Chapter 20
If all white cats died, what proportion of the kittens in the next generation would be white? (Figure 20.3)
Answer:In the example of Figure 20.3, the frequency of the recessive white genotype is 0.16. The remaining 84 cats (out of 100) in the population are homozygous or heterozygous black. If the 16 white cats died, they will not contribute recessive white genes to the next generation. Only heterozygous black cats will produce white kittens in a 3:1 ratio of black to white. Homozygous x homozygous black and homozygous x heterozygous black cats will have all black kittens. Since there are 36 homozygous black cats and 48 heterozygous black cats, with a new total of 84 cats, the new frequency of homozygous black cats is 36/84 or 43%, with the heterozygous black cats now comprising 57% of the population. If p2 = 0.43, then p = 0.65 (approximately), then 1- p = q, and q = 0.35. The frequency of white kittens in the next generation, q2, is 0.12 or 12%.
Why might the frequency of green toads not increase in the next generation, even if color differences have a genetic basis? (Near Figure 20.9)
Answer: Differential predation might favor brown toads over green toads, green toads might be more susceptible to disease, or green toads might be less able to tolerate variations in climate, among other possibilities.
What evolutionary change in body size might you expect? If the number of eggs laid per day was not affected by body size, would yourprediction change? (Figure 20.9)
Answer: Since the intermediate-sized water strider has the highest level of fitness, it would be expected that the intermediate size would become more prevalent in the population. If the number of eggs laid per day was not affected by body size, the small water striders would be favored because of their tendency to live longer than their larger counterparts.
Would you expect the frequency ofcopper tolerance to be affected bydistance from the mine site? (Figure 20.10)
Answer: Yes. The frequency of copper tolerance will decrease as distance from the mine increases.
What would happen if after 20 generations, experimentersstarted keeping flies that moved toward the light anddiscarded the others? (Figure 20.15)
Answer:The proportion of flies moving toward light (positive phototropism) would again begin to increase in successive generations.
As improved medical technology leads to decreased infantmortality rates, how would you expect the distribution ofbirth weights in the population to change? (Figure 20.16)
Answer: The distribution of birth weights in the human population would expand somewhat to include more babies of higher and lower birth weights.
How do these results depend on the manner by which the guppy predators locate their prey? (Figure 20.18)
Answer: Guppy predators evidently locate their prey using visual cues. The more colorful the guppy, the more likely it is to be seen and thus the more likely it will become prey.
What might explain the lack of change in winning speeds? (Figure 20.19)
Answer:Thoroughbred horse breeders have been using selective breeding for certain traits over many decades, effectively removing variation from the population of thoroughbred horses. Unless mutation produces a faster horse, it remains unlikely that winning speeds will improve.