Position within the geographic range, relative local abundance and developmental instability

Local population abundance often declines with increasing distance from the center of the species' geographic range. This pattern has been hypothesized to parallel the decline in the probability of encountering favorable niche requirements, implying that local abundance is primarily a reflection of local environmental conditions. In this study we evaluate a potential corollary of this hypothesis. We postulate that local conditions that depress abundance are also likely to perturb development. Accordingly, we predicted that spatial patterns in abundance should be mirrored by spatial patterns of developmental instability. To test this prediction we compared 38 contemporary and Pleistocene populations, belonging to 30 mollusc species. Samples were collected at a single site along the Gulf of California, differentially located in relation to the center of each species' geographic range. We identified, as boundaries to triangular bivariate distributions, a negative relationship between developmental instability and relative abundance, and a positive relationship between instability and relative distance from the center of the range. Overall, however, the observed pattern deviated from the expected in that the majority of low-abundance peripheral populations exhibit relatively low levels of developmental instability.