Reinforcement of pre-zygotic isolation and karyotype evolution in Agrodiaetus butterflies

Speciation by degrees Speciation is among the most important and poorly understood areas in evolutionary biology. One controversial aspect is reinforcement, in which natural selection favours mate discrimination when incipient species are still capable of hybridizing, but with reduced fitness. Recent studies have questioned several of the classic examples of reinforcement in natural populations; once part of darwinian orthodoxy, reinforcement was relegated to the margins. But a new study puts it back on the agenda. An analysis of the evolutionary history of Agrodiaetus, an unusual group of butterflies with variable chromosome numbers, shows that when closely related species occur together, they almost always differ in wing colour. This is a character critical in mate choice and colour differentiation occurs primarily between young and closely related species, arguing in favour of reinforcement as a mechanism generating diversity.