b) Not all populations experience equal amounts of genetic drift or non-random m
ID: 19404 • Letter: B
Question
b) Not all populations experience equal amounts of genetic drift or non-random mating. Evolutionary biologists use the effective population size, Ne, to estimate the size of an ideal population that would undergo the same amount of genetic drift or show effects of non-random mating as the actual population. Smaller populations have more drift. The value of Ne can be determined from:
Ne = 4 NmNf / Nm + Nf
Where Nm is the number of males in the actual population and Nf is the number of females.
If one population of elk has 2 males and a harem of 6 females and another population in the same
region has 1 male and 11 females. Which population is more likely to experience genetic drift? Why?
(keywords to look up: Sewall Wright, effective population size, and Fisherian ratio)
Explanation / Answer
a) According to the Hardy-Weinberg principle, the frequency of alleles in a population remains constant over generations as long as no natural selection occurs, ie. the population is in genetic equilibrium. Since we are only concerned with frequency, the size of the population (in this case) is irrelevant, and the 18 individuals are meant to throw us off. The two alleles, p and q, are related by the equation (p+q)2 = p2 + 2pq + q2 = 1. We can substitute M for p and q for M' (the assignment is arbitrary and switching the two leads to the same result). Thus, MM corresponds to p2, MM' corresponds to 2pq, and M'M' corresponds to q2. If we plug this in, then we get MM = 0.72 = 0.49 Dark, MM' = 2 x 0.7 x 0.3 = 0.42 Mottled, and M'M' = 0.32 = 0.09 Green. Notice that 0.49 + 0.42 + 0.09 = 1.
Answer: The frequencies are: 0.49 Dark, 0.42 Mottled, and 0.09 Green.
b)
For the population with 2 males and 6 females, Ne = 4x2x6/2 + 6 = 30
For the population with 1 male and 11 females, Ne = 4x1x11/1 + 11 = 55
Since the smaller effective population undergoes greater genetic drift, the population with 2 males and 6 females is the answer.
Answer: The population with 2 males and 6 females.
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