How to Define a Species ?
Origin of new Species - Hybridization - Reproductive Isolation - Adaptive Radiation

When discussing the origin of new species, some thoughts should be devoted to the definition of a species, because not everybody uses the same. In taxonomy, the phenic definition of a species is used, i.e. individuals identical in most of their important features, occupying a uniform area, and distinguishable from related groups by clear discontinuities are grouped. Despite the fact, that this is no foolproof approach, it has its merits and has proven to be practical in classifying plants. It is still the basis of plant and other organisms’ systematics.

Under the influence of genetics, a species was defined as a reproductive unit, that covers individuals actually or potentially capable of reproducing with each other. In other words, a species is the sum of interbreeding individuals, and thus the most inclusive unit of normal biparental reproduction. A species consists often of geographically separated populations. A population of individuals unrestrictedly capable of intermating is called a panmictic population. The sum of all genes, or, more precisely, all alleles, that can be combined by mating is called a gene pool.

Species are usually separated by discontinuities. They are subject to reproductive isolation. Discontinuities constitute barriers, that prevent, or at least inhibit, an interspecific gene flow and gene exchange. This is advantageous, because such genomes do usually not co-operate harmoniously, and reproductive success would be drastically reduced. The isolation mechanisms are therefore essential for stabilizing a species’ gene pool and the improvement of its fitness.

The approach population genetics chooses to define a species is appropriate for use in evolutionary biology, although it disregards a number of aspects:

1. Many unicellular and a few multicellular organisms reproduce mainly asexually by dividing. Sexual reproduction is rare, or has in the case of quite a number of species not yet been recorded. Despite the absence of interbreeding, these organisms are grouped into species, too. Many higher and lower plants are capable of vegetative reproduction. Progeny derived by vegetative reproduction forms a clone of genetically identical individuals (apart from newly emerged mutations).

2. In the case of higher plants, a number of intraspecific reproductive barriers are particularly important. They include autogamy, sterility factors, different degrees of ploidy or mechanical barriers preventing pollination (like heterostyly). See the following chapter for further examples.

3. As allopolyploidy shows, related species do not necessarily form (fertile) hybrids. Such cases are not frequent, but they occur and play, especially in angiosperms, an important role in the development of new species able to open up new habitats (adaptive radiation).

4. Reproductive barriers between species function often only under certain conditions, but may collapse, if the conditions are altered. As a consequence, new combinations of genes may result, that may be better adapted to the new environment.

These arguments provide good evidence for the species being a dynamic unit. Reproductive barriers, although efficient, are not absolutely inflexible.