The species is the basic taxonomic category used by biologists to classify living things. The other categories (genus, family, order, class and phylum) are considered artificial and arbitrary. On the other hand, we tend to regard the species as natural, obvious, similar to a concept when the represented objects are living beings. But we will not enter here into the famous problem of universals, nor wonder on whether concepts (and species) really exist or are mere constructs of the human mind.
The classic definition of a species is: a set of living beings that share common characteristics and can interbreed, giving rise to fertile offspring. Notice that the use of the word interbreed implies that the living things in question use sexual reproduction. This leads us to ask whether the concept of a species should be restricted to living beings with this type of reproduction, or it can be extended to those that reproduce otherwise, such as prokaryotes and some eukaryotes. This question can be answered in several ways:
- Species exist whenever there is life: all living beings belong to a species that differs clearly from other species
- Species exist only when they occupy different ecological niches: for example, different species of insects that feed on different plants or animals.
- Species exist only when there is sexual reproduction: the species definition given above makes no sense for living beings that reproduce asexually, therefore those beings have no species.
Biologists disagree on which of these alternatives is best, although some observations on asexual beings seem to favor the third.
- Prokaryotes: they reproduce by bipartition. After some time, a single individual becomes two. Traditionally they have been classified into species, following the usual nomenclature, e.g. Escherichia coli. However, bacteria are able to exchange genetic information in the form of plasmids, small circular DNA strands, even among quite different species. Some biologists believe that the set of all bacteria forms a genetic continuum, a field of life without evident divisions in different species.
- Parthenogenetic plants. In many groups the two types of reproduction coexist, but in Alchemilla, a rosacea that typically reproduces asexually, 300 species are distinguished that differ in very slight details. It is thought that they could actually make a genetic continuum, where we could go from one species to the next with a few DNA changes. Among plants, there are other such examples.
- Parthenogenetic animals. They are few and frequently arise as hybrids of two species that reproduce sexually, so they use to be as different from one another as their ancestors are.
In conclusion, it looks like the concept of species is linked in some way with sexual reproduction. Where reproduction is asexual, living beings often form a genetic continuum where division in species may be arbitrary. But the data are often contradictory and counterexamples are found almost everywhere. For instance, there are groups with sexual reproduction that form a genetic continuum. The field is quite open for research.