Peripatric speciation is a special version of the allopatric speciation mode and happens when one of the isolated populations has very few individuals. Here’s a very hypothetical example of how the peripatric speciation mode works, returning to our intrepid fruit flies venturing off the mainland on a bunch of rotting bananas. We pick up their story as their banana bunch is washed up on an island:

1. Double disaster: Not only are the island fruit flies now geographically isolated from their mainland relatives, but only a few larvae have survived the harrowing journey to end up colonizing the island.

2. Rare genes survive: These few survivors just by chance carry some genes that are rare in the mainland population. One of these rare genes happens to cause a slight variation in the mating dance. Another causes a slight difference in the shape of male genitalia. This is an example of the founder effect.

3. Gene frequencies drift: These small differences, which are rare on the mainland, drift to fixation in the small population on the island over the course of a few generations (i.e., the entire island population ends up having these genes).

4. More changes: As the island population grows, the unique reproductive features on the island result in a cascade of changes caused by sexual selection. These changes optimize, or at least improve, the fit of male and female genitalia to one another and female sensitivity to nuances of the mating ritual. Flies also experience natural selection that favors individuals better suited to the climate and food of the island.

5. Speciation: After some generations, the island flies become reproductively isolated from the mainland flies. Peripatric speciation has occurred.

In peripatric speciation, small population size would make full-blown speciation a more likely result of the geographic isolation because genetic drift acts more quickly in small populations. Genetic drift, and perhaps strong selective pressures, would cause rapid genetic change in the small population. This genetic change could lead to speciation.

The essential characteristic of this mode is that genetic drift plays a role in speciation. There are likely many cases where a population is split into two unequally-sized populations and they become separate species. However, it is very difficult for us to tell after the fact what role genetic drift played in the divergence of the two populations — so gathering evidence to support or refute this mode is challenging.