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much of the discussion on rapid population divergence tended to focus on the kinds of environmental or geographic circumstances that might promote rapid evolution. However, in recent years, the attention has shifted to situations where intraspecies and intragenomic conflicts can lead to rapid evolution of genes (Rice, 1998). Such conflicts arise whenever natural selection favors alleles that are penetrant under some circumstances (such as in one sex), even though those same alleles may reduce other components of fitness that are manifest in other contexts (such as in the other sex). Genomic conflicts can lead to tit-for-tat, or arms-race, evolution between groups of genes within the same genome. In “Inter-Locus Antagonistic Coevolution as an Engine of Speciation: Assessment with Hemiclonal Analysis” (Chapter 3), William Rice et al. explore this issue directly by developing a model evolutionary system with Drosophila melanogaster. In this system, individual haploid chromosome complements (hemiclones) are drawn, using genetic tricks that are possible with Drosophila, from a longstanding laboratory population. Once isolated, each hemiclone can be measured, by replicating in combination with other hemiclones, for its net effect upon fitness of particular phenotypes. The approach allows a careful assessment of the selection gradient and additive genetic variance for traits that enhance fitness in males but reduce fitness in their female mates.

The findings that rapid evolution of genes, including that caused by genomic conflict, can lead to the formation of reproductive barriers notwithstanding, there remains the question of how much gene flow can be tolerated between diverging populations if speciation is to occur (Wright, 1940). Not even populations with many rapidly evolving genes can be expected to become reproductively isolated from other populations if gene flow rates are high. With this point in mind, Francisco Ayala and Mario Coluzzi, in “Chromosome Speciation: Humans, Drosophila, and Mosquitoes” (Chapter 4), explore models in which recombination suppressors, such as chromosomal inversions, can enhance the opportunity for adaptive divergence in the face of gene flow between parapatric populations (Coluzzi, 1982; Noor et al., 2001; Rieseberg, 2001). These models are more plausible than those in which chromosomal inversions enable divergence by causing low hybrid fitness, and they are supported particularly by recent evidence from Drosophila and Anopheles.

Regardless of the rates and roles of genetic changes that contribute to divergence, there remain very large questions about the phenotypic manifestation of these changes. Mary Jane West-Eberhard, in “Developmental Plasticity and the Origin of Species Differences” (Chapter 5), addresses these questions and stresses that phenotypes may derive from genes in ways that are highly contingent upon other genes and upon environmental circumstances. Thus, single new alleles or new environmental circum-

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