Speciation

When natural selection and mutations are adding new alleles occur in a population, it spreads throughout that population, but because the species have been separated they go through different changes of alleles. The separation caused by the barrier of the Colorado River stopped gene flow between the species. Thus, overtime they have evolved into two completely different species and the genetic code for the two populations has become more incompatible.

In allopatric speciation there is a geographic barrier like a river or mountain that separates populations keep them from being able to interbreed or share the same gene pool, eventually, over time there can be a change in the separated population from mechanisms like natural selection or genetic drift which can prevent them from interbreeding even if they were brought together