Stephen is a PhD student in the Ecology & Evolutionary Biology department. Before coming to the Powell and Caccone Labs, he earned degrees from Columbia University and the University of St Andrews, and was a program manager at the American Museum of Natural History. His dissertation research focuses on the relationship between genome evolution and population size. Classical interpretations of the nearly neutral theory of molecular evolution predict that reduced efficiency of selection in small populations will lead to the accumulation of deleterious mutations within the genomes of these populations, leading to increased mutational load and declining fitness. Other evolutionary models, however, predict that stable populations of different sizes will not differ in amino acid substitution rates, and that increased rates of beneficial mutations will stop mutational meltdowns except in extremely small populations.
Stephen is testing these theoretical predictions by comparing whole genome sequences of phocid seals, a clade that ranges in population size from some of the rarest large mammals on the planet (the monk seals) to the most abundant (the Weddell and crabeater seals), and includes species that have undergone extreme bottleneck events (the elephant seals). This sequencing effort also includes producing a genome sequence for the recently extinct Caribbean monk seal, using specimens from natural history collections.