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populations of tortoises), we used a laboratory island population to assess the evolutionary principles that underlie inter-locus antagonistic coevolution between the sexes. Our finding, that after hundreds of generations of coevolution we can detect an ongoing arms race between the sexes, supports the conclusion that perpetual arms races occur in nature and contribute substantially to the genetic divergence that leads to reproductive isolation and speciation.

A study such as ours could not feasibly be carried out in nature and therefore is possible only in the context of laboratory island populations. For example, the fact that we were able to detect heritabilities among hemiclones of only 2.4% for female resistance illustrates the substantial statistical power of this approach. The genetic measurements that we obtained for standing genetic variance and heritability took advantage of a broad array of genetic tools that are available only in laboratory populations of D. melanogaster. So, in general, we see two options: (i) study only natural populations and wait for technology to advance to the point that experiments such as ours are possible in situ, or (ii) study laboratory island populations where these experiments are possible today. We see a clear advantage to the second option.

We believe that the process of biotic evolution has basic underlying principles that apply to manmade microcosms just as they do to natural ecosystems. If we want to estimate the current or historical trajectory of a natural population, then we need to study that population in situ. But, if we want to understand the evolutionary principles that underlie evolution in nature, rather than specific evolutionary histories, then we can study them just as effectively, and in general more so because of reduced technical constraint, in the context of laboratory island populations. There is the danger that newly constructed laboratory populations will display misleading transients, and, as a result, caution is needed when interpreting results from laboratory populations that have not coevolved over a protracted number of generations (Sgrò and Partridge, 2000). Nonetheless, laboratory island populations make possible evolutionary analysis that cannot be achieved in nature and thereby provide an essential complement to direct studies of populations in nature.

CONCLUSIONS

The allopatric model of speciation, as originally articulated by Dobzhansy (1937) and Mayr (1942), requires genetic divergence among physically isolated populations. Although sequence data are available for only a small number of genes that cause reproductive isolation, the available data indicate that these genes evolve rapidly under positive Darwinian selection [see article by H. A. Orr, “The Genetic Basis of Reproductive



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