Question

An acute viral disease infects a population of chipmunks, and only those homozygous for allele b are resistant. (Those individuals heterozygous or homozygous for allele B are susceptible and suffer 100% mortality).

A. If the initial frequency of allele b=0.2, what is the frequency (ignoring mutation) one generation after the introduction of this disease?

B. What is the mechanism of evolutionary change in this example?

Answer 1

A.The individuals with 'bb' allele will only survive.Which means 'BB' and 'Bb" will die after the introduction of the disease.The initial allele frequencies of b=0.2 and B= 0.8. Because the individuals with BB and Bb succumb to the disease, next generation we see only 'bb' individuals in the population. The allele frequency is 1. Because all individuals survived are 'bb'.

B. 'bb 'individuals got selected by nature. It is not a random change in allele frequency. Because 'bb' individuals have an advantage over 'BB' and 'Bb' individuals, the allele frequency of 'b' has drastically increased in very short period of time. Such change where one extreme genotype is preferred over others is directional selection. In the population, 'bb' individuals are only 0.04 frequency. Which means 4% of individuals. But in the next generation, due to selection their % became 100%. So , it is directional selection.