Geographic patterning of variation in segment number in geophilomorph centipedes: clines and speciation

Since their origin as a metameric group, arthropods have diversified consid erably in their number of segments. Present-day geophilomorph centipedes pr ovide a model system for investigating the evolutionary origins of this div ersification, because they exhibit intraspecific variation in segment numbe r. (This variation is, however, derived; it is not a plesiomorphic conditio n within the Chilopoda.) Previous studies have shown that there are signifi cant differences in segment number between populations within several geoph ilomorph species. In one (arguably two) species, it has been demonstrated t hat there is a particular form of geographic patterning of the variation, n amely a latitudinal dine, with the segment number decreasing with increasin g distance north. Here, we provide additional data on four more species, al l of which show evidence of a latitudinal dine in either one or both sexes. It is therefore becoming clear that this is a general phenomenon, applying widely (perhaps universally) across the Geophilomorpha, a group consisting of some 1000 known species. It may be that latitudinal dines are a frequen t part of the speciation cycle in this group.