1Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, 2Department of Anthropology, The Ohio State University
April 17, 2020 , Platinum Ballroom
The plesiadapiform primate, Phenacolemur fortior, is hypothesized to have evolved from Phenacolemur praecox in the Bighorn Basin, Wyoming, through rapid sympatric speciation, with intermediate stages known stratigraphically between the two species. This evolutionary transition occurred during a faunal turnover event (Biohorizon A) in the Bighorn Basin. P. fortior went extinct and was replaced by P. citatus before a second faunal turnover event (Biohorizon B). An ancestor-descendant relationship between P. praecox and P. citatus has been proposed based on similar morphology and size, suggesting that P. citatus arose by allopatric speciation. This study tests whether neutral evolution can explain morphological evolution observed during and between each speciation event. We calculate a rate of morphological evolution, based on the ratio between inter- and intra-species variation in height and protoconid height of p4 and length and width of p4, m1, m2, and m3, between early and late individuals of P. praecox, P. fortior, and P. citatus and over both lineages. Results show that changes in height and protoconid height of p4 and lengths and widths of all cheek teeth within and between species are consistent with strong stabilizing selection (rate <0.000001). Moreover, there is evidence of a release of the selective pressure (rate >0.000001) in both lineages associated with the biohorizons, which resulted in increased rates of morphological change for these tooth dimensions. This suggests that changes resulting in species-level divergence can occur under a process of relaxed stabilizing selection and highlights the importance of the biohorizons for their disruptive effects.