Evolution in one of the world’s most abundant organisms, Prochlorococcus

Prochlorococcus is one of the most abundant organisms on the planet, and this abundance has the potential to give it unique evolutionary properties. Very large idealized populations experience very little drift. This relationship has caused many authors to assume that Prochlorococcus genomes have little to no drift, but care needs to be taken to understand how real populations of Prochlorococcus differ from idealized populations and how this affects their evolution. Part of this thesis is devoted to understanding a parameter called the effective population size, which is meant illustrate the amount of drift in real, non-idealized populations. Using our knowledge of effective population size we can then investigate the relative affects of selection and drift throughout the genome. We can use knowledge about which sites evolve primarily by selection to better understand interpretations of common genome statistics. Finally, I apply my findings about interpretations of genome statistics to literature on the evolution of Prochlorococcus genome size.