Science Without Species: Doing Science with Tree-Thinking

Abstract

The focus of this volume is speciesism. While the concepts of species and speciation remain the focus of a great deal of research, it is worth exploring how in recent decades evolutionary biology has, in several ways, moved away from species as the key unit of analysis of biological questions. I begin by outlining how phyloge­netic comparative methods have become essential methodological tools in statistical analyses of relationships between traits. Species are not statistically independent observations, because the reality is that they are related, genetically and statistically, on a phylogenetic tree. Phylogeny also plays a key role in modern analyses of spatial patterns in biodiversity, and in fact relying on phylogenetic biodiversity measures can avoid a number of problems created by attempting to impose a uniform species rank across different continents and clades. Similarly, a major challenge in modern studies of diversification and extinction concerns the units of analysis and how they are defined and recognized. Both “genus” and “species” are human-defined ranks imposed on the phylogenetic tree. The phylogenetic tree is the more fundamental reality that is produced by the macroevolutionary process, and it could include every level of gradation of genetic and morphological divergence. Once ranks are imposed upon it, a variety of methodological problems are created as scientists attempt to make these ranks standardized and comparable across different datasets and timescales. I outline how phylogenetic thinking might help provide a solution. I conclude with other examples where cutting-edge science is done with phylogenies without much need of the “species” rank—for example, in the battle against Covid-19.