What type of population would evolution occur most rapidly?

Answer 1

Smaller populations or populations split by some sort of geographic barrier.

In small populations gene flow and genetic drift would shift allele frequencies back and forth very rapidly as the population would be subject to the vagaries of the environment and the small size population effects of random effects and various sized gene flows into the populations gene pool. No selection for adaptive traits here and not enough allele shift for speciation., though enough for evolution

If a large enough population is split, say by a river or mountain range, then you have different mutations offering up different selective opportunities and if these populations remain split long enough they may lose the ability to interbreed and become two different species. This without the wash out of diversity that would plague small populations.

Answer 2

it all depends the way things are set up