Biological Species Concept

Ernst Mayr


The Biological Species Concept can be stated as: biological species are groups of interbreeding natural populations that are reproductively isolated from other such groups.

The Biological Species Concept answers the Darwinian Why question, Why are there species? Why do we not find in nature simply an unbroken continuum of similar or more widely diverging organism? An organization of the diversity of life into species permits the protection of harmonious, well balanced, well adapted gene pools, an indiscriminate interbreeding of individuals, would lead to an immediate breakdown of these harmonious genotypes, for this reason the concept is not applicable to organisms that don't form sexual populations, their genotype does not require any protection because it is not threatened by destruction through out crossing.

By what devices is the integrity of a species being maintained? Dobzhansky introduced the term isolating mechanisms for these devices defining them as agents that hinder the interbreeding of groups of individuals or reduces to zero the frequency of exchange of genes between the groups. There is a enormous diversity of such devices : sterility genes, chromosomal incompatibilities, ecological exclusion, behavioral properties that facilitate the recognition of conspecifics.

What a biologist encounters in nature are populations of organism, the task is to assign these populations to species. This requires two operations a.) to develop a concept of what a species is, resulting in the definition of the species category in the Linnean hierarchy an b.) to apply this concept when combining populations into species taxa.

The species taxon refers to a concrete zoological or botanical object consisting of a classifiable population of organism. They are particulars, individuals or biopopulations they can be described and delimited against other species taxa.

The species category indicates a rank in the Linnaean hierarchy, it articulates the concept of the biological species and is defined by the species definition.

Is the species a class or an individual? "The species taxon -any species taxon- is an individual with any member of that particular species being a part of the species. The species category however is a class. It is the class, the species of wich are the species taxa. "(Mayr, 1976)

The criticisms of the Biological Species Concept are directed against the decisitions made in the delimitation of species taxa, in contiguous interbreeding populations it causes no difficulties. However, the criterion of interbreeding would seem to be inapplicable in the delimitation of species wherever isolated populations are involved, populations isolated either in time or space. The basic difficult is that populations isolated evolve as independent gene pools and some of them are on their way to becoming a new species or are actually have passed this threshold. But we must take an inference on the basis of all available data and criteria( analysis of their genetics, molecular biology tools, nature of their isolation mechanism ) as to how far along they have proceed on the way to becoming a separate species. However in most ambiguous situations it is useful to treat allopatric populations of doubtful rank as subspecies , trinomials provide information of closest relationship and allopatry, In ornithology the convention has developed to call strongly differentiated allopatric populations, allospecies.

The biological species concept deal with the definition of the species category , and only has meaning where a gene pool comes into contact with gene pools of other species that is at a given locality and at a given time (the nondimensional situation), However, because species taxa have an extension in space, the species status of noncontiguous populations must be determined by inference. Also species taxa have an extension in time, thus evolving is not such a species criterion, species do not differ in this respect from other living entities.