The environments of the isolated ponds differed enough that natural selection and perhaps genetic drift acted on the separate populations. Eventually the fish in the different ponds may have diverged so much genetically that they could no longer interbreed even if brought together. In this way geographic isolation of fishes in Death Valley probably led to the formation of new species. Geographic isolation, plus reproductive isolation, probably is the usual cause of the formation of new species. Reproductive Isolation Sometimes groups of organisms within a population become isolated genetically without prior geographic isolation.
Reproductive isolation is the inability of formerly interbreeding organisms to produce offspring.
Reproductive isolation can arise through disruptive selection. Remember that in disruptive selection the two extremes of a specific trait in a given population ate selected for. The wood frog and the leopard frog have become reproductively isolated , possibly as a result of disruptive selection. Though the wood frog and the leopard frog sometimes interbreed in captivity, they do to interbreed where the ranges overlap in the wild. The wood frog usually breeds in early April, and the leopard frog usually breeds in mid-April.