Guppies and Population Growth

In the second stream, the rapid infusion of new fish almost completely eliminated pure residents—an outcome conservationists usually hope to avoid. That result suggests “a slow trickle of immigration might be preferable,” Fitzpatrick says

Genetically speaking, populations of species that are separated from one another begin to diverge fairly rapidly. In as many as eight generations, visible and measurable morphological differences appear. On a genetic level even one or two germ stations may begin to drift, through epigenetics and the influence of the environment on survival of offspring, among other variables.

Realistically, this means that any sudden influx of new genes changes the population. In severe cases, as above, it can overwhelm the original culture altogether. And cause extinction, with the best of motivations.