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Some such groups argue passionately about the superiority of their preferred concept over the alternatives. However, other groups argue just as passionately in favor of different species concepts. In addition, the species problem seems to be getting worse rather than better, which is to say the number of alternative species concepts has been growing rather than diminishing.

Moreover, judging by the increasing numbers of critiques and proposed alternatives, Mayr's species definition, although still perhaps the most widely adopted, seems to be less popular now than 20-30 years ago. The existence of diverse species concepts is not altogether unexpected, because different concepts are based on properties that are of greatest interest to different subgroups of biologists (18).

For example, biologists who study hybrid zones tend to emphasize reproductive barriers, whereas systematists tend to emphasize diagnosability and monophyly, and ecologists tend to emphasize niche differences. Paleontologists and museum taxonomists tend to emphasize morphological differences, and population geneticists and molecular systematists tend to emphasize genetic ones.

Nevertheless, for those biologists who are able to set aside their own personal investments and research interests, all of the concepts seem to have some merit. It is certainly the case that all are based on important biological properties.

The reconciliation of alternative and incompatible species concepts derives from the recognition of a more general concept of species that is shared by all contemporary species concepts and definitions (18, 30, 31). All modern species concepts and definitions conform to this general species concept and can therefore be considered variants of it.

This general species concept, not Mayr's more restricted species definition, is the true biological species concept (see below). Several influential discussions of the general species concept that became established during the Modern Synthesis emphasized the correspondence of species with metapopulations or gene pools.

Species were equated with groups of interconnected populations that form an extended reproductive community and an unevenly distributed but unitary gene pool or field for gene recombination.

The equation of species with metapopulations or gene pools is evident in a number of species definitions from the period of the Modern Synthesis, including those proposed by several of the most influential contributors to that movement.

For cases involving purely asexual reproduction, there are two possibilities regarding species. One possibility is that purely asexual organisms do not form species (8, 38, 39).

The other possibility is there are processes other than the exchange of genetic material, such as natural selection, that determine the limits of species in purely asexual organisms (22, 40, 41). Both of these views are consistent with the equation of species with metapopulations. Either asexual organisms do not form metapopulations, and therefore they do not form species, or they do form metapopulations (as the result of some process or processes other than interbreeding), and therefore they also form species.

The general metapopulation concept of species is also evident in species definitions that describe species as lineages rather than as populations. On the one hand, populations can be considered to extend through time, in which case a population is equivalent to a lineage.

Alternatively, populations can be considered to exist at an instant in time, in which case a population is equivalent to an instantaneous cross section of a lineage, and a lineage corresponds to a continuous series of populations (the ancestral-descendant sequence of populations in Simpson's definition).

Populations as lineages (modified from ref. The three shaded distributions (a-c) represent cross sections of the time-extended population lineage at three different times.

The population itself can be interpreted either as the 3D (time-extended) object, in which case it is equivalent to the lineage, or as one of the 2D (time-limited) objects, in which case it is equivalent to a cross section of the lineage.

The important point is that virtually all contemporary definitions of the species category are based on a common general concept of species: the concept of species as (segments of) metapopulation lineages (18, 30, 31). Definitions that describe species as populations simply view species over some relatively short interval of time, usually the present.

In contrast, definitions that describe species as lineages tend to view species over longer time intervals. Virtually all contemporary species definitions (by which I mean those advocated by some contemporary group of biologists) conform to this general metapopulation lineage concept of species (18). They differ with regard to emphasizing (in addition to different temporal perspectives) the theoretical concept itself vs.

They also differ with regard to the properties of metapopulation lineages that are considered necessary for those lineages to be regarded as species. Although this corollary is now often taken for granted, it is important to recognize that it represents a significant departure from an older view of the species category.

Under the older view, the species category was simply a rank in the hierarchy of taxonomic categories. More specifically, the taxa at all levels in the hierarchy were viewed as being of the same basic kind, namely, groups of organisms that shared particular traits (3), but they were assigned to different ranks to indicate differences in relative inclusiveness.

Species were included within genera, genera were included within families, and so forth. In contrast, the view of species that emerged during the Modern Synthesis was that species are fundamentally different from the taxa above and below them in the taxonomic hierarchy. Species differ from genera (for example) not only in inclusiveness but also in kind. Species are metapopulation lineages, whereas genera are groups of species sharing a relatively recent common ancestry.