Table 1.

What Is a Species: A Biological (and Nomenclatural) Dilemma

“There is no consensus on how to define a species, and likely never will be.”19 Despite this discouraging preamble, we will try to present some basic information. The following definitions are among the most diffused in the species-definition debate over the last 50 years.

Biological species concept30

“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”

Diagnostic concept 31

A species can be defined as “the smallest aggregation of population (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals.”31 According to this definition, a species is limited by a definite set of characters, which, traditionally, are morphological.

Genealogical species concept32

A species is represented by populations that constitute a single group, without any exclusive subgroup. All the members of the group share a common ancestor (monophyly).

Ecological species concept33

“A species is a lineage (or a closely related set of lineages), which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range. A lineage is a clone or an ancestral-descendent sequence.”

There are unsolved difficulties in applying any of the definitions listed above, whatever the adopted: “every group differs in the biological criteria impacting species divergence, setting up a sliding scale from well defined to problematic species.”34