Conservation strategies are often directed to individual species or for habitats identified as species-rich and therefore tend to assume that most individuals of a particular species have been assigned correctly. Many biologists argue that identifying species is far from straightforward.
The biological species concept (BSC), defines species as “…groups of actual or potential interbreeding natural populations that are reproductively isolated from other similar groups.
Although conceptually simple, the BSC has several defects, for example, a literal interpretation does not allow hybridization. In addition, BSC cannot categorize species that have asexual reproduction or by self-fertilization.
Over 20 different species, concepts can be found in the literature. One alternative to the BSC that has been gaining support in recent years is the phylogenetic species concept (PSC). This defines species as groups of individuals that share at least one unique derived trait and is often interpreted as a species being the smallest identifiable monophyletic group of organisms within which there is a parental pattern of ancestry.
PSC avoids the problem of asexual reproduction to some extent, but has been criticized for dividing organisms based on traits that may be of little biological relevance, and also for creating an overwhelmingly large number of species. In addition, two groups identified as separate species under the PSC may retain the potential to reproduce with each other. Then they would no longer be monophyletic and would therefore need to be reclassified as a single species.
The PSC tends to identify more species than the BSC. A review of 89 studies concluded that the PSC identified 48.7 percent more species than the BSC.
As the increasingly popular PSC replaces BSC as the most common species concept, the number of endangered species will increase and the geographical distribution of many will decrease. This in turn would lead to a large-scale reassessment of numerous conservation programs, for example, the location of high-profile biological hotspots that host large numbers of endemic species may change depending on the number of species in a given region.
The Biological Species Concept
The biological concept of species, generally accepted, though sometimes reluctantly, by evolutionary biologists, treats species as reproductively isolated populations. Such populations can consist of smaller groups that are also populations, including local ones. One species is reproductively isolated from all other species, and therefore cannot intersect with them.
However, the subpopulations of a species have the ability to interbreed with each other either directly or indirectly. Such mixing is not a property of individual organisms. It is the exchange of genetic material or gene flow across the population.
The following formal definition of species category is from Mayr: Groups of interbreeding natural populations that are reproductively isolated from other similar groups. Such formal definitions tend to be misleading, especially since their meaning depends on the definition of other terms and the theoretical context in which they are used.
The biological concept of species is linked to the theory of speciation. It is generally accepted that, under normal circumstances, populations must be geographically isolated (allopatric) before such divergence can occur to the extent that they remain distinct even in the same location (sympatric).
The evolution of such intrinsic isolation mechanisms is equated with speciation and serves as a basis for determining species status in terms of reproductive isolation. However, the biological species concept does not rule out the possibility of sympatric speciation, i.e. without geographic isolation.
Reproductive isolation between whole species is often confused with the inability of two organisms within a species to produce offspring. The definition includes populations under natural conditions: artificial crosses produced in the laboratory are not important.
Likewise, some degree of hybridization does not contradict the definition as long as the species remain distinct and are allowed to evolve separately. A problem that is not so easy to rule out arises from the fact that allopatric populations are regarded as parts of the same species as long as they are potential, not only actually, capable of interbreeding.
The concept of recognition emphasizes what holds species together rather than what separates them. Organisms are said to share a “Specific Mate Recognition System” (SMRS) that allows them to recognize conspecifics of the appropriate sex. According to this concept, species are equivalent to reproductive populations and collectively to biological species.
However, there are two important differences. First, a population can, at least in principle, vary geographically in the way mates are recognized. Second, a single lineage can develop a different SMRS.
The concept of cohesion emphasizes the point that something actually holds species together. This determination of cohesion is precisely the role of sex in the biological concept of species. It has been suggested that something other than sex could play this role. That such a thing actually exists has yet to be proven.
The concept of evolutionary species emphasizes the point that species are not only population groups, as they exist at any time: they are also lineages, and the fact that they develop is very important.
Cladistic species concept
Cladistic species concepts have been advocated by those who reject biological species as an appropriate entity for studying relationships between lineages. There is no agreement on what this unit should be, but it is generally part of a biological species.
With this concept, they argue that what others refer to as subspecies or local populations should elevate their taxonomic rank to that of a full species. This move would have many serious implications. The number of species names would increase enormously.
The similarity may be morphological, physiological, genetic or otherwise.
The morphological concept of species treats species as classes of organisms that are similar in their anatomy and other morphological characteristics. difficult to use.
The genetic concept of species treats species as classes of organisms with similar genes. The physiological concept of species treats species as types of organisms that share the physiological ability to mate and produce fertile offspring. It is important not to confuse this concept of physiological species with the concept of biological species.
The concept of phenetic species is based on the idea of general similarity espoused by the school of numerical phenetic taxonomy that flourished in the 1960s. The idea was that a large sample of attributes, carefully assembled and controlled by the techniques processed would yield groups that could be treated as taxa. However, to arrive at a definition of a species, one would have to make an arbitrary and subjective decision about how much the organisms would differ in order to call them species.
Many biologists, therefore, advocate a less dramatic approach in which multispecies concepts are retained as long as it is clear which concept is being used; some situations lend themselves to BSC, others to PSC, while others (e.g., those involving many unicellular taxa or parasites) lend themselves to a very different approach.
This tactic has the advantage of being well balanced but suffers from the uncertainties related to variable taxonomic criteria.