The Evolution of Populations Chapter 23 The Evolution of Populations Questions prepared by Jung Choi Georgia Institute of Technology Michael Dini Texas Tech University

Genetic Variation The original source of all genetic variation is _____. natural selection sexual reproduction independent assortment recombination mutation Answer: e

mutation

Evolution In evolutionary terms, an organism's fitness is measured by its _____. stability in the face of environmental change contribution to the gene pool of the next generation genetic variability mutation rate health Answer: b

contribution to the gene pool of the next generation

Allele Frequencies Red short-horned cattle are homozygous for the red allele, white cattle are homozygous for the white allele, and roan cattle are heterozygotes. Population A consists of 36% red, 16% white, and 48% roan cattle. What are the allele frequencies? red = 0.36, white = 0.16 red = 0.6, white = 0.4 red = 0.5, white = 0.5 Allele frequencies cannot be determined unless the population is in equilibrium. Answer: b 6

red = 0.6, white = 0.4

Cystic Fibrosis 1 The frequency of cystic fibrosis, a recessive genetic disease, is 1 per 2,500 births among Northern Europeans. Assuming random mating, what is the frequency of carriers? 1/2,500, or about 0.04% 1/50, or about 2% 1/25, or about 4% The frequency cannot be calculated because selection violates Hardy-Weinberg assumptions. Answer: c The frequency of carriers is 2pq. The allele frequency, q, is 1/50 since qq = 1/2500. P is close to 1. You may want to discuss why option d does not apply. 8

1/25, or about 4%

Cystic Fibrosis 3 The frequency of cystic fibrosis is extremely low among Asians and Africans. What may explain the relatively high frequency of cystic fibrosis among Northern Europeans? bottleneck effect founder effect genetic drift directional selection Heteozygote advantage Answer: e The first three can explain the presence of harmful alleles in small populations, but probably not in Northern Europeans. Directional selection is unlikely to maintain a harmful allele at a relatively high frequency. Balancing selection (heterozygote advantage) is the most likely answer, analogous to the high frequency of the sickle-cell disease allele in equatorial African populations. 10

Heteozygote advantage

A high proportion of the cats on Key West have extra toes (polydactyly). What is the most likely explanation? high rate of mutation founder effect bottleneck effect directional selection for extra toes Answer: b This question asks students to apply an understanding of the different mechanisms of evolution to formulate a hypothesis that explains an actual situation. In this case, history tells us that the so-called Hemingway cats descended from a six-toed cat brought to the island by a ship captain in the 1800s. 12

founder effect