How does arthropod segment number evolve?--Some clues from centipedes.

Studies of intraspecific variation in the number of trunk segments of geophilomorph centipedes provide clues as to how different species of arthropods, and whole clades in some cases, come to be characterized by different segment numbers. However, although previous work in this area has revealed an interesting geographical pattern-a latitudinal cline in which segment number decreases with increasing latitude-the causality of the cline remains obscure. Is it because of selection on genetically based variation, or is it a result of a form of phenotypic plasticity in which the segmentation process is directly affected by a latitude-correlated factor such as temperature? Here, we provide some indirect evidence for plasticity. If the cline is indeed a plastic one, a paradox arises, because the cline mirrors interspecific variation-geophilomorph species with more northern ranges typically have fewer segments than those from further south-but interspecific differences cannot arise from nonheritable variation. We propose a resolution of this apparent paradox via a model in which genetic and environmental factors interact through selection acting on developmental reaction norms.