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| Taxonomic categories |
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.
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| Escherichia coli |
- 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.
Manuel Alfonseca
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