This weeks blog (also posted on my blog) is a departure from fish, but is about a recent paper of mine that uses phylogenetic comparative methods to test hypotheses for body size and scale evolution among Sceloporus lizards.
Oufiero, C.E.$, G.E.A. Gartner$, S.C. Adolph, and T. Garland Jr. 2011. Latitudinal and climatic variation in scale counts and body size in Sceloporus lizards: a phylogenetic perspective. In press Evolution. DOI: 10.1111/j.1558-5646.2011.01405.x
$ These authors contributed equally
This summer the lab has a reading group on phylogenetic comparative methods, where we are reading through some of the classic phylogenetic papers discussing the various methods. This past week we focused our attention on phylogenetic generalized least squares methods or PGLS. This method was introduced by Grafen in 1989, and although it wasn’t initially a common phylogenetic comparative approach, has seen more use in recent years. For those not familiar with this method, it utilizes a regression approach to account for phylogenetic relationships. In this method the phylogeny is converted to a variance-covariance matrix, where the diagonals in the matrix represent the “summed length of the path from the root of the tree to the species node in question (Grafen 1992).” That is, how far each tip is from the root; in an ultrametric tree the diagonals in the variance-covariance matrix will all be the same. The off diagonals represent the “shared path length in the paths from the root to the two species (Grafen 1992)”. In other words, the off diagonals are the distance from the root to the last common ancestor for the two species. Similar to independent contrasts, this method assumes Brownian motion evolution; however, unlike independent contrasts PGLS assumes the residual traits are undergoing Brownian motion evolution, whereas independent contrasts assumes the characters themselves are undergoing Brownian motion evolution. The other main difference in PGLS is the use of raw data instead of computing independent contrasts. In short, the PGLS approach is similar to a weighted regression, where the weighted matrix is the variance-covairnace matrix based on the phylogeny of the group, and assuming the same phylogeny will produce the same results as independent contrasts.
So what does this have to do with size, scales and Sceloporus? Well, in a recent study we used a PGLS approach to examine patterns of body size and scale evolution in relation to latitude and climate among Sceloporus lizards. Sceloporus (fence and spiny lizards) are a group of more than 90 species of lizards found from Central America up to Washington State in the U.S. Throughout their range they experience a diversity of habitats, from deserts to tropical forests to temperate forests; and have been used in many studies examining physiological ecology, life history evolution and thermal biology. In our study we used Sceloporus to test two hypotheses for the evolution of morphology. 1) Lizards exhibit an inverse Bergmann’s Rule, with larger individuals found at lower latitudes and/or warmer climates. 2) Lizards from hotter environments will exhibit fewer and thus larger scales to aid in heat dissipation; whereas lizards from colder environments will exhibit more/smaller scales to aid in heat retention. There has been conflicting results for these hypotheses in the literature, and latitude has often been used as a proxy for climate. However, one of the unique things about our study is the incorporation of multivariate techniques to describe habitat. We use latitude as a predictor as well as climatic variables (temperatures, precipitation and a composite aridity index Q), and also utilize principal component analysis to characterize habitat. We therefore can test for specific climate predictors of these traits without assuming that higher latitudes necessarily equate to colder environments.
To test our hypotheses we gathered data on 106 species and populations of Sceloporus from the literature and museum specimens. We obtained latitude from the literature and source maps, and climate date from the International Water Management Institute’s World Water and Climate Atlas (http://www.iwmi.cgiar.org/WAtlas/Default.aspx). Using a recent phylogenetic hypothesis for Sceloporus (Wiens et al. 2010) we examined the relationship between maximum snout-vent length with latitude and 5 climatic predictors under three models of evolution (no phylogenetic relationships (OLS), Brownian motion (PGLS) and a model in which the branch lengths are transformed in an Ornstein-Uhlenbeck process (RegOU). To examine hypothesis 2 we examined a multiple regression with dorsal scale rows as the dependent, body size as a covariate and latitude or one of the 5 climatic predictors as independents. We also compared results with principal components 1-3 as predictors of dorsal scale counts.
So what did we find? First, we found that phylogenetic models (PGLS or RegOU) were always better fit than non-phylogenetic (OLS) based on likelihood ratio tests and AICc scores. We also found that as latitude increases mean and minimum temperatures decrease, as well as precipitation and aridity, but maximum temperature tends to increase. Thus, lizards from this group found at higher latitudes may be experiencing more desert like environments.
For hypothesis 1, we found support for the inverse of Bergmann’s Rule when viewed from a climatic perspective; larger lizards were found in areas with higher maximum temperatures, but not at lower latitudes. We also found that larger lizards were found in more arid environments.
Our results for hypothesis 2 were a little more complex. We did not find support for the first part of hypothesis 2, lizards with fewer scales were not found in hotter environments. We did find support for the second part of hypothesis 2, lizards with more scales are found in environments with lower minimum temperatures. We also found a positive effect of latitude, and a significant negative effect of aridity (with lizards with more scales inhabiting more arid environments). Results with principal components were also consistent, with PC1 (a latitude/temperature axis) having a significant negative effect on scale count; and PC2 (a maximum temperature/precipitation axis) having a significant positive effect.
Our results suggest several things. First, latitude alone may not be an accurate description of the environment organisms face, particularly at the finer spatial scales over which an individual species may exist. Second, we found support for the inverse of Bergmann’s Rule at the inter-specific level, which has also been found to be a consistent trend intra-specifically in some ectotherms (see Ashton and Feldman 2003). Finally, our analyses suggest that both temperature and precipitation (hence aridity) are important to the evolution of scale counts in this group. These findings also suggest that scale size may be important for other physiological processes, such as evaporative water loss (lizards in more arid environments may have more/smaller scales to reduce rates of evaporation through the skin as has been suggested by Soulé and Kerfoot 1972 ). Examining the relationship of morphological traits that may function in physiological processes may provide insight into how these organisms may respond to global of climate change.