-new species formed from a small pop isolated at the edge of a larger pop
-version of allopatric & happens when one of the isolated pops has very few individuals
-ex: intrepid fruit flies venture off mainland on bunch of rotting bananas, banana bunch washes up on island:
1. now geographically isolated
2. rare genes survive (by chance carries rare gene from mainland and causes variation in mating dance and another to male genitalia (founder effect))
3. gene frequencies drift- those diffs drift to fixation over generations
4. more changes- sexual selection causes unique reproductive features
-changes improve male/female genitalia fit, and nat. sel. favors those suited to island climate & food.
5. speciation- flies reproductively isolated from mainland flies