The most significant forces driving allopatric speciation are...

Question

a. different genetic drifts affecting each population

b. behaviors that prevent the two populations from mating.

c. different selective pressures affecting each population

d. anatomical differences that prevent the two populations from mating

Answer 1

Allopatric speciation refers to two (or more) populations that evolve in reproductive isolation due geographic or physical barriers (like an ocean, great distances, a huge desert etc). So, quite frankly, the answer is not there. The only reasonable one is A, "different genetic drifts affecting each population". Because of geographic isolation, populations have a smaller gene pool and will evolve different physical and/or biological characteristics.

Answer 2

It would depend on the specific circumstances.  Both a. and c. can be significant forces in allopatric speciation.  Both b. and d. can drive speciation as well, but are not the ultimate cause, more likely the result of a. or c.

Allopatric speciation occurs when there is a barrier separating two populations from breeding and since they are not interbreeding, genetic differences accumulate separately in each population.  These differences can be the result of both genetic drift and different selective pressures.  It depends on the specific situation which is more significant.

Answer 3

I'm with the first two answers given by other people.   The correct answer is not in your list.

Allopatric speciation is when two subpopulations get *geographically* isolated.

As such, the answer is both a and c.  

Genetic drift can affect the two populations differently (since the geographic separation prevents genes from moving between the two populations) ... especially if the two populations are small.

And if they are in different regions they can be (but don't have to be) affected by different selective pressures.