Gillespie and Oxford set out to solve this mystery: the happy-face spiders on different islands independently evolved similar morphs and the same 2 yellow:1 non-yellow ratio — but why? They reasoned that there must be some advantage to maintaining this frequency within the different populations — some kind of selection could act as the driving force for retaining the balance between the yellow and non-yellow morphs.
Could it be that the yellow morph has an advantage in terms of camouflage and that natural selection has made it more frequent than the other morphs? Gillespie and Oxford rejected this hypothesis. After all, if this were the case, then the yellow morph would have greater reproductive success and over time, would become more and more frequent in the population. And that’s not what they observed. The populations remained consistently at a 2 yellow:1 other ratio.
Could it be that sexual selection has something to do with it? Perhaps red morphs are better at attracting mates than others, but yellow morphs are better camouflaged. These two competing patterns of selection, sexual selection decreasing the frequency of yellow morphs and predation increasing the frequency of yellow morphs, could balance each other out and end up maintaining the ratio. Gillespie and Oxford also rejected this hypothesis based on two pieces of evidence: (1) the spiders cannot see color, and (2) they are nocturnal. Color blind spiders finding each other in the dark are unlikely to choose a mate based on a smiling red abdomen! So sexual selection probably doesn’t have much to do with it. The possible explanation remained elusive.