A is for aardvark and Z is for zebra, and anyone will tell you those are separate animals.
We grow up learning that species are completely distinct; a lion is different than a tiger, which is different than a bear. But what really makes animals different? Scientists use morphology, phylogeny, evolution, and genetics to differentiate between the levels of families, species, and subspecies.
However, this differentiation is one of the most controversial topics in evolutionary biology and has occupied scientists for centuries. There are at least 24 different species concepts, each with different types of supporting evidence.
The Biological Species concept claims that an animal is a distinct species if it is reproductively isolated from another animal. For example, horses and donkeys can interbreed, but that mule cannot produce its own offspring. There is no gene flow past the first generation, and so the horse and the donkey are isolated.
There are several types of reproductive isolation including behavioral isolation, mechanical isolation, gametic isolation, and hybrid infertility. Each of theses types of isolation prevents a species from mixing their genes with members of different species.
However, take a Chihuahua and a Great Dane. They are mechanically isolated—a breeder who wants a chihua-dane must use artificial insemination. The two breeds are reproductively isolated…does that mean that they are different species? Wolves and domestic dogs produce fertile offspring, and can continue to interbreed with dogs or wolves although they are separate species, according to classic phylogeny.
Other species concepts continue to muddy the waters by drawing lines between species using criteria that rarely hold true for other species concepts. But why do we have to have species at all? Why do scientists insist on putting every animal into discrete boxes based on arbitrary standards?
In the domain of science, it is important to have ways to talk about things in the world, like species. However, those ideas often get taken so far that scientists get disgruntled when they cannot figure out if a dog is a subspecies of wolf or a completely separate species.
Maybe species are like colors on a color wheel. The colors that blend together represent the genes flowing between those species, so there is a complete continuum between two species for some animals. Some animals stand alone as their own color.
The aardvark, for example, is the only living member of the order Tubulidentata. If it were represented by yellow, it would be completely distinct from orange and green, with no tangerine-yellow or lime green on either side. Other animals like dogs and wolves might be represented as violet and indigo, with a gradient between them. They are separate species because they each have a set of defining genetic and morphological characteristics that are not the same, but they are not completely isolated from each other.
Maybe it is a question that doesn’t have an answer—the scourge of science. How about a species is a species until it’s not?