Intraspecific variation, its geographic structure, and the relationship between variation and duration, with examples from Cambrian trilobites and recent fiddler crabs

Phenotypic variation is fundamental to evolutionary change because it influences not only the connectivity of populations but also the adaptability and extinction susceptibility of taxa. Despite the potential importance of morphological variation in structuring evolutionary patterns, little is known about how relative differences in intraspecific variation and its geographic structure contribute to differences in species longevity. This study investigates the relationship between species-level variation, geography and longevity, first, by exploring patterns of geographic variation in extant and fossil arthropods using geometric morphometrics and, second, by quantifying relative variation, geographic range size, and duration in a clade of Upper Cambrian pterocephaliid trilobites from the Great Basin, USA. Results show that in extant North American fiddler crabs, species with more intraspecific variation are not necessarily more widespread but do express more geographic variation. In pterocephaliids, geographic range size and duration are positively correlated, but duration and intraspecific variation are negatively associated with one another. In particular, longer lived species tend to have smaller amounts of intraspecific variation and shorter lived species have more variation. Phylogenetic effects were explored and found not to determine the association between these variables, though the distribution of geographic range size, and to some extent, intraspecific variation, show phylogenetic signal. In light of the results from the fiddler crab study, one possible explanation for these results is that species with more variation tend to have shorter durations because the greater variation reflects higher rates of morphological evolution. These results have implications for the processes driving morphological evolution and how those processes pattern the distribution of species in the fossil record, both across time and space.