Defining a species
A species is often defined as a group of individuals that actually or potentially interbreed in nature.
In this sense, a species is the biggest gene pool possible under natural conditions.
For example, these happy face spiders look different, but since
they can interbreed, they are considered the same species: Theridion
|That definition of a species might seem cut
and dried, but it is not in nature, there are lots of places
where it is difficult to apply this definition. For example,
many bacteria reproduce mainly asexually. The bacterium shown
at right is reproducing asexually, by binary fission. The definition
of a species as a group of interbreeding individuals cannot
be easily applied to organisms that reproduce only or
Also, many plants, and some animals, form hybrids in nature. Hooded crows and carrion crows look different, and largely mate within their own groups but in some areas, they hybridize. Should they be considered the same species or separate species?
If two lineages of oak look quite different, but occasionally form hybrids with each other, should we count
them as different species? There are lots of other places where the boundary of a species is blurred. It's
not so surprising that these blurry places exist after all, the idea of a species is something that we humans
invented for our own convenience!