The Aurelian had hoped to write a short, witty piece about speciation in butterflies. He thought he understood the issues, but just to be sure, started reading and re-reading books and papers on the subject.
A useful pocket definition of species is that it represents a group of interbreeding natural populations that are reproductively isolated from other such groups. Typically the appearance of new species is accompanied by the establishment of reproductive barriers. There are a number of ways in which this can come about.
For years, the Aurelian had been attracted to the ice-age refugia theory. This suggested that during the ice-age, populations of European butterflies retreated to the remaining areas of the continent that were still relatively unaffected, viz Southern Spain, Southern Italy, the Balkans and Greece.
After many thousands of years, as the ice slowly retreated, the different species returned to their previous haunts throughout continental Europe. However, the butterflies of species A that had “over wintered” in Spain had changed over time, and had changed in different ways to the butterflies of species A that had “over wintered” in Italy. Changed enough that they could no longer interbreed, and thus had diverged into two similar, but different species, Aa and Ab.
This theory goes a long way to explaining why we have so many pairs of similar but slightly different species of butterfly. Only apparently it’s all bollocks. Recent comparative laboratory studies show that these species diverged before the ice-age that supposedly provided the separation necessary for speciation. They were already different.
So there goes that theory.