Chromosomal rearrangements and speciation

Several authors have proposed that speciation frequently occurs when a popu lation becomes fixed for one or more chromosomal rearrangements that reduce fitness when they are heterozygous. This hypothesis has little theoretical support because mutations that cause a large reduction in fitness can be f ixed through drift only in small, inbred populations. Moreover. the effects of chromosomal rearrangements on fitness are unpredictable and vary signif icantly between plants and animals. I argue that rearrangements reduce gene flow more by suppressing recombination and extending the effects of linked isolation genes than by reducing fitness. This unorthodox perspective has significant implications for speciation models and for the outcomes of cont act between neospecies and their progenitor(s).