Reading Dawkins’ “The Greatest Show on Earth” yesterday, I was reminded of this letter from Darwin to Wallace:
I crossed the Painted Lady and Purple sweetpeas, which are very differently coloured varieties, and got, even out of the same pod, both varieties perfect but not intermediate. Something of this kind I should think must occur at least with your butterflies & the three forms of Lythrum; tho’ those cases are in appearance so wonderful. I do not know that they are really more so than every female in the world producing distinct male and female offspring…
Maybe if they’d pursued this topic a bit more, Darwin and Wallace could have rejected the blending theory of inheritance that was prevalent at the time and duplicated Mendel’s discovery of particulate inheritance. But to what did “your butterflies” refer?
I’m pretty sure I found the answer today. In the Sumatra chapter of Wallace’s “Malay Archipelago” he discusses polymorphism in the buttefly, Papilio memnon. The females of this species occur in two very different forms, one of which mimics another species that birds don’t eat. Wallace notes that females of either form produce offspring of both forms, with no intermediates.
I wondered whether anyone has worked out the genetic basis of this polymorphism. “Identification of doublesex alleles associated with the female-limited Batesian mimicry polymorphism in Papilio memnon” found ” two allelic sequences of the doublesex gene that corresponded exactly with the mimetic and non-mimetic female phenotypes.” The image below. showing male and two female versions, comes from their paper. No mention of Darwin’s landmark letter to Wallace, though.