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Inquiry Questions

The Hardy–Weinberg equilibrium. In the absence of factors that alter them, the frequencies of gametes, genotypes, and phenotypes remain constant generation after generation.
If all white cats died, what proportion of the kittens in the next generation would be white?
Answer: This question can be answered in many ways. Here's one way: with the white cats dead, the new allele frequencies would be B: p = 0.71 and b: q = 0.29. Thus, the probability of getting a white kitten, whose genotype is bb, would be q2 = 0.08.
Here's another way of looking at the same question: The only way that a white kitten could be born would be if two heterozygotes mated. The proportion of heterozygotes in the population would be 0.48 / (0.48 + .036 = 0.84) = 0.57. Thus, the probability that two heterozygotes would mate is 0.572 = 0.32. The probability that a kitten would be white if its parents are both heterozygotes is 1/4. Thus, 0.32 * 0.25 = 0.08, the frequency of white kittens in the next generation.
Selection to match climatic conditions. Frequency of the cold-adapted allele for lactate dehydrogenase in a type of fish (the mummichog, Fundulus heteroclitus) decreases at lower latitudes, which are warmer.
Why does the allele frequency change from north to south?
Answer: Mean water temperature increases with latitude. Thus, the further north, the more advantageous is the cold-adapted allele.
FIGURE 21.10
Body size and egg-laying in water striders. Larger female water striders lay more eggs per day, but also survive for a shorter period of time. As a result, intermediate-sized females produce the most offspring over the course of their entire lives and thus have the highest fitness.
What evolutionary change in body size might you expect? If the number of eggs laid per day was not affected by body size, would your prediction change?
Answer: Yes. In that case, selection on longevity would favor small individuals. In the absence of any selective pressure in the opposite direction, this directional selection would be expected to lead to the evolution of smaller body size.
FIGURE 21.11
Degree of copper tolerance in grass plants on and near ancient mine sites. Individuals with tolerant alleles have decreased growth rates on unpolluted soil. Thus, we would expect copper tolerance to be 100% on mine sites and 0% on non-mine sites. However, prevailing winds blow pollen containing nontolerant alleles onto the mine site and tolerant alleles beyond the site's borders.
Would you expect the frequency of copper tolerance to be affected by distance from the mine site? How would your answer change depending on whether you were upwind or downwind from the mine site?
Answer: The frequency of copper tolerance should decrease with distance from the mine site because the further from the mine, the less pollen from the mine will be blown in. Copper tolerance should decline with distance both up- and downwind from the mine, but, for a given distance, the frequency should be considerably lower upwind (occasionally, pollen will move in that direction due to wind shifts and other reasons).
FIGURE 21.16
Directional selection for negative phototropism in Drosophila. Flies that moved toward light were discarded, and only flies that moved away from light were used as parents for the next generation. This procedure was repeated for 20 generations, producing substantial evolutionary change.
What would happen if after 20 generations, experimenters started keeping flies that moved toward the light and discarded the others?
Answer: Evolution would change directions and the trend would be toward
increased tendency to fly toward light.
FIGURE 21.17
Stabilizing selection for birth weight in human beings. The death rate among babies (red curve; right y-axis) is lowest at an intermediate birth weight; both smaller and larger babies have a greater tendency to die than those around the most frequent weight (blue area; left y-axis) of between 7 and 8 pounds.
As improved medical technology leads to decreased infant mortality rates, how would you expect the distribution of birthrates in the population to change?
Answer: If babies far from the mean (either large or small) have higher survival rates, then selection against alleles for small or large size would not be as strong, and the expected results would be a broadening of the frequency distribution of birth weight in newborns.
FIGURE 21.19
Evolutionary change in spot number. Guppies raised in low-predation or no-predation environments in laboratory greenhouses had a greater number of spots, whereas selection in more dangerous environments, such as the pools with the highly predatory pike cichlid, led to less conspicuous fish. The same results are seen in field experiments conducted in pools above and below waterfalls (photo).
How do these results depend on the manner by which the guppy predators locate their prey?
Answer: If predators detect fish by smell or by feeling water currents (or any other non-visual sensory mode), then brightly colored fish would not be at a selective disadvantage. Other traits (odor, ability to move without creating much disturbance in the water) would, instead, be the target of selection.
FIGURE 21.20
Selection for increased speed in racehorses is no longer effective. Kentucky Derby winning speeds have not improved significantly since 1950.
What might explain the lack of change in winning speeds?
Answer: No genetic variation exists anymore, so winning speeds cannot be increased.

Self Test

1). Which of the following is not an assumption of the Hardy–Weinberg equilibrium?
    a). Mating occurs preferentially.
    b). The size of the population is large.
    c). There is no migration.
    d). There are no mutations.
Answer: a

2). In a population of red (dominant allele) or white flowers, the frequency of red flowers is 91%. What is the frequency of the red allele?
    a). 9%
    b). 30%
    c). 91%
    d). 70%
Answer: d

3). Which of the following describes gene flow?
    a). random mating
    b). migration
    c). genetic drift
    d). selection
Answer: b

4). Which of the following conditions is not needed for natural selection to occur in a population?
    a). Individuals must be able to move between populations.
    b). Variation must be genetically inherited.
    c). Certain variations allow an individual to produce more offspring that survive in the next generation.
    d). There must be variations in the phenotypes of individuals in the population.
Answer: a

5). Which of the following is the ultimate source of genetic variation in a population?
    a). gene flow
    b). assortive mating
    c). mutation
    d). selection
Answer: c

6). Natural selection can be countered by which of the following?
    a). genetic drift
    b). gene flow
    c). mutation
    d). All of the above can counter natural selection in some way.
Answer: d

7). The maintenance of the sickle cell allele in human populations in central Africa is an example of
    a). gene flow.
    b). heterozygote advantage.
    c). genetic drift.
    d). nonrandom mating.
Answer: b

8). What would happen in the U.S. if malaria once again became a widespread disease?
    a). Over time, the sickle cell allele would become more prevalent in the population.
    b). Many individuals in the population would die of malaria.
    c). Individuals who were heterozygous for the sickle cell allele would be less susceptible to malaria.
    d). All of these events would occur.
Answer: d

9). ____________ operates to eliminate intermediate phenotypes.
    a). Directional selection
    b). Disruptive selection
    c). Stabilizing selection
    d). Random chance
Answer: b

10). When transplanted to streams above waterfalls, guppy populations evolved more spots because
    a). predators are not present.
    b). they consumed different sources of food.
    c). spots make guppies harder to see against their background.
    d). all of the above
Answer: a

Test Your Visual Understanding

1). Match the following descriptions with the correct panels in the figure.
    a). Phenotypically similar individuals mate.
    b). Individuals migrate.
    c). Space exploration expands with the settlement of a population of 100 individuals on Mars.
    d). A nuclear power plant dumps boiling water into a reservoir, killing all bacteria except those that contain a heat shock gene.
    a).(b) Nonrandom mating
    b).(a) Gene flow
    c).(c) Genetic drift
    d).(d) Selection

Applying Your Knowledge

1). Consider a human population that is similar to the ideal Hardy–Weinberg population in that it is very large and generally random-mating. Although mutations occur, they alone do not lead to great changes in allele frequencies. However, migration occurs at relatively high levels-perhaps 1% per year. The following data describe relative numbers of individuals bearing the two alleles of the MN blood group:
MM MN NN Total
Individuals 1787 3037 1305 6129
Do these data suggest that some factor is disrupting the Hardy–Weinberg proportions of the three genotypes? What are the allele frequencies of M and N?
The frequencies of the phenotypes are:
p2 = 1787 / 6129 = 0.29 or 29%
q2 = 1305 / 6129 = 0.21 or 21%
2pq = 3037 / 6129 = 0.5 or 50%
Using a Punnett square you would expect a population of 25% homozygous dominant, 25% homozygous recessive, and 50% heterozygous. The frequencies found in this population are close to the expected values that would suggest that the population is in Hardy–Weinberg equilibrium.
The allele frequencies of M (p) and N (q) are:
p2 = 0.29 so p equals the square root of 0.29 or p = 0.54
q2 = 0.21 so q equals the square root of 0.21 or q = 0.46
According to the Hardy–Weinberg equilibrium (p + q) = 1 and using a Punnett square, the expected allele frequencies should be .50 for each allele and in this example: 0.54 + 0.46 = 1
This supports the conclusion that the population is in Hardy–Weinberg equilibrium such that there is no migration or selection is disrupting this equilibrium.

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