Chapter 20

1. In science, the word “theory” does not mean just a guess or an untested idea. Instead, theory refers to a well-tested body of knowledge that explains the facts that we observe in the natural world. The millions of observations biologists make of living and fossil organisms every year demonstrate the factual basis of evolution. We can observe changes over time in the extensive fossil record of the Earth, just as we can observe the process of evolution at work in living natural populations and in controlled laboratory experiments. Observing that evolution occurs, however, does not by itself explain how evolution occurs. “Evolutionary theory” refers to the body of knowledge about the processes of evolution and our models of how those processes work. For example, using mathematical modeling, a biologist can show that we expect all biological populations of a finite size to evolve by genetic drift. That does not mean, however, that other processes are not resulting in evolutionary change as well. Charles Darwin’s major contribution was to argue that the process of natural selection was a major factor in the evolution of populations of living organisms over time. That idea has been tested many thousands of times by many thousands of biologists since Darwin’s time, and natural selection has been repeatedly shown to be important in the evolutionary change that biologists have observed, and continue to observe, across life on Earth.

2. Although most individual bacteria would die upon exposure to an antibiotic, bacteria that could survive short-term exposure would multiply rapidly after the antibiotic treatment ended. Over time, the population of bacteria would evolve resistance to the antibiotic, as any mutations that allowed survival would increase in frequency. The full treatment is judged to be effective against virtually all the bacteria in the population. If no bacteria survive the full course of the antibiotic, then the population cannot evolve resistance. If the treatment is stopped short, then there is an increased likelihood that some of the bacteria (those with the greatest antibiotic resistance) will survive, and the population of bacteria will evolve increased antibiotic resistance.

3. Humans select traits in domestic plant and animal populations based on our interest in the trait, rather than on how it affects the natural reproductive rate or survivorship of the organisms. Many of the traits selected by humans would not be advantageous in wild populations. For example, humans have selected many cattle breeds for high body fat and high body weight. These traits result in large calves, which in turn result in calving difficulties for cows. Ranchers often have to assist in the birth of such calves, because the calf (and likely its mother) would often die without such assistance. In a natural population, there would be selection for smaller calf size and birth weight, which would increase the successful reproductive rate and survivorship.

4. Behaviors can respond to environmental cues that are predictive of future conditions, and these behaviors can be selected for if they are under genetic control. For example, day length becomes shorter as we move closer to winter, so individual mammals have a survival advantage if they respond to shortening days by going into hibernation. In this case, the environmental cue (day length) is predictive of future environmental conditions (the cold of winter). The traits exist in the present because these associations (as between shortening day length and the approach of winter) have existed for a long time.

5. Natural selection cannot act when there is no effect on the effective reproductive rate of the organism. Diseases such as Alzheimer’s usually occur long after the reproductive years have passed. As long as the disease does not affect the relative likelihood of the survival of the affected person’s offspring (as a result of reduced parental care, for example), we would not expect natural selection to lead to any reduction in Alzheimer’s disease in human populations.

1. Mutations provide the genetic variation on which all the other evolutionary processes act.

2. Neutral traits (traits that confer neither an advantage nor a disadvantage) are free to increase or decrease in a populations by drift alone. In small populations, chance effects have a large role. If the difference in fitness between individuals is relatively small, and there are few individuals in a population, then which individuals survive and reproduce is likely to be a result of factors that do not relate directly to the fitness of the organism. Under these conditions, even some mildly deleterious traits are expected to become fixed in the population over short periods of time.

3. Self-fertilization reduces the frequency of heterozygotes but does not change the allele frequencies in a population. By contrast, sexual selection (nonrandom mating in preference for a particular phenotype) produces a directional change in the population. Hence, the population evolves.

4. A small population might be under strong selection for a particular trait that is favored locally. However, if there is extensive gene flow from neighboring populations where the particular trait is not favored, then selection for the trait in question will be overwhelmed by the gene flow from the surrounding populations.

1. 1. Frequency of allele a: 0.60; of allele A: 0.40;

   2. Frequency of genotype aa: 0.40; of genotype Aa: 0.40; of genotype AA: 0.20;

   3. Expected frequency of genotype aa: 0.36; of genotype Aa: 0.48; of genotype AA: 0.16

2. 2pq = 2(0.2)(0.8) = 0.32

   Since the observed frequency of heterozygotes is lower than the frequency predicted by Hardy–Weinberg expectations, any of the following are reasonable explanations of this pattern:

   • Toads from two or more adjacent subpopulations could be breeding in the same pond. The frequencies of alleles in each population may differ, resulting in high frequencies of different alleles in each subpopulation. In other words, the assumption of no gene flow has been violated.

   • Toads may not be breeding randomly within the population. For example, if closely related toads are more likely to mate with each other than with distantly related toads, then fewer heterozygotes would be expected in population (a form of inbreeding).

   • Heterozygous individuals may be at a disadvantage in the population (e.g., fewer heterozygotes may survive to adulthood). This would represent a violation of the Hardy–Weinberg assumption of no selection.

3. Presumably, the observed population is large enough to calculate that the low frequency of heterozygotes is significant. However, if the present generation of toads was produced by a small number of parents in the previous generation, then random effects of a small population size could explain the observed pattern.

1. Stabilizing selection results in a reduction in variation within the population and an increase in frequency of the modal phenotype. Directional selection results in a change in the modal phenotype of the population in one direction. Disruptive selection results in a population with a bimodal distribution of phenotypes.

2. As shown in Figure 20.13, mortality increases with both smaller and larger birth weight compared to the optimal birth weight of about 7.5 pounds. Significantly smaller babies are more likely to be premature or undernourished, and thus less likely to survive. Significantly larger babies may cause difficulties in delivery, putting both the baby and mother at greater risk and stress during childbirth. Thus, babies close to the modal birth weight are most likely to survive and grow into adults, thereby increasing the relative frequency of alleles for intermediate birth size in the population.

3. Almost any phenotype of an organism that differs markedly from its close relatives is likely the result of directional selection. Among the many thousands of possible examples, consider the long necks of giraffes, the long trucks of elephants, the great body size of whales, the large brain of humans, the great height of giant sequoias, and the large floating leaves of water lilies. Directional selection obviously can result in reduction in size as well, so also consider the tiny body size of hummingbirds compared to other birds, or the tiny leaf size of duck weed compared to other flowering plants. Undoubtedly, you can think of many other examples.

1. The heterozygotes between the sickle-cell and normal alleles may have an advantage in defense against malaria. The normal allele may be able to function normally, whereas the sickle-cell allele provides protection against the malaria parasite.

2. A large population of humans could be tested for infection with malaria, and the frequency of infection, and the consequences of the infection, could be compared in people who do versus do not carry the sickle cell gene as heterozygotes. If the heterozygous individuals have an advantage in malaria resistance, we would expect them to exhibit lower infection rates or reduced effects of infection.

3. High genetic variation leads to more opportunities for the presence of beneficial alleles or traits. If the environment suddenly changes, and beneficial traits are already present in the population, then selection can increase the frequency of those traits and the population can rapidly evolve. However, high genetic variation does not guarantee that appropriate beneficial alleles will already be present in the population.

1. Insects may be physiologically constrained by their system of respiration, which may not be able to support as large a body as that of birds. This is an example of an historical constraint that limits insect body size.

2. Anything that resulted in sudden, widespread environmental change would be likely to affect many species simultaneously. Examples include a meteorite or comet impact, widespread volcanic activity, or sudden climatic changes.

1. Four groups of moths show clear indications of long wing tails, and each of these groups is closely related to moths that lack wing tails. Therefore the trait must have evolved in parallel at least four times. The long wing tails are present in 11 species on the tree, but some of these species are closely related to one another, and so the trait could have evolved in the common ancestor of each closely related species cluster. Therefore the trait appears to have originated about four times independently.

2. Although the long wing tails appear to have evolved four times independently, there is evidence that directional selection continued in many species as the trait evolved. For example, notice the parallel changes from green to yellow to red (indicating increasing length of wing tails) in many of the species.

1. For C. philodice, 43.2% of all viable males are heterozygous, so 56.8% must be homozygous. To get expected numbers of heterozygous and homozygous mating males, we multiple the expected proportions (from all viable males) by the total number of mating males sampled. Therefore we expect to see (0.432)(50) = 21.6 heterozygous mating males, and (0.568)(50) = 28.4 homozygous mating males. If we repeat the same calculations for C. eurytheme, we expect (under the given assumption) to see (0.478)(59) = 28.2 heterozygous mating males and (0.522)(59) = 30.8 homozygous mating males.

2. Chi-square calculations for C. philodice:

   Genotype	Expected (E)	Observed (O)	O – E	(O – E)2	(O – E)2/E
   Heterozygotes	21.6	31	9.4	88.36	4.091
   Homozygotes	28.4	19	–9.4	88.36	3.111

   The sum of the last column gives the chi-square test statistic: 7.202. Since this value is greater than the critical value (P = 0.05) of 3.841, the observed results are significantly different from the expectations at P < 0.05. In other words, we can reject the null hypothesis and conclude that the proportions of each genotype (heterozygotes and homozygotes) of mating males are significantly different from the proportions of these genotypes seen among all viable males in C. philodice.

   Chi-square calculations for C. eurytheme:

   Genotype	Expected (E)	Observed (O)	O – E	(O – E)2	(O – E)2/E
   Heterozygotes	28.2	45	16.8	282.17	10.005
   Homozygotes	30.8	14	–16.8	282.17	9.162

   The sum of the last column gives the chi-square test statistic: 19.167. Since this value is greater than the critical value (P = 0.05) of 3.841, the observed results are significantly different from the expectations at P < 0.05. In other words, we can reject the null hypothesis and conclude that the proportions of each genotype (heterozygotes and homozygotes) of mating males are significantly different from the proportions of these genotypes seen among all viable males in C. eurytheme.

3. (0.75)¹⁶ is approximately 0.01002. Therefore the investigators would need to analyze at least 16 larvae from each batch of eggs to judge the genotype of the father with 99 percent certainty. An easy way to find this answer is to multiply 0.75 × 0.75, and then multiply the answer by 0.75, and continue until the result is approximately 0.01. Keep track of the number of times you multiply by 0.75 to find the appropriate sample size.

1. A. carolinensis from islands with introduced A. sagrei have significantly larger toepads with more lamellae compared to lizards from islands without A. sagrei. As the A. sagrei were only introduced to the islands in 1995, these differences in foot structure appear to have arisen since that time. The fact that the toepads have evolved so quickly indicates that there is strong selection for larger toepads with more lamellae in lizard populations on the invaded islands.

2. The common garden experiment confirms that the observed differences have a genetic basis, and are not due to different expression of the same genes on the two sets of islands. If the lizards raised in the common garden experiment had not shown the same level of differences that were observed in the wild populations, then the observed changes could not be attributed to evolution, which refers to genetic changes in populations over time.

3. The most important evolutionary process in this example is selection. Given that tree-top Anolis typically have larger toepads compared to ground-dwelling species, it is logical that lizards with larger toepads and more lamellae are more likely to survive and reproduce in tree tops (compared to lizards with smaller toepads and fewer lamellae). By living longer, lizards with larger toepads will produce more offspring over time, and so their genes will be increasingly represented in subsequent generations.

4. In this case, individuals of A. carolinensis with smaller toepads and fewer lamellae would have an advantage on the ground and on low perches on the invaded islands. Therefore, we would predict that the evolutionary change would occur in the opposite direction; the average toepads would become smaller on the invaded islands compared to un-invaded islands.

   • Selection for larger toepads appears to be occurring.

   • Mutation is certainly occurring in the populations (as it does in all species), although it likely has a very small effect at this time scale.

   • The populations on each island are likely to be fairly small, so drift is occurring.

   • Gene flow among populations is unknown, but it is likely to be low, since new invasions of islands appear to be rare.

   • Non-random mating among the lizards may be occurring, although there is no evidence of this process described in the experiment.

5. The other four processes of evolution (mutation, drift, gene flow, and nonrandom mating) are likely occurring as well, although they are less important to this example:

   • The variation in toepad size and number of lamellae would not exist without mutations in the genes that produce these structures, so mutation was critical for introducing genetic variation into the populations. But mutations occur very slowly, so very few new mutations that affect toepad size would be expected over 15 years.

   • Genetic drift is certainly occurring on the islands, because the populations on each island are limited. However, drift would not produce a consistent directional effect in toepad size across islands, and so it cannot account for the consistently larger toepads in A. carolinensis on invaded islands.

   • There was no attempt to measure gene flow among the islands in this experiment. But the fact that A. sagrei only occurred on islands where it was introduced suggests that inter-island movement of lizards is low and not likely a major factor in the study.

   • Lizards might choose mates based on their toepad size (non-random mating), which would affect the distribution of toepad size in the population, producing more variation on which selection could then act. However, there is no evidence for this process in the described experiment.