Predictors of intraspecific morphological variability in a tropical hotspot: comparing the influence of random and non‐random factors

Abstract

Aim We describe patterns of skull size and shape variation in an Atlantic forest endemic rodent to test the influence of genetic structure, historical and environmental variables upon intraspecific morphological variability. Location South America, Brazil, Atlantic forest. Methods We analyse subtle differences in skull morphology of Akodon cursor through geometric morphometrics applied to 324 individuals from 12 localities distributed throughout the species range. Using cytochrome-b gene (cyt-b) sequences from 125 individuals (38 localities), we describe underlying patterns of genetic structure and transform them into distance measures that are included in our morphological analyses. We estimate the relative importance of genetic structure, historical variables and environmental variables on skull size and shape through mixed model selection and Akaike's information criterion. Results Geographical patterns in skull size are mainly explained by non-random factors related to primary productivity and precipitation, whereas spatial shifts in shape correlate with mitochondrial divergence. Cytochrome-b data revealed a phylogeographic break around the Jequitinhonha River, yet striking morphological shifts were observed further south. Differences in palaeostability between regions, and the configuration of rivers, appear as secondary sources of explanation for observed patterns. Main conclusions Multiple forces explain morphological variation within A. cursor. Teasing apart the effects of local adaptation and gene flow may be difficult, but is a key to improve our understanding of the drivers of intraspecific morphological variation. Our findings support the view that size is a more labile feature than shape, and that it may more easily break away from constraints imposed by gene flow. The combination of random and non-random factors, together with documented breaks in the distribution of the Atlantic forest over the Late Quaternary, accounts for the majority of morphological differences observed in A. cursor.