Why did each island have a 2:1 ratio of yellow to other morphs? A first hypothesis involved dispersal. As an example, imagine that we start out with two populations with different ratios of blue to orange individuals. One population is half blue spiders and half orange, and the other is heavily biased towards orange individuals. What would happen to the populations if there were no movement between them, in other words, if there were no dispersal? Click the “Start” button below to see what happens to our two populations of 24 spiders over 12 generations.

The ratios in each population change independently and remain quite different from each other. But what would happen if individuals were allowed to move between populations, in other words, if dispersal between populations were common? Click the button below to find out.

Because some genes are being exchanged every generation, each population ends up with the same ratio of blue to orange! Gillespie and Oxford thought that this mechanism might explain the consistent 2:1 ratio they found among the happy-face spiders. They based a first hypothesis on this idea.

Gillespie and Oxford turned to molecular data to test this hypothesis.