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fitness (P = 0.029), corroborating the significant positive net-selection gradient on this trait deduced from the measures of male adult lifetime offspring production from the larger sample of 35 hemiclones.

In sum, we found significant additive genetic variation among hemiclones for remating rate in both males and females, but the net-selection gradient on this trait was positive in males and negative in females. To look for independent genetic variation for remating rate in males and females, we constructed a bivariate plot of remating rate of the same hemiclones when expressed in males vs. females. No significant correlation was found (r = −0.175, P = 0.316), indicating that remating rate in the two sexes is controlled by different genetic variation. Because there is independent genetic variation for remating rate in the two sexes, and because it is selected in opposite directions in each sex, we conclude that this trait presently is evolving in opposite directions in the two sexes and therefore that sexually antagonistic coevolution for mating rate is currently in evidence in this laboratory island population.


In the above section, we used hemiclonal analysis to provide evidence that (i) females have genetic variation for resistance to male-induced harm, (ii) resistance contributes substantially to total genetic variation for net fitness, (iii) propensity to remating strongly influences the degree of female resistance, and (iv) there is unique genetic variation for remating rate in males and females that is selected and evolving in opposite directions in the two sexes. These data provide support for the hypothesis that perpetual inter-locus, intersexual arms races contribute to rapid genetic divergence among allopatric populations, and owing to the phenotypes that coevolve (reproductive behavior, physiology, and anatomy) are likely to be contributing to the specific genetic divergence that leads to reproductive isolation and speciation.

The data that we described, however, came from a laboratory island population rather than directly from nature. Some might argue that such populations are too artificial and hence tell us nothing about evolution in nature. We disagree. We cannot statistically extrapolate from our laboratory island population to natural populations of D. melanogaster because our laboratory population is not a random sample from the natural environment. We can, however, use laboratory island populations to make inferences about the fundamental principles of evolution and then use logic to extrapolate to the process of evolution in nature. Just as Darwin (1859) used his study of island tortoise populations to deduce general evolutionary principles (rather than extrapolate to specific continental

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