial founders. Although rare, such events are occasionally observed, such as when a mass of vegetation carrying several iguanas washed up on the shore of the Caribbean island of Anguilla in 1996, leading to the establishment of a population that still occurs there to this day. Page 403 In a similar way, isolated human populations begun by relatively few individuals are often dominated by genetic features characteristic of their founders. Amish populations in the United States, for example, have unusually high frequencies of a number of conditions, such as polydactylism (the presence of a sixth finger). 13 The bottleneck effect Even if organisms do not move from place to place, occasionally their populations may be drastically reduced in size. This may result from flooding, drought, epidemic disease, and other natural forces, or from changes in the environment. The few surviving individuals may constitute a random genetic sample of the original population (unless some individuals survive specifically because of their genetic makeup). The resulting alterations and loss of genetic variability have been termed the bottleneck effect . The genetic variation of some living species appears to be severely depleted, probably as the result of a bottleneck effect in the past. For example, the northern elephant seal, which breeds on the western coast of North America and nearby islands, was nearly hunted to extinction in the nineteenth century and was reduced to a single population containing perhaps no more than 20 individuals on the island of Guadalupe off the coast of Baja, California (figure 20.6). As a result of this bottleneck, the species has lost almost all of its genetic variation, even though the seal populations have rebounded and now number in the tens of thousands and breed in locations as far north as near San Francisco. Figure 20.6 Bottleneck effect: case study.Because the Northern Elephant Seal (Mirounga angustirostris) lives in very cold waters, these, the world's largest seals, have thick layers of fat, for which they were hunted nearly to extinction late in the nineteenth century. At the low point, only one population remained on Guadalupe Island, with perhaps as few as 20 individuals; during this time, genetic variation was lost. Since being protected, the species has reclaimed most of its original range and now numbers in the tens of thousands, but genetic variation will only recover slowly over time as mutations accumulate. Any time a population becomes drastically reduced in numbers, such as in endangered species, the bottleneck effect is a potential problem. Even if population size rebounds, the lack of variability may mean that the species remains vulnerable to extinction—a topic to which we will return in chapter 58. Selection favors some genotypes over others As Darwin pointed out, some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior. We describe the results of this process as selection (see figure 20.4 e). In artificial selection, a breeder selects for the desired characteristics. In natural selection, environmental conditions determine which individuals in a population produce the most offspring. 14 Evolution by natural selection occurs when the following conditions are met: 1. Phenotypic variation must exist among individuals in a population.Natural selection works by favoring individuals with some traits over individuals with alternative traits. If no variation exists, natural selection cannot operate. 2. Variation among individuals must result in differences in the number of offspring surviving in the next generation. This is natural selection. Because of their phenotype or behavior, some individuals are more successful than others in producing offspring. Although many traits are phenotypically variable, individuals exhibiting variation do not always differ in survival and reproductive success. 3. Phenotypic variation must have a genetic basis. For natural selection (see item 2) to result in evolutionary change, the selected differences must have a genetic basis. Not all variation has a genetic basis— even genetically identical individuals may be phenotypically quite distinctive if they grow up in different environments. Such environmental effects are common in nature. In many turtles, for example, individuals that hatch from eggs laid in moist soil are heavier, with longer and wider shells, than individuals from nests in drier areas. Page 404 When phenotypically different individuals do not differ genetically, then differences in the number of their offspring will not alter the genetic composition of the population in the next generation, and thus, no evolutionary change will have occurred. It is important to remember that natural selection and evolution are not the same—the two concepts often are incorrectly equated. Natural selection is a process, whereas evolution is the historical record, or outcome, of change through time. Natural selection (the process) can lead to evolution (the outcome), but natural selection is only one of several processes that can result in evolutionary change. Moreover, natural selection can occur without producing evolutionary change; only if variation is genetically based will natural selection lead to evolution. Selection to avoid predators The result of evolution driven by natural selection is that populations become better adapted to their environment. Many of the most dramatic documented instances of adaptation involve genetic changes that decrease the probability of capture by a predator. The caterpillar larvae of the common sulphur butterfly Colias eurytheme usually exhibit a pale green color, providing excellent camouflage against the alfalfa plants on which they feed. An alternative bright yellow color morph is reduced to very low frequency because this color renders the larvae highly visible on the food plant, making it easier for bird predators to see them (see figure 20.4 e). One of the most dramatic examples of background matching involves ancient lava flows in the deserts of the American Southwest. In these areas, the black rock formations produced when the lava cooled contrast starkly with the surrounding bright glare of the desert sand. Populations of many species of animals occurring on these rocks— including lizards, rodents, and a variety of insects—are dark in color, whereas sanddwelling populations in surrounding areas are much lighter (figure 20.7). 15 Figure 20.7 Pocket mice from the Tularosa Basin of New Mexico whose color matches their background.Black lava formations are surrounded by desert, and selection favors coat color in pocket mice that matches their surroundings. Genetic studies indicate that the differences in coat color are the result of small differences in the DNA of alleles of a single gene. Predation is the likely cause for these differences in color. Laboratory studies have confirmed that predatory birds such as owls are adept at picking out individuals occurring on backgrounds to which they are not adapted. Selection to match climatic conditions Many studies of selection have focused on genes encoding enzymes, because in such cases the investigator can directly assess the consequences to the organism of changes in the frequency of alternative enzyme alleles. Often investigators find that enzyme allele frequencies vary with latitude, so that one allele is more common in northern populations, but is progressively less common at more southern locations. A superb example is seen in studies of a fish, the mummichog (Fundulus heteroclitus), which ranges along the eastern coast of North America. In this fish, geographic variation occurs in allele frequencies for the gene that produces the enzyme lactate dehydrogenase, which catalyzes the conversion of pyruvate to lactate (seesection 7.8). Biochemical studies show that the enzymes formed by these alleles function differently at different temperatures, thus explaining their geographic distributions. The form of the enzyme more frequent in the north is a better catalyst at low temperatures than is the enzyme from the south. Moreover, studies indicate that at low temperatures, individuals with the northern allele swim faster, and presumably survive better, than individuals with the alternative allele. Selection for pesticide and microbial resistance A particularly clear example of selection in natural populations is provided by studies of pesticide resistance in insects. The widespread use of insecticides has led to the rapid evolution of resistance in more than 500 pest species. 16 The cost of this evolution, in terms of crop losses and increased pesticide use, has been estimated at $3–8 billion per year. In the housefly, the resistance allele at the pen gene decreases the uptake of insecticide, whereas alleles at the kdr and dldr genes decrease the number of target sites, thus decreasing the binding ability of the insecticide (figure 20.8). Other alleles enhance the ability of the insects' enzymes to identify and detoxify insecticide molecules. Figure 20.8 Selection for pesticide resistance.Resistance alleles at genes such as pen and kdr allow insects to be more resistant to pesticides. Insects that possess these resistance alleles have become more common through select
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