Variable segment number in centipedes: population genetics meets evolutionary developmental biology.

The case studies of population genetics focus on intraspecific variation, but most cases--at least where the variation is polymorphic--deal with characters that are not directly linked to organismic structure or ontogeny. Conversely, the case studies of evolutionary developmental biology focus directly on structure/ontogeny, but usually involve only interspecific comparisons. To integrate these complementary approaches, it is desirable to have a model system that permits study of intraspecific variation in development, using a character whose genetic basis either is already known or can be elucidated. Segment number in geophilomorph centipedes is proposed as a possible model system of this kind. Segment number is variable in natural populations of geophilomorphs, while in the other centipede orders it is fixed, either completely (scutigeromorphs, lithobiomorphs), or at least within species (scolopendromorphs). Statistical analysis of data on the extent of variation in different geophilomorph species suggests that segment number may be of selective importance, rather than the variation being merely an inevitable consequence of the difficulty of achieving a high degree of repeatability when there is a large number of segments.