Evol Ecol Res 14: 503-517 (2012)     Full PDF if your library subscribes.

Occupation of environmental and morphological space: climatic niche and skull shape in Neotoma woodrats

Jorge Soberón and David Martínez-Gordillo

Biodiversity Institute and Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, Kansas, USA

Correspondence: J. Soberón, Department of Ecology and Evolutionary Biology, University of Kansas, Dyche Hall, 1345 Jayhawk Blvd., Lawrence, KS 66045, USA.
e-mail: jsoberon@ku.edu


Background: Theoretical and empirical studies suggest that in some circumstances niche evolution may be very slow – a phenomenon called ‘niche conservatism’. Evidence for niche conservatism comes mainly from studies of niches whose axes are climatic variables with broad ranges of spatial covariance (coarse-grained niche variables). The geographic area inside the physiologically viable climatic extreme values (i.e. climate tolerances) is usually large and may expose populations to a wide range of finer-grained selective pressures.

Hypothesis: Coarse-grained niche variables lead to different evolutionary patterns than do finer-grained niche variables.

Data: Skulls from museum specimens of rats of the genus Neotoma provided data on lateral, ventral, dorsal, and mandibular aspects. Climate data were assembled for each place where a specimen had been caught.

Key assumption: Rats of the genus Neotoma respond to selective pressures that are coarse-grained in space, whereas their cranial dimensions respond to fine-grained selective pressures. The volume of a minimally enclosing ellipsoid in the principal component (PCA) space of either climatic or morphological variables is a surrogate for the species’ breadth of occupation of climatic and morphological spaces.

Methods: We measured and analysed overall variance in climatic variables and morphology using geometric morphometry methods. We analysed ellipsoid volumes, together with summaries of skull shape and climatic tolerances, as to the influence of phylogeny on patterns of variation. And we also searched for contrasting patterns of morphological and climatic features.

Conclusions: Patterns in climatic and morphological variables were different. Climatic PC axes were mostly uncorrelated with morphological PC axes. Ellipsoid volumes of the climatic variables were significantly smaller than those of the morphological variables. Blomberg’s K did show that the evolution of most of the PC axes (four morphological and three climatic) cannot be distinguished from Brownian motion. However the evolution of two PCs – the PC dominated by dryness and the one dominated by ventral view of the skull – have been restricted by phylogeny.

Keywords: fundamental niche, geometric morphometry, Neotoma, niche conservatism, North America, realized niche, skull.

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