Friday, August 8, 2008
Exhibit Hall CD, Midwest Airlines Center
Dena L. Grossenbacher, Graduate Group in Population Biology, University of Claifornia, Davis, Davis, CA, Samuel D. Veloz, Climate Change & Quantitative Ecology, Point Blue Conservation Science, Petaluma, CA, Jason P. Sexton, School of Natural Sciences, University of California, Merced, Merced, CA and Justen B. Whittall, Biology, Santa Clara University, Santa Clara, CA
Background/Question/Methods The relationship between niche evolution and speciation has been intensively studied in recent years to reveal how ecological factors may influence the diversification of taxa. In stark contrast to historical perspectives, which focused primarily on the importance of geographic barriers for promoting speciation, it has been suggested that ecological interactions manifested through niche evolution may play an important role in speciation events. To investigate the importance of niche evolution in speciation we studied the plant genus Mimulus which contains approximately 150 species with a center of diversity in California. Using recent advances in the accuracy of niche modeling while accounting for phylogenetic history in Mimulus, we asked 1) what is the rate and direction of niche evolution over time and 2) is niche evolution associated with speciation? We hypothesize that if niche evolution is correlated with speciation, we should see increased rates of niche divergence associated with recently diverged taxa. If, on the other hand, niche evolution is not involved in speciation, we would expect niches to evolve randomly over time and at a roughly constant rate according to the principles of Brownian motion. Evolutionary relationships in Mimulus were determined using a Bayesian phylogenetic analysis of a four gene dataset including both nuclear and chloroplast loci. To produce a phylogeny with branchlengths proportional to time we used penalized likelihood in r8s. The niche of each species was predicted using Maxent, an environmental niche modeling algorithm, by combining point occurrence locations (from online herbarium records) with environmental layers derived from geographic information systems (GIS). We estimated the degree of niche overlap among all species and compared the results to the relative age of divergence while accounting for shared ancestry.
Results/Conclusions
We found that niche overlap between species decreases sharply with increasing branch length for recent speciation events, suggesting that niche divergence and speciation are initially correlated. The initial correlation may also reflect an allopatric process of speciation due to historical autocorrelations between space and environments. If, however, the pattern is simply due to allopatric speciation, the pattern detected here conflicts with other studies which have shown niche conservatisim among taxa that have undergone allopatric speciation. Our study provides evidence for the importance of niche evolution in relation to speciation events and demonstrates innovative techniques for integrating phylogenetics and ecological niche modeling for studying ecological and evolutionary questions.
