Ana Marcela Florez Rueda
This document attempts to describe the evolutionary species concept and to provide criteria to delimit evolutionary species. The evolutionary species concept was first stated by Simpson in 1961 and restated by Wiley in 1981 as “An evolutionary species is a single lineage of ancestor-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate”. Following Hening1 evolutionary species are “logical individuals bounded by speciation events with origins, existence and ends” 1 , they have an origin by cladogenesis, undergo evolution by anagenesis, and disappear by extinction. There has to be a “correlation with the species and the number of cladogenetic events that have occurred during the course of descent within a clade”1, this definition ties species to speciation and seeks for process, “divorcing species from cladogenesis destroys de distinction between tokogeny (nonhierarquical descent) and phylogeny ( hierarquical descent)”1 and this distinction is one of the premises of phylogenetic analysis.
The species problem involves different questions one is the species concept per se (an idea of what kind of entity species are) and the other is the operational question (a methodological approach to recognizing species in a particular case)2. Several authors1,2,3 have argued for the distinction of these two questions considering it the root of the “species problem”. Operational concepts (that try to answer both questions at once) are not universal because all operational species criteria will fail in some cases3 due to missing data, or simply inapplicability, consider the biological criteria of reproductive isolation, it is not applicable to asexual species, and sometimes it is impossible to determine. But the operational question is indeed the most important and accurate operational methods are a necessity. The ESC does not provide a method for the delimitation of species, but based on it one can choose the right method to delimit evolutionary species, there is plenty of empirical data and methods that can be brought to the question of whether a group of specimens is worthy of being hypothesized as parts of an evolutionary species, and when we consider this we can see the links between this concept and other species concepts.
The operational concepts must be reinterpreted as delimitation criteria. There are more than 20 species concepts which may have a delimitation criteria, each one of them should be evaluated to see if the species that delimits are evolutionary species, here I will only evaluate four species concepts/delimitation criteria. Biological delimitation agrees with the ESC because it proves lineage independence and identity, in the few cases when this kind of data is available it should be the primary criteria, but this is as previously stated a non universal delimitation criteria, and probably the less applicable, discerning potential reproductive barriers can be difficult, time consuming, expensive, and fraught with error. Homology and genealogical concordance are delimitation criteria that agree with ESC these are embodied in the PSC (sensu Mishler and Theriot), and GSC (sensu Baum and Shaw), they take into account the process of cladogenesis, and prove lineage independence, contrary to biological delimitation this methodology is widely applicable to all the taxa and there is a lot of data available. Delimitation criteria based on PSC (sensu Wheeler and Platnick) might result in every diagnosable subpopulation be called a species ignoring its genealogy, with terminals that may have tokogenetic relationships, making the delimited species unuseful for phylogenetic analysis, this approach is at odds whit the ESC, and thus prohibited.
The ESC provides an universal idea of what The Species is, but accurate operational methods are a necessity, hopefully biologist are now in a position to shift its attention away from the endless debate about the definition of The Species and “focus instead on estimating accurately the boundaries and numbers of species and studying the diverse processes involved in their origin and maintenance”. 2
1. Wheeler, Q. D. and Meier, R. (Editors). 2000. Species Concepts and Phylogenetic Theory: A Debate. New York: Columbia University Press.
2. de Queiroz, K. 2005. Different species problems and their resolution. Bioessays 27:1263-1269.
3. Wiens, J. J., and M. R. Servedio. 2000. Species delimitation in systematics: inferring diagnostic differences between species. Proceedings of the Royal Society of London, Series B 267:631–636.