You are part of a team of scientists exploring a small, newly discovered island. This island is located approximately 150 miles off the coast of the mainland. Preliminary evidence suggests that humans have never populated the area and have likely had almost no presence on the island. In your work cataloging the various organisms on the island, you have discovered a new population of birds. Their gross appearance is very similar to a known species on the mainland. Genetic data also suggest that there is a close relationship between the new population and the species on the mainland. However, the coloration pattern of the newly discovered birds is distinct. The mainland birds are primarily brown with splotches of red on the back and wings. The individuals in the new population of birds are largely red with patches of deep purple on the wings and small crests on the top of their heads. Such crests and patches of purple are known to exist in the mainland species but are rare.
Speculate on possible explanations of these observations. Hypothesize on the past events that may have led to this phenomenon and also link your explanation to the underlying molecular basis of inheritance.
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We must begin by considering how the island population bird began. While 150 miles is not far for a migratory bird species to travel, the fact that this is a newly discovered population indicates that these birds are resident on the island and do not migrate. The fact that they are genetically similar to a mainland species is good evidence that they are descended either from that species or from a recent common ancestor. It is likely that the island population shows the results of the Founder Effect, which occurs when a population is descended from a small group of founding individuals. In this case, it seems likely that the island population is descended from just a few birds who were blown out to sea in a storm or who wandered off course during a migration, and it is likely that the founders carried the alleles for the crest and color patches which are present but uncommon in the mainland population. Since the founders carried uncommon alleles, the entire island population, which is composed of their descendants, all have these alleles, making them common in the island population.
Another potential driving force behind the differences is Mate Choice. If the crests and/or the color patches are favored when individuals are choosing mates, this could result in the desirable characteristic becoming more common in the population over time. This effect is generally more pronounced in small populations such as the one on the island because the number of possible mates is so limited.
A third consideration is Genetic Drift, yet another result of small population size. In genetic drift, the frequency of an allele can change simply due to random chance. Consider a population of 1000 of these birds, of whom half carry the allele for the crest. If a storm kills half the population, what are the chances that only birds with the crest allele survive? It's possible, but not very likely to happen. Now think about a population of just six birds, again with half carrying the crest allele. Now what are the odds of only crested birds surviving if a storm kills three birds? This scenario is much more likely.
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