Our research uses integrative studies of adaptive radiation to address two longstanding questions about biological diversification: (1) What factors underlie major biodiversity patterns?  and (2) What processes contribute to the formation of new species?  Most of our work is focused on Anolis lizards (anoles), a clade of nearly 400 ecologically diverse species that has undergone repeated adaptive radiations across the neotropics.  Building on more than three decades of intense anole research, our work provides new insights by combining molecular phylogenetic and population genetic analyses, large ecological and phenotypic datasets, and, most recently, experimental genetic studies.  Our research can be divided into two, inter-related topics: (1) use of phylogenetic comparative methods to investigate major patterns of species diversity and (2) integrative studies that study the process of species formation within and among closely related taxa.

Explaining Patterns of Species Diversity

As theories of adaptive radiation emerge, a major focus of ongoing work is the identification of general patterns of diversity and their underlying explanations (reviewed in Glor 2010).  Such patterns include the evolutionary species-area relationship and the tendency for clades to experience a period of rapid diversification before declining, presumably as ecological opportunities are exhausted.  Our work addresses these and other patterns using cutting-edge phylogenetic analyses.  One component of this work involves reconstruction of new phylogenetic trees from phylogenomic datasets and a second uses these trees to test hypothesized patterns of diversity.

Phylogeny of Anolis: In collaboration with the anole genome sequencing project, we have generated the first phylogenetic trees for Anolis from a multi-locus nuclear DNA sequence dataset (Alföldi et al., 2011).  The new trees generated by this work provide extraordinary new insights on anole diversification and a strong foundation for our future work.  In addition to conducting more sophisticated analyses of this dataset, we are currently expanding upon sampling that already includes 94 species and ~20,000 bp of DNA sequence data from more than 40 loci.

Testing Patterns of Biological Diversity: In collaboration with Dan Rabosky, we have recently used a suite of new maximum likelihood based phylogenetic methods to show that the evolutionary species-area relationship seen among anoles on large Greater Antillean islands is driven by island-size specific carrying capacities rather than differences in diversification rate (Rabosky and Glor 2010).  A companion project recovered corresponding diversity-dependent declines in divergence of adaptively significant morphological trait (Mahler et al. 2010).  Together, these studies provide some of the strongest available support for the prediction that ecological opportunity ultimately limits diversification during adaptive radiation.  We are following up this work with more expansive comparative analyses of diversification across taxa from the West Indies and other islands.

Species and Speciation

A new wave of research is now focused on disentangling the relative contributions of geographic isolation, natural selection, sexual selection and other processes to speciation during adaptive radiation.  Two previous discoveries motivate our ongoing work in this area.  First, surveys of mtDNA variation showed that many widespread anole species are characterized by extensive geographic genetic differentiation, consistent with the presence of cryptic species or incipient speciation (Glor et al. 2003, 2004, 2005, Kolbe et al. 2004, 2007).  Second, studies of morphometric and ecological variation revealed that geographically distinct populations within islands may be adapted to local environmental conditions (Glor et al. 2003, Knouft et al. 2009, Losos et al. 2009).  Together, these results set the stage for ongoing research that uses GIS-based methods to identify the ecological basis for species boundaries and integrates geographic, phenotypic, adaptive, reproductive and genomic data to investigate the process of species formation.

The Ecological Basis for Species Boundaries: The recent availability of georeferenced locality data and GIS data layers characterizing climatic and environmental variation permits widespread investigation of the contribution of ecological processes to the origin and maintenance of species boundaries.  Using new metrics to quantify and compare this data and resulting environmental niche models, we have diagnosed significant environmental divergence between geographically distinct populations (Warren, Glor, and Turelli 2008, 2010) and tested whether specific types of environmental barriers are contributing to the evolution of species boundaries (Glor and Warren 2011).  As part of several new collaborative efforts, we are expanding this work to include comparative studies of species distributed across the West Indies.

Integrative Studies of Speciation: Our most ambitious project involves developing one group of anoles – the distichus group of trunk anoles – as a model system for integrative studies of speciation during adaptive radiation.  This fascinating group is distributed across Hispaniola and the Bahamas and includes six species and nearly twenty subspecies, the latter diagnosed primarily differences in dewlap color and pattern.  By exploiting the fact that this group appears to include populations at varying stages of the speciation process, we are able to carefully test a range of possible diversity-generating mechanisms, including geographic isolation, local adaptation, character displacement, reinforcement, and sensory drive.

Up to this point, our work on the distichus group has produced three major discoveries: (1) phenotypically distinct subspecies found in the Dominican Republic are associated with deeply divergent mtDNA haplotype clades, consistent with incipient or cryptic speciation (Glor and Laport 2011); (2) nuclear gene flow may be sharply reduced where mitochondrially, phenotypically, and ecologically distinct populations come into contact, suggesting ongoing speciation across an ecological gradient (Ng et al. 2009, Ng and Glor, In press); (2) dewlap color is both heritable and adaptive to local signaling conditions, consistent with the core predictions of sensory drive models of speciation.  We are now in the early stages of expanding this work to include more comprehensive sampling of the nuclear genome and detailed experimental studies of the evolution of reproductive isolation.

Population Genomics: Through genomic scans of hundreds of AFLP loci sampled across hundreds of individuals, we have already recovered preliminary evidence for the type of genomically complicated speciation observed among other recently diverged taxa, where species differences result from differentiation of a relatively limited fraction of the genome while other portions of the genome move relatively freely between diverging populations.  Over the coming months, these AFLP loci will be sequenced using next generation sequencing, to provide further insight on the genomics of speciation.  We are particularly interested in identifying which regions of the genome are involved species level differentiation, and whether these regions are influenced by natural selection.

Experimental Hybridization: Experimental work on reproductive isolation in model organisms has established a number of important patterns and mechanisms of speciation.  My lab is using a series of no-choice crosses to investigate the evolution of reproductive isolation among phenotypically and genetically distinct populations in the distichus group.  This experiment began last winter with 240 animals from two subspecies evenly divided among four cross types (two within subspecies crosses and the two reciprocal subspecific crosses).  These breeding experiments have yielded more than 1,400 eggs, and we are currently rearing hundreds of the resulting offspring.  It is already clear that, while hybrid crosses produce large numbers of viable eggs and offspring, they also generate significantly more infertile eggs than pure crosses.  After monitoring offspring from this F1 cross through adulthood, we will use these individuals to test general patterns of sterility and inviability through backcrossing experiments.  These studies will represent the most comprehensive work of its kind conducted with squamates, and will fill an important gap in knowledge acquired from studies of other taxa.

Leave a Reply