Considerable debate still exists bout the precise nature of the mechanisms involved in the process of natural selection. In 23 May Nature, Patrik Nosil and colleagues from Simon Fraser University, Burnaby, Canada, show that divergent selection for host adaptation, and not genetic drift, has promoted the parallel evolution of sexual isolation in the walking-stick insect Timema cristinae (Nature 2002, 417:440-443).

Nosil et al. studied populations of T. cristinae from eight study sites in the Santa Ynez Mountains of California, and recorded the distribution of the two genetically determined color-pattern morphs (the colors match the color of the host plants).

They observed that the geographic distance between populations was positively correlated with the degree of sexual isolation of T. cristinae. However the degree of sexual isolation observed between populations was not correlated with genetic distances between pairs of populations. In addition, the pairs of populations...

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