Determining the processes that drive speciation is a central question for the study of evolutionary biology, and research has generally focussed on adaptive speciation and speciation driven by sexual selection as independent processes. Adaptive evolution occurs when ecological selection shapes the evolution of traits for optimal fitness or performance in specific ecological contexts. Alternatively, sexual selection results from divergent selection of the traits that distinguish the best mates. Our new paper shows that in the Australian sand dragon complex (Ctenophorus maculatus and allied taxa) – both work together with geography to drive speciation repeatedly, and we suggest in a specific sequence.
(Thanks to Stewart Ford for the image above)
Figure 1: Distribution of lineages within the Ctenophorus maculatus species complex, including drawings of the dorsal surface (showing body proportions and dorsal markings) and visual social signals (i.e., ventral throat and chest pattern). Dorsal drawings are scaled to mean male snout-vent length (see C. m. dualis for scale bar). Extent of the arid zone is also shown (dotted line). Illustrations by Corrine Edwards.
In our new paper out today in The American Naturalist we studied how the evolution of adaptive and social signaling traits relates to geography in closely related lizards from the Australian arid zone (see our Publications page). We found that lineages diverged as heterogeneous arid habitats developed and expanded in Australia. As species invaded distinct ecological niches repeatedly and independently within arid regions, both adaptive and social signaling traits convergently evolved in response to ecological selection in these new habitats.
Figure 2: Time-calibrated species tree for the lineages within the Ctenophorus maculatus species complex. Posterior probabilities are displayed (above/below nodes), as are median clade divergence times (95% highest posterior density in brackets and bars). Scale bar is in millions of years before present. Boxes indicate significant niche divergence (above branches) and niche conservatism (below branches) for different niche axes. Also shown are the scaled dorsal views and visual social signals. Illustrations by Corrine Edwards.
But striking variation in visual social signaling traits not only resulted from adaptive evolution. Rather, geographic overlap between lineages explains greater variation in visual signals than ecological context. These results suggest that divergent visual signals reinforce the species boundaries initiated by adaptive evolution, driving dramatic variation in these traits. Such divergent signals allow avoidance of maladaptive hybridization between ecologically distinct species. Thus, we found that interactions between adaptive and social signal evolution during speciation depended on geographic context.
Figure 3 (part of 4 in paper): Proportions of variance independently explained by the environment (white), spatial overlap (dark gray), and the phylogenetic relationships among lineages (light gray) for visual (B) signaling traits. Circle overlap shows the proportion of variance explained by combinations of these factors, with unexplained variance listed outside circles.
This project represented years of work, and for me at least was challenging and exciting for a number of reasons. It changed the way I do science, the way I think about science and it pushed me to learn complex new analytical methods that I had never before used – as well as develop some novel tools. One of the exciting things about this paper was a novel way of graphing phylogenetic uncertainty onto disparity-through-time (DTT) plots (Harmon et al. 2003) that I developed. It allowed me to look at where phylogenetic uncertainty was concentrated in time and how that might affect the interpretation of trait DTT. The R code implementing this can be found here.
Figure 4 (part 3 in paper): Disparity-through-time (DTT) plots for visual signaling traits in the Ctenophorus maculatus species complex. The median observed relative subclade trait disparity is shown (black line; fig. 2), in addition to phylogenetic uncertainty from 1,000 posterior trees (red lines; aqua dotted line indicates median). The median (yellow dotted line) and 95% confidence interval from 1,000 simulations of the null Brownian motion model from each of 1,000 posterior trees (gray shading) is also shown. Time is in millions of years before present.
It also changed the way I viewed the review process for scientific publication. For me, the review process felt more like a collaboration between myself, my co-authors, the reviewers (MRE Symonds and one anonymous reviewer), the associate editor (SJ Steppen), and the editor-in-chief (JL Bronstein). All with the common goal of getting the best out of the research in the paper. It was the MOST constructive and enjoyable review-revise experience that I have ever had at a journal. Thank you The American Naturalist!
Another reason this paper was exciting was that I got to work with my sister, Corrine Edwards. She is an amazingly talented artist whose drawings (yes they are drawings, not photos!) appear throughout the paper.