Question

You identify five endemic species of mammals living on an island a short distance from the mainland. You also identify each species' closest relative on the mainland. Two alternatives exist to explain this group of species:
Hypothesis 1. The ancestors of the current island species dispersed from the mainland to the island and subsequently diverged on the island (dispersal).
Hypothesis 2. The island was originally part of the mainland but a rise in sea level resulted in its being cut off, with the result that the mammals on the island diverged from the ones on the mainland (vicariance).
How would you distinguish between these two hypotheses after sequencing the same segment of DNA in representatives of all ten species?

A) If the level of genetic divergence for all five species pairs is similar, then vicariance is the likely cause

B) If the level of genetic divergence for all five species pairs is similar, then dispersal is the likely cause.

C) If the level of genetic divergence differs for at least two of the five species pairs, then dispersal is the likely cause.

D) If the level of genetic divergence differs for each of the five species pairs, then vicariance is the likely cause.

Answer 1

Speciation by vicariance occurs when a population that was initially widely distributed is separated by a newly formed barrier like a river or a mountain range. Here hypothesis 2 assumes that vicariance is what led to speciation in the five mammal species found on the island.

Speciation by dispersal occurs when a part of the initial population crosses a pre-existing barrier to colonize a new region. Here hypothesis 1 assumes dispersal.

Since vicariance theory assumes that all the island species got separated from the mainland populations at the same time, the genetic divergence between all five species pairs would be expected to be similar. Dispersal does not make this assumption as a species can jump across the barrier at any point in time and then diverge from the mainland population. Knowing these facts, let us evaluate all the four options.

A) If the level of genetic divergence for all five species pairs is similar, then vicariance is the likely cause- This is false. Since the selection pressures on each species is different, they need not necessarily diverge at the same pace even though they got sepearted from the mainland populations at the same time.

B) ) If the level of genetic divergence for all five species pairs is similar, then dispersal is the likely cause – This is false as species can disperse at different time ponts.

C) If the level of genetic divergence differs for at least two of the five species pairs, then dispersal is the likely cause – This is true.

D) If the level of genetic divergence differs for each of the five species pairs, then vicariance is the likely cause – This is also false, as we cannot assume that genetic divergence won’t be similar for species which separated from the mainland at the same time. Here dispersal would be the more likely cause.

Hence, option C is the right answer.

I hope this helps ?