Species Cohesion in the Age of Discordance

Since the Modern Synthesis, it is common to claim that species are cohesive entities (Mayr 1973, Templeton 1989, Barker 2007). But what does this mean? Broadly speaking, cohesion is a sort of uniformity among organisms of a species (Ereshefsky 2001). This uniformity manifests itself at a time, as organisms are genotypically and phenotypically similar to each other, but also across time, as they share an evolutionary fate (Wiley 1980, Barker & Wilson 2010). In this paper, I discuss species cohesion in the light of phylogenetic discordance (Doolittle & Bapteste 2007). Discordance is prevalent in evolution and refers to the fact that most species histories do not map on to the histories of the genes they contain (Mallett et al. 2015). Moreover, phylogenetic discordance promotes genetic polymorphism within species and, relatedly, differences in the evolutionary fate of conspecific organisms. Hence, discordance raises the following issue concerning species cohesion: why are species cohesive entities despite their having high rates of discordance? In other terms, why doesn’t discordance prevent cohesion? In this paper, I provide a theoretical framework for analyzing the relation between cohesion and phylogenetic discordance. This framework allows biologists and philosophers to tackle the above questions and to make sense of species cohesion in the face of discordance.