Does Everything Have to be a Species?

Evolution determines ancestry by reference to a phylogenetic tree. Phylogenetic trees are built based on homologous traits. Homology refers to similarity in the traits. Therefore, in the evolutionary view, gross overall similarity, in either genetics or morphology, is determining ancestry. This idea matters for our discussion because speciation, in an ultimate sense, is a question of ancestry. Many species concepts appeal in some sense or another to a most recent common ancestor.