Phylogenetic and Environmental Components of Morphological Variation: Skull, Mandible, and Molar Shape in Marmots (Marmota, Rodentia)

Abstract

Abstract. The phenotype is a product of its phylogenetic history and its recent adaptation to local environments, but the relative importance of the two factors is controversial. We assessed the effects of diet, habitat, elevation, temperature, precipitation, body size, and mtDNA genetic divergence on shape variation in skulls, mandibles, and molars, structures that differ in their genetic and functional control. We asked whether these structures have adapted to environment to the same extent and whether they retain the same amount of phylogenetic signal. We studied these traits in intra-and interspecific populations of Eurasian marmots whose last common ancestor lived 2-5 million years ago. Path Analysis revealed that body size explained 10% of variation in skulls, 7% in mandibles, and 15% in molars. Local vegetation explained 7% of variation in skulls, 11% in mandibles, and 12% in molars. Dietary category explained 25% of variation in skulls, 11% in mandibles, and 9% in molars. Cyt b mtDNA divergence (phylogeny) explained 15% of variation in skulls, 7% in mandibles, and 5% in molars. Despite the percentages of phylogenetic variance, maximum-likelihood trees based on molar and skull shape recovered most phylogenetic groupings correctly, but mandible shape did not. The good performance of molars and skulls was probably due to different factors. Skulls are genetically and functionally more complicated than teeth, and they had more mathematically independent components of variation (5-6-in skulls compared to 3-in molars). The high proportion of diet-related variance was not enough to mask the phylogenetic signal. Molars had fewer independent components, but they also have less ecophenotypic variation and evolve more slowly, giving each component a proportionally stronger phylogenetic signal. Molars require larger samples for each operational taxonomic unit than the other structures because the proportion of within-taxon to between-taxon variation was higher. Good phylogenetic signal in quantitative skeletal morphology is likely to be found only when the taxa have a common ancestry no older than hundreds of thousands or millions of years (1% to 10% mtDNA divergence)-under these conditions skulls and molars provide stronger signal than mandibles. The phenotype is the product both of phylogenetic history and recent adaptation to local environments, but the relative importance of the two factors in explaining morphometric variation is contested (MacLeod and Forey 2002). Some studies have concluded that morphology is an unreliable indicator of phylogenetic relationships because of capricious, adaptation-driven homoplasy (e.g., Graur et al. 1997;Collard and Wood 2001;Stone and Cook 2002) or because of lack of differentiation among genetically distinct populations (e.g., Barratt et al. 1997; Mayer and von Helvernsen 2001;Roca et al. 2004). But other studies have found significant phylogeographic structuring in morphometric data among closely related taxa (Thorpe 1976;Straney and Patton 1980;Hausser 1984;Malhotra and Thor...

Taxonomic identification using geometric morphometric approach and limited data: an example using the upper molars of two sympatric species of Calomys (Cricetidae: Rodentia)

“…A study on the outline of the first molar of extinct murine rodents demonstrated that different diet groups could be distinguished through geometric morphometric analysis (Cano et al 2013). However, in marmots and some Didelphidae it has been demonstrated that diet is weakly correlated with molar shape (Caumul andPolly 2005, Chemisquy et al 2015).…”

“…Limitations in analyses owing to insufficient sample size are common, particularly in paleontological samples (Matthews and Stynder 2011), as it was in ours. Is important to point out that geometric morphometrics and features from the molars can be used in other types of studies beyond taxonomic identifications, as for example: correlating changes in molar shape with paleoclimate aspects (McGuire 2010), phylogenetic (Caumul and Polly 2005), morphological evolution and environmental variations (Renaud and Van Dam 2002).…”

Cranial phenotypic variation in Meriones crassus and M. libycus (Rodentia, Gerbillinae), and a morphological divergence in M. crassus from the Iranian Plateau and Mesopotamia (Western Zagros Mountains)

“…Bookstein 1991;Rohlf & Marcus 1993;Adams & Rohlf 2000), this approach was followed in this study. As the skull phenotype is genetically and functionally more complex than teeth (Caumul and Polly 2005;CordeiroEstrela et al 2008), it probably represents relevant biological variation of underlying natural groups. Table 1).…”

“…Inter-group skull and bulla size differences between all the M. crassus groups, as defined for the shape analysis at an intra-specific level, and M. libycus were tested by a Kruskal-Wallis test (with Bonferroni correction) using PAST. As the ventral cranium contains a larger number of evolutionarily independent components of variation (Caumul & Polly 2005) as well as the main part of the tympanic bulla, only the size differences obtained from the ventral view are plotted in Fig. 5.…”

“…The distinction, however, of the OTU's is not as clear as the dorsal view data show. Although the ventral view does not reveal a clear distinction either, the results are incorporated in this study, as the ventral cranium contains a larger number of evolutionarily independent components of variation and is known to be the most informative view (Dobigny et al 2002;Caumul & Polly 2005).…”