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The species is the basic unit of . It represents a population (or group of populations) of organisms that is evolutionarily distinct from all others, and which is which has embarked upon its own evolutionary path. Essentially, the idea of a species is connected to the idea of - interchange of genetic material occurs between members of one species (referred to as conspecific), but not between members of different species. As such, the species is distinct from all other kinds of in that it represents something that has, to some extent, an actual objective "existence", rather than just being an abstract and arbitrary construct.
In practice, of course, the species is not the simple, straightforward unit that biologists seem to pretend that it is. There are certain issues with the application of the concept of species, and a whole host of different definitions have been suggested for what a species actually is. None of these is universally applicable, and different species concepts may actually be more effective for different taxa.
For instance, the concept of species is not really applicable to asexually reproducing organisms. Obviously, gene flow does not normally occur between such organisms, which would arguably make every individual its own species. Because such a situation would obviously not be conducive to practical investigation, researchers on asexual organisms recognise clusters of similar individuals as "species". However, these "species" concepts (such as the bacterial species concept, which regards individuals with less than 5% genetic difference from each other as conspecific) are arbitrary constructs, unlike species in the proper sense.
The most familiar species concept is undoubtedly the biological species concept, popularised by Ernst Mayr. This defines a species as a cluster of interbreeding or potentially interbreeding organisms reproductively isolated from all other such clusters. While this is the species definition that you will see in most textbooks, and possibly the one that the largest number of researchers will support, there are significant practical issues with this concept. For a start, the ability to interbreed has not been tested for representatives of most species, and simply could not be. Not only is it not feasible or possible to maintain and breed many organisms in the laboratory, the sheer number of experiments that would be required would be prohibitive. For any given number (n) of populations, the bare minimum of required trials would be 2n (one male and one female of each population mated with the appropriate sex of every other population). Considering that there are estimated to be up to 30 million species living today, the issues are obvious. Also, there is the question of how to define "interbreeding", as this is a factor of time. Make the time frame recognised as the present too small, and species collapse (because not all individuals are breeding all the time, much as they might like to be). Make it too large, and all organisms on the planet would be connected by interbreeding (I admit that these are completely facetious examples, but I think my point still stands). Finally, reproductive isolation of populations is not a yes/no question. Many organisms belonging to different species can potentially interbreed and produce fertile offspring, but are not going to under normal circumstances due to differences in breeding behaviour and other factors. Under the biological species concept, what are we going to make of the western () and Clark's () grebes, two closely related and still interfertile species whose members won't breed with each other at the beginning of the breeding season, but become more willing to accept mates of the wrong species as the season progresses and the window for any sort of breeding becomes smaller (Nuechterlein & Buitron, 1998)?
A concept that has arisen as a strong contender to the biological species concept in recent years is the phylogenetic or diagnostic species concept, which defines a species as the smallest diagnosable population of organisms sharing a distinct pattern of descent. This concept removes most of the practical issues mentioned above, and is arguably what the majority of taxonomists have actually been doing over the years, even when they claimed to be using the biological species concept. However, the diagnostic species concept is not without its issues. For instance, it has led to a great deal of , as most taxa previously recognised as under the biological concept become full species under the diagnostic concept (after all, subspecies are defined by being distinguishable from each other - it is most likely that the concept of "subspecies" is inapplicable under the diagnostic species concept). Also, there is no lower limit on how fine the characters used to distinguish species are. What have previously been regarded as the North Atlantic, North Pacific and Southern Hemisphere populations of the right whale () are all but indistinguishable morphologically, but have been demonstrated to possess relatively minute genetic differences. Some authors have therefore suggested recognising the North Pacific and Southern Hemisphere populations as separate species. Finally, it is debatable whether the diagnostic species concept would help with the grebe example above, as the two species do not have fully "distinct patterns of descent".
While the last example is commonly called the "phylogenetic species concept", I prefer to call it the "diagnostic species concept" because there is another species concept that has been suggested under the name of "phylogenetic species concept", and I prefer to reserve that name for this one. This latter concept basically states that a species is the smallest possible monophyletic taxon. This concept has not been widely accepted, because most authors would not agree that a species is necessarily monophyletic. Imagine a widespread species found throughout a continent (say, North America). A small group of individuals from that species becomes isolated on an island (say, Hawaii), and their descendants, in adapting to the new habitat, rapidly become radically different from their ancestors and become a new "species", while the population on the mainland continues as before. Under the phylogenetic species concept, this would require that the ancestral species no longer be recognised as it was, because it would no longer be monophyletic. This is despite the fact that it has not changed in any recognisable feature, and its size means that there is no guarantee that gene flow would have homogenised the population since the island group became isolated. This last point also reflects the fact that it is quite likely that the concept of "monophyly" is inapplicable at the species level, because monophyly requires descent from a single ancestor, and the species, almost by definition, represents the level at which reticulate descent within a population becomes less important than vertical descent between populations.
Finally, the recognition species concept would define a species as a cluster of organisms whose members recognise other members of the species as potential mates. This concept is only really applicable to species that have reasonably complex features connected with breeding, such as behaviour or primary or secondary sexual characteristics (and the less charitably minded might suggest that it implies that ugly individuals aren't conspecific with anybody). However, it is potentially useful in taxa that do have such features, especially taxa such as were species divergence may happen too quickly for other species concepts to be practical or applicable. This would, for instance, probably be the best concept for dealing with the grebe problem mentioned earlier.
Nuechterlein, G. L., & D. Buitron. 1998. Interspecific mate choice by late-courting male western grebes. Behavioral Ecology 9: 313-321.
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