From T.I.G.E.R.S. Preservation Stations, Barefoot Landing, Myrtle Beach, S.C. And Parrot Jungle Island, Miami, Fla. Why aren”t horses and donkeys the same species? All members of a species that live in particular area at a given time. All of the alleles of a gene that exist in a population. Evolution = Change in the frequencies of alleles in a population over time. Why is the frequency of the sickle-cell gene different in North America versus Africa?