phylogenetic grading

Phylogenetic grading is an approach used to infer the evolutionary history of behaviors by comparing extant species and arranging their behaviors from least to most complex. This method assumes that simpler, or more "primitive," forms of a behavior are ancestral. However, this assumption is problematic because complexity does not always increase over evolutionary time, and species with simpler behaviors are not necessarily less adapted or further from ancestral forms. In reality, complexity is driven by survival and reproductive success, and some species with simpler behaviors today may have descended from ancestors with more complex behaviors. Therefore, viewing living species' behaviors as rungs on a ladder of increasing complexity is an inappropriate oversimplification.1