Differences in (G + C) content between species: a commentary on Forsdyke's"chromosomal viewpoint" of speciation

Forsdyke (1999) has recently argued that differences in (G + C)%, or G + C content, may trigger new species formation. He further argues that the geni c model has shortcomings that can be overcome by his "chromosomal" thereaft er, "G + C") model. We disagree on several counts. First, we do not accept that the genic model has the shortcomings suggested by Forsdyke. There is a n abundance of empirical support for the contribution of individual genes, as well as of mapped chromosomal regions, to post-zygotic reproductive isol ation land Haldane's rule). Further, we argue that the G + C model suffers from the same theoretical difficulties as other speciation models based on underdominance. We also question the evidence Forsdyke uses to support his model. Finally, we describe analyses of G + C content in a well-studied mod el system of speciation (the Drosophila melanogaster species complex), the results of which are incompatible with the G + C model. Thus, while Forsdyk e's G + C model cannot be explicitly ruled out, it is not directly supporte d by empirical data. In contrast, the genic model is well supported by empi rical data, holds up on theoretical grounds, and does not require any assis tance from the G + C model. (C) 2001 Academic Press.