Now Gillespie and her colleagues understood a little more about the evolutionary history of the happy-face spiders, but they still didn’t understand the 2:1 ratio. The proteins and DNA sequences together demonstrated that very few spiders moved between populations on different islands. This ruled out dispersal as an explanation for the similar morphs and morph frequencies on different islands. What else might explain the consistent ratio? Perhaps learning more about the genetics of these happy-face patterns would reveal something.

Studying the genes, adding to the mystery

Gillespie and Oxford turned to breeding experiments to identify the genetic mechanism behind color pattern formation.

They started with spiders from Maui, bred individuals of known parentage, and counted the number of offspring of each morph. This type of breeding experiment is a common method used to figure out how genes produce a particular trait such as color morph.

Selective breeding between individuals of known parentage should result in predictable patterns of color morphs in the offspring. And in fact, that was the case for the Maui spiders. The frequency of color patterns in both male and female offspring was consistent with what you would expect if color pattern were passed from parent to offspring at a single gene on a chromosome.

However, mysteriously, the same breeding experiment on the spiders from Hawaii yielded quite different results! Some of the color patterns occurred only in female offspring and others only occurred in males. This indicated that the genetic component for color morphs on Hawaii was linked to gender!

The genetic mechanisms were different between the spider populations on Maui and Hawaii. The spider populations had evolved the same color patterns and the same color pattern frequencies, but they’d done it in totally different ways! Why did all the islands independently evolve the same set of color pattern traits?