Chapter 6: Population Genetics 2: Migration, Drift, Non-random Mating

1.	Conservation managers often try to purchase corridors of undeveloped habitat so that larger preserves are linked into networks. Why? What genetic goals do you think the conservation managers are aiming for? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
2.	Figure 6.32 shows a measure of genetic differentiation between populations, as a function of geographic distance. The data are from human populations in Europe. Genetic differentiation has been calculated based on loci on the autosomes (inherited from both parents), the mitochondrial chromosome (inherited only from the mother), and the Y chromosome (inherited only from the father). Note that the patterns are different for the three different kinds of loci. Keep in mind that migration tends to homogenize allele frequencies across populations. Develop a hypothesis to explain why allele frequencies are more homogenized across populations for autosomal and mitochondrial loci than for Y—chromosome loci. Then go to the library and look up the following paper, to see if your hypothesis is similar to the one favored by the biologists who prepared the graph: Seielstad, M. T., E. Minch, and L. L. Cavalli—Sforza. 1998. Genetic evidence for in humans. Nature Genetics 20:278–280. [Part of title deleted to encourage readers to develop their own hypotheses.] [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
3.	Loss of heterozygosity may be especially detrimental at MHC loci, because allelic variability at these loci increases disease resistance. Surveys of microsatellite loci show that the gray wolves on Isle Royale, Michigan are highly inbred (Wayne et al. 1991). This wolf population crashed during an outbreak of canine parvovirus during the 1980s. How might these disparate facts be linked? How could you test your ideas? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
4.	If you were a manager charged with conserving the collared lizards of the Ozarks, one of your tasks might be to reintroduce the lizards into glades in which they have gone extinct. When reintroducing lizards to a glade, you will have a choice between using only individuals from a single extant glade population, or from several extant glade populations. What would be the evolutionary consequences of each choice, for both the donor and recipient populations? Which strategy will you follow, and why? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
5.	Bodmer and McKie (1995) review several cases, similar to achromatopsia in the Pingelapese, in which genetic diseases occur at unusually high frequency in populations that are, or once were, relatively isolated. An enzyme deficiency called hereditary tyrosinemia, for example, occurs at an unusually high rate in the Chicoutimi region north of Quebec City in Canada. A condition called porphyria is unusually common in South Africans of Dutch descent. Why are genetic diseases so common in isolated populations? What else do these populations all have in common? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
6.	Remote oceanic islands are famous for their endemic species–unique forms that occur nowhere else (see Quammen 1996 for a gripping and highly readable account). Consider the roles of migration and genetic drift in the establishment of new species on remote islands. How do plant and animal species become established on remote islands? Do you think island endemics are more likely to evolve in some groups of plants and animals than others? Consider a new population that has just arrived at a remote island. Is the population likely to be large or small? Will founder effects, genetic drift, and additional waves of migration from the mainland play a relatively large or a small role in the evolution of the new island population (compared to a similar population on an island closer to the mainland)? Do your answers help explain why unusual endemic species are more common on remote islands than on islands close to the mainland? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
7.	Why are the rates of silent—site substitutions reported in Table 6.1 higher than the rates of replacement substitutions? Why do pseudogenes have the highest rates of evolution? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
8.	By using the start codon AUG as a guidepost, researchers can determine whether substitutions in pseudogenes correspond to silent changes or replacement changes. In contrast to most other loci, the rate of silent and replacement changes is identical in pseudogenes. Explain this observation in light of the neutral theory of evolution. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
9.	Why did Kimura and Ohta develop the nearly neutral model of molecular evolution? According to the neutral and nearly neutral theories, how does population size affect the rate of molecular evolution? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
10.	When researchers compare a gene in closely related species, why is it logical to infer that positive natural selection has taken place if replacement substitutions outnumber silent substitutions? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
11.	What is codon bias? Why is the observation of nonrandom codon use evidence that certain codons might be favored by natural selection? If you were given a series of gene sequences from the human genome, how would you determine whether codon usage is random or nonrandom? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
12.	The complete genome sequence of humans is available now and the sequence of chimpanzees will be available soon. Outline how you would analyze homologous genes in the two species to determine which of the observed sequence differences result from drift and which result from selection. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
13.	Recall that the fourth chromosome of Drosophila melanogaster does not recombine during meiosis. The lack of genetic polymorphism on this chromosome has been interpreted as the product of a selective sweep. If the fourth chromosome had normal rates of recombination, would you expect the level of polymorphism to be different? Why? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
14.	As we have seen, inbreeding can reduce offspring fitness by exposing deleterious recessive alleles. However, some animal breeders practice generations of careful inbreeding within a family, or "line breeding," and surprisingly many of the line—bred animals, from champion dogs to prize cows, have normal health and fertility. How can it be possible to continue inbreeding for many generations without experiencing inbreeding depression due to recessive alleles? (Hint: Responsible animal breeders do not breed animals known to carry deleterious traits.) Generally, if a small population continues to inbreed for many generations, what will happen to the frequency of the deleterious recessive alleles over time? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
15.	In the mid-1980s, conservation biologists reluctantly recommended that zoos should not try to preserve captive populations of all the endangered species of large cats. For example, some biologists recommended ceasing efforts to breed the extremely rare Asian lion, the beautiful species seen in Chinese artwork. In place of the Asian lion, the biologists recommended increasing the captive populations of other endangered cats, such as the Siberian tiger and Amur leopard. By reducing the number of species kept in captivity, the biologists hoped to increase the captive population size of each species to several hundred, preferably at least 500. Why did the conservation biologists think that this was so important as to be worth the risk of losing the Asian lion forever? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
16.	In this chapter we saw that in many cases, gene frequencies in small populations change at different rates than in large populations. As a review, state whether the following processes will typically have greater, smaller, or similar effects on evolution in small versus large populations: Selection Migration Genetic drift Inbreeding New mutations per individual New mutations per generation in the whole population Substitution of a new mutation for an old allele ixation of a new mutation [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.