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variation among hemiclones; and (iv) measurement of the selection gradient acting on phenotypic variation among hemiclones. We apply hemiclonal analysis to test the hypothesis that there is ongoing antagonistic coevolution between the sexes in the D. melanogaster laboratory model system and then discuss the relevance of this analysis to natural systems.

There is widespread agreement among evolutionary biologists that the allopatry model is responsible for generating much of the species diversity presently found within sexually reproducing lineages (Coyne and Orr, 2004; Dobzhansky, 1937; Futuyma, 1986; Gavrilets, 2004; Mayr, 1942; Muller, 1942; White, 1978). Although substantial empirical evidence supports the operational steps of the basic allopatry model (Rice and Hostert, 1993), there is no general consensus regarding the relative importance of alternative evolutionary processes that drive the specific genetic divergence that leads to reproductive isolation. Because allopatry necessitates that populations be physically separated, there can be no direct selection for reproductive isolation and therefore it must develop as a pleiotropic byproduct of the genetic differences that accrue due to the independent evolution of populations.

At a minimum, recurrent mutation and random genetic drift of neutral variation lead to genetic differentiation among allopatric populations. However, natural selection can greatly accelerate the rate of genetic divergence. Van Valen (1973) used paleontological evidence to conclude that adaptation to the physical environment is asymptotic (declining in rate with time). Rapid adaptation occurs when a population initially experiences a new physical environment, but the rate of evolution slows as the population becomes progressively more adapted to the prevailing physical conditions. The rate of adaptation diminishes with time as the lag-load (the reduction in mean population fitness due to the average trait of a population differing from its optimal value) of the population decreases. In contrast, adaptation to the biotic environment is expected to be nonasymptotic when enemies (for example, predator and prey, host and pathogen, or resource competitors) become locked in a perpetual arms race of adaptation and counteradaptation. In this case of interspecific antagonistic coevolution, the lag-load of a species does not diminish with time because adaptive progress is continually eroded due to counter-evolution by enemy species.

An analogous cycle of antagonistic coevolution can take place between genes that reside within the genome of a single species (inter-locus antagonistic coevolution). In this case, adaptive allelic replacement at one

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