Geographic variation in skull shape of the water rat Scapteromys tumidus (Cricetidae, Sigmodontinae): isolation-by-distance plus environmental and geographic barrier effects?

Fernando M Quintela,Rodrigo Fornel,Thales R.O Freitas

doi:10.1590/0001-3765201620140631

Abstract

The geographic variation in skull size and shape of the swamp rat Scapteromys tumidus was examined in samples from eight geographic clusters in almost of its distribution in southern Brazil and Uruguay. For analysis we used two-dimensional geometric morphometric methods for dorsal, ventral and lateral views of the skull. The geometric descriptors showed no significant differences in skull size between geographic clusters, while differences in shape were highly significant. We found a significant and moderate correlation between geographic and morphological distances, corroborating the isolation-by-distance model. Samples from the Rio Grande do Sul central coastal plain were the most differentiated, segregating completely from all other samples in canonical variate analysis for the dorsal view. The most visible variable regions in skull were the zygomatic arch (mainly the squamosal root of zygomatic) and the lateral braincase borders. Once correlation between geographic and morphological distances were not strong, it is possible that other factors (environmental heterogeneity and/or geographic barriers) may are acting in S. tumidus skull differentiation.

Highlights

Microevolution is a process of small evolutionary changes within populations of a determined species (Avise 2000)
We found marked geographic variation in shape of the water rat Scapteromys tumidus skull
The Mantel’s test showed a significant and regular correlation between the geographic and morphological distances, which corroborates with the isolation-by-distance model

Summary

Introduction

Microevolution is a process of small evolutionary changes within populations of a determined species (Avise 2000). Such changes in genetic and morphological characters within and among populations characterize different evolutionary units over geographic space (dos Reis et al 2002a). In relation to morphological traits, intraspecific variations occur as a result of genetic diversity, selection, reproductive isolation and other factors. Microevolutionary events, are the driving force of macroevolution (evolution beyond species level) processes, resulting in speciation (Avise 2000). Intraspecific morphological variation can be accessed by using methods of traditional and geometric morphometric analysis

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