What is a species?

Simply put, a species is group of similar individuals capable of exchanging genes or interbreeding. However, this simple definition misses a lot of the nuance and detail of what makes a species distinct. Although an important mechanism of evolution, adaptation does not always result in a new species, although it can be involved sometimes. The process of new species being formed is called speciation.

Although being able to define a species is a critical aspect of biology, the actual definition of what makes a species is still not totally settled, and is still the subject of research and debate. There are two main explanations of what species are:

Isolating mechanisms

The biological species concept

This concept focuses on the ability, or lack thereof, of exchanging genes between individuals.

This makes it possible to state that species are "groups of interbreeding populations which are reproductively isolated from other such groups" (Ernst Mayer 1942). There are several ways that organisms can be prevented from making babies with others - as you can see in the table below, these can be pre-zygotic (before the zygote) or post-zygotic (after the zygote develops).

An example of this is the mule. Mules are the infertile hybrids between a daddy horse and a mammy donkey. The only way to produce a mule is to breed a horse and a donkey; you cannot mate two mules together, as they are infertile. Using the table opposite, this would be an example of hybrid infertility, which is a post-zygotic isolating mechanism.

The main issue with the biological species concept is that it does not explain all of the observations that can be made in the wild. Under this concept, species should not be able to produce fertile hybrids. But, in recent years, biologists have discovered much greater amounts of hybridization than was thought possible - this has been especially true when it comes to plants.

The ecological species concept

Botanists have been aware of this higher rate of hybridization for a long time - in some studies, up to 50% of the plants sampled can be difficult to define by genetic isolation means. This hybridization can be a long term issue - fossil data shows that balsam poplars and cottonwood trees have been phenotypically distinct species for 12 million years, yet they have often produced hybrids, even through to modern times. There are also lots of examples of animal hybridization events being not uncommon, with one survey suggesting that up to 10% of the world's 10,000 bird species known to have hybridized. However, hybridization is not rampant in the animal world, and is quite rare, but it happens enough to cast doubt on whether reproductive isolation is the only force keeping species separate.

The ecological species concept proposes that the differences between species are maintained by natural selection; each species has adapted to its niche in the environment, and selection keeps those species stable. Hybrids have little effect, as their alleles are selected against as soon as they start breeding with the other, non-hybrid members of the species.

The video below nicely sums up the topic of speciation:

Genetic Drift

Natural selection reinforces the reproductive isolation of species, by selecting for certain phenotypes. However, there is another mechanism at play. Random changes in small populations, founder effects and bottlenecks can all lead to changes in characteristics that CAN THEN lead to reproductive isolation. So, it's not just natural selection at play, you also have to think of its little brother: genetic drift. See the video for more!