SPECIATION is the primary process of cladogenetic evolution. It is generally agreed that genetic divergence sufficient to produce reproductive isolation among populations is acquired in the allopatric state1–4. One of the major problems of evolutionary genetics is, then, characterisation of the genetic differences which actually produce the reproductive and sexual isolation characteristic of species. A partial answer to this problem has been provided by estimating the degree of structural gene divergence over various taxonomic levels—populations, subspecies, semispecies, sibling species and morphologically distinguishable species5,6. The general picture emerging from these studies is moderate structural gene differentiation at the subspecies level. Once reproductive isolation has been achieved, little additional change seems to be required to establish sexual isolation between subspecies. There are, however, exceptions to this generalisation7,9, and they do not seem to be infrequent. Speciation may occur as a result of chromosomal rearrangements, accompanied by little, if any, detectable change in the structural genes usually studied.