18 Species Concepts and Speciation: Facts and Fantasies

Abstract

The idea that evolution takes place at varying rates has a long history, as does the argument that evolutionary change is concentrated at points of speciation. It was however not until 1972, with the publication of a seminal paper by Eldridge and Gould, that these two ideas were brought together and became widely discussed under the term punctuated equilibrium and viewed as an alternative to “phyletic gradualism.” If speciation is indeed the engine of evolution, it is necessary to ask what precisely species are. There is widespread acknowledgment that the Evolutionary Species Concept, first proposed by Simpson in 1961, reflects the reality of species: a species is a unitary evolving lineage. The debate about the definition of species is essentially concerned with how best to operationalize this concept. I show here that the closest operational definition is the so-called Phylogenetic Species Concept in which a species is the minimal cluster of individuals that is diagnosably distinct from other such clusters. There is, importantly, no such thing as an “amount of difference” necessary for species status. Speciation is usually assumed to be allopatric, but interesting cases have been made that other modes, such as sympatric, are not only feasible but also potentially important; speciation also occurs by hybridization. Nor can phyletic speciation be discounted. Speciation may occur very rapidly indeed. I finally discuss how different modes of speciation could be detected in the fossil record.