Our research in comparative evolutionary biology focuses on the evolution of the multivariate phenotype. Work centers on three interrelated areas: 1) developing new evolutionary tools and analytical approaches for quantifying evolutionary changes in multivariate phenotypes, particularly for geometric morphometric data, 2) understanding macroevolutionary patterns of phenotypic diversity and rates of phenotypic evolution, and 3) understanding microevolutionary patterns of morphological change and their selective and historical causes. Some recent projects in each of these research arenas are described below.
Evolutionary theory, Morphometrics, and methods development
Our theoretical work focuses on the development of new evolutionary tools and analytical methods for assessing biological patterns and processes. Much of our efforts focus on methods for understanding the evolution of the multivariate phenotypes. The evolution of multi-dimensional traits (such as shape) is complex, and considerable effort is required to properly merge the mathematical properties of such traits with current evolutionary theory. For instance, we recently developed an analytical framework for assessing phenotypic change of multi-dimensional traits that provides a means of comparing evolutionary trajectories for two or more lineages (Collyer and Adams, 2007; Adams and Collyer 2007; Adams and Collyer 2009; Collyer and Adams, 2013). This method furthers the study of 'evolutionary modes' for multivariate data; enabling the assessment of convergence and parallelism in a multivariate context (e.g., Adams, 2010; Adams and Nistri, 2010). Other recent work provided a likelihood approach for comparing evolutionary rates for two or more phenotypic traits on a phylogeny (Adams, 2013).
Our current theoretical work focuses on the intersection between phylogenetic comparative biology and geometric morphometrics. Current evolutionary theory allows the comparison of phylogenetic rates of evolution for single, univariate traits or sets of independent traits, but complex multi-dimensional traits such as shape cannot be evaluated with existing approaches. To alleviate this shortcoming I have recently developed a phylogenetic rate parameter for shape data (Adams, 2014). Other current theoretical projects include: mathematically generalizing methods for measuring phylogenetic signal for multi-dimensional traits, developing distance-based PGLS approaches, and evaluating morphological integration in a phylogenetic context.
MacroevolutionAry patterns and processes
At the macroevolutionary level, we utilize a comparative phylogenetic approach to understand phenotypic diversification, and the tempo and mode of evolutionary change. Some of our recent work examined the evolution of body size clines in amphibians, where we showed no body size clines are apparent in Plethodon, and by summarizing all available data, we found no evidence of body size clines in salamanders or even amphibians (Adams and Church 2008; 2011). Other recent work examined the interplay between selection and development in driving morphological evolution in Italian Hydromantes salamanders (Adams and Nistri, 2010). This work discovered multiple evolutionary shifts in ontogenetic trajectories of morphological traits, which resulted in ontogenetic convergence across species of Italian Hydromantes salamanders, due in part to strong selection on adult foot morphology, which appears adapted for the unique climbing behavior exhibited in these cave-dwelling animals (Adams and Nistri, 2010).
Other macroevolutionary work examines phylogenetic patterns and processes across the family plethodontidae. We observed that rates of species diversification and rates of morphological evolution are uncorrelated (Adams et al. 2009), though species richness does correlate with rates of phenotypic diversification (Rabosky and Adams 2012); a pattern consistent with diversity-dependence. We also found no phylogenetic association between morphological variation and microhabitat use across the family (Blankers et al. 2012). This work has important theoretical implications, as it demonstrates a complex relationship between the tempo of morphological change, rates of species diversification, and habitat use across lineages.
Our current macroevolutionary work examines the tempo and mode of morphological evolution in two different taxonomic systems: salamanders and scallops. In salamanders, we are using a phylogenetic comparative approach to examine the tempo and mode of morphological evolution for several ecologically-relevant morphological traits, across two distinct salamander lineages experiencing distinct selective regimes (eastern North American Plethodon and Italian Hydromantes). This work allows us to utilize a naturally occuring system to test a priori predictions about the evolutionary process and the extent of morhpolgoical change within and between lineages. Our phylogenetic comparative work on scallops (in collaboration with Dr. J. Serb) examines the tempo of evolution of shell shape in relation to several functional and behavioral groups that exist across this lineage (e.g., Serb et al. 2011).
A major goal in evolutionary biology is to determine how proximate selective forces drive patterns of phenotypic evolution. Our work has yielded valuable insights into how species interactions and adaptation to local environment both impact the evolution of phenotypic adaptations, and influence broad-scale community dynamics and subsequent phenotypic diversification. For example, work on plethodontid salamanders has discovered that biotic interactions, notably competition, are a major selective force driving morphological change in Plethodon (particularly head shape: e.g., Adams and Rohlf 2000; Adams 2004; Maerz et al. 2006; Adams et al. 2007; Adams 2010). Historical factors such as local adaptation and contingency also play a strong role in this group, with the interplay between the two generating complex phenotypic responses (e.g., Arif et al. 2007; Myers and Adams 2008; Deitloff et al. 2013).
Our current work in this area examines microevolutionary patterns of morphological change across geographic space and across multiple species to determine the relative importance of biotic interactions and environmental influences on phenotypic diversity. Second, we are synthesizing patterns across ecological systems to discover what processes generate repeated patterns of evolutionary change in plethodontid salamanders, and under what circumstances. Initial work in this area shows that in some instances, repeatable patterns of phenotypic change can occur in Plethodon (e.g., Adams 2010), providing a link between microevolutionary change and macroevolutionary diversification in this group.