Question

For each of the scenarios below:

a) state why evolution is occurring

b) state whether mutation, natural selection, genetic drift or gene flow appears to be the mechanism of evolution

c) and describe why you picked a particular mechanism of evolution

Scenario 1: Researchers have been studying an isolated, small population of wolves (fewer than 30 individuals) on Isle Royale in Lake Superior for the past 30 years. This year they find that there are only 16 individuals left in the population and only 1 of these individuals is a female.

Scenario 2: A large population of colonially nesting (large groups of individuals nest together in a colony) shorebirds varies in plumage color from very white to dark gray. During the breeding season, a train derailment causes a toxic substance to flow into a lake where the colony is located. Approximately 75% of the mating individuals in the colony die. During the following breeding season, researchers notice that almost all of the individuals in the population have medium to dark gray plumage.

Answer 1

Scenario 1 -- A group of less than 30 wolves are breeding among themselves. They are isolated from other population of wolves so, there is no chance of genetic exchange with new individuals. The breeding and interbreeding among the same individuals must have resulted in inbreeding depression . This results in reduced biological fitness in the wolf population. A population bottleneck has occurred with no chance of introduction of new genetic material. Moreover, due to inbreeding a deleterious allele gets a chance to accumulate.

The reduction in the wolf population is a result of continuous inbreeding which has reduced the biological fitness of the population resulting in death of individuals. The lack of biological fitness is more evident in females as only one female has survived among the 16 individuals.

b] In scenario II, the exposure to the toxic chemical substance resulted in the death of 75% of the shorebirds. Because the offsprings in the next breeding season , show medium to dark grey plumage, we can assume that the shorebirds with white plumage were more susceptible to the toxic chemical. Most of the birds which survived, must have been with grey plumage. Also, exposure to the toxic chemical must have caused mutation in the gene controlling plumage colour due to which the grey colour has become the dominant colour.

c] The mechanism of evolution is mutation and to a certain extent , natural selection.. This is so because in the first case there is no possibility of genetic drift or gene flow because of the seclusion of the wolf population.The inbreeding could have resulted in the accumulation of the deleterious allele.

In the second case, the exposure to the toxic substance has resulted in the change in the plumage colour of the shorebirds. We know that exposure to chemicals can cause mutation in the genes and there is evidence to suggest that the birds with grey plumage had a better chance to survive. This could be natural selection.