Clarke and Sheppard's Supergene Theory

Clarke and Sheppard proposed an additional gradualist ("Fisherian") idea: that the linkage within the supergene had been built up bit by bit. Theoreticians have now shown that this is unlikely: a long period of polymorphism would be required, and, most likely, the most favourable morph would have become fixed before linkage had time to evolve. Instead, the genes must almost certainly have been tightly linked to start with.

Pre-existing developmental supergenes do not now seem as unlikely as they seemed when Fisher, Sheppard and Clarke started writing. Many developmental switch genes, such as the homeotic gene clusters, are in fact "multigene families", tandem duplications whose functions have diverged. If this is true also for genes controlling wing colour development (none have yet been cloned or sequenced), it  seems likely that the "supergene" in P. memnon could also be a multi-gene family. Alternatively, linked developmental genes could have been initially widely spaced, but became trapped within single-mutation inversions which reduce recombination because of crossover suppression (see CHROMOSOMAL EVOLUTION).

We will know a lot more about this when someone sequences actual genes that cause mimicry. The race is on in a number of laboratories here and in the USA!


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