To really understand the concept of ‘species’, we first need to have a look at the biological meaning of the word. In biology a species is a group of organisms that are capable “of reproducing fertile offspring under natural circumstances”.
What does this mean? Sometimes it is very clear that individuals are from different species. Often we look at appearances, and by doing so it is clear that a mouse and a snake are different species. The fact that both mouse and snake have different genes, different information ‘how to build them’ causes these differences in appearance. These differences also ensure that they can’t reproduce together. But sometimes genes and appearances look very much alike, so it’s harder to tell them apart.
It’s even getting more confusing when two individuals of different species can reproduce. A famous example is crossing a horse with a donkey; the result is a mule or a hinny. So despite the fact that these two animals normally don’t reproduce, technically speaking, it is possible. But the offspring is rarely fertile; you can’t breed with mules or hinnies.
Although horses and donkeys have a lot of similarities and they can reproduce, there not the same species, because their offspring isn’t fertile.
The previously described method of how to divide individuals into species, although clear, isn’t always that easy. When do we speak of natural circumstances? And what if offspring can be fertile, but in the second or third generations, they aren’t anymore? Interesting questions but they are too complex to elaborate in more detail. Let’s have a look at the term subspecies instead.
Within a species there are often small differences. These are no obstacle for reproduction or fertility of the offspring. However these differences do have genetic causes. Are there visible differences in a group of individuals of the same species? Than you may divide the species even further to the level of sub-species. These differences can occur for example when a group individuals of a species is isolated from the rest of the population. This isolation can cause this group to develop small genetic differences over time. The animals could develop a different kind of fur because by a colder climate, or they could become much bigger because of good food availability. If the conditions for reproduction and fertile offspring are still valid, then we’re dealing with a subspecies.
These descriptions of species and subspecies clearly show that distinguishing between subspecies isn’t easy and there is no fixed framework. When can we speak of a genetic difference that can be observed? And how can it be observed? Is it only visible under the microscope or can it be seen with the naked eye as well? And is the difference big enough to label it a subspecies? These are difficult questions for biologists and there is currently no consensus on this topic.
How about species and subspecies within gorillas? When you have a look at the list below, with the Latin names included, it becomes quite clear. All have the name ‘gorilla’. This is what we call the genus. By looking at the genus level you can identify what kind of animals we’re talking about. The second part of the name tells you something about the species of gorilla you’re dealing with. The third part finally tells you which subspecies. When we look at the gorillas that are on display in European zoos, we look at the Gorilla (genus) gorilla (species) gorilla (subspecies). This gorilla is commonly known as western lowland gorilla.
Western gorilla (Gorilla gorilla):
Eastern gorilla (Gorilla beringei):