What is a species? At first, that seems like a simple question. Species is probably the most commonly discussed rank of the taxonomic hierarchy, but it is by no means simple.
The most prevalent definition of a species relies on the biological species concept. To be a species, individuals must be able to interbreed and produce viable offspring. In other words, the offspring must also be able to produce more offspring. For example, horses and donkeys are able to mate, but their offspring (mule) are sterile. Therefore, donkeys and horses are different species, but that is obvious by looking at them. What if you couldn’t tell by looking at them? Even with the sophistication of DNA technology, most species descriptions rely on morphology, or what the organisms look like.
There could be a variety of reasons why individuals cannot, or do not, interbreed. If two species are separated by a geographic feature, such as the Appalachian Mountains or the Mississippi River, they will not be able to interbreed because they are spatially disjunct. Over time, the species will probably change and become more and more different. What if two species breed at different times of the year? They will not be able to interbreed because the don’t breed at the same time. Or what if two species do live in the same place, but have unique calls or genitalia that are incompatible? They can’t interbreed. What about two species of plants that have different numbers of chromosomes? They won’t be able to reproduce viable offspring.
Sometimes species are difficult to distinguish just by looking at them. For example, the Western meadowlark and the Eastern meadowlark look virtually identical and their ranges overlap. However, their songs are different and they don’t interbreed, despite looking alike. Some organisms are called ‘ring species’. Imagine a species of snakes at lives around a lake. Surrounding the lake are different subspecies that are adapted to the local conditions and each of these subspecies can breed with the neighboring subspecies until you reach a point where the ends of the rings overlap. There the two subspecies cannot breed. Where did the snake population shift from one species to another? Are they all the same species? And what about species where individuals look radically different from each other? In many organisms, males and females look very different. For example, in some species of iguana there are three different morphotypes: the big males, the small females, and the small males. Unless you studied their behavior, it would be difficult to determine whether they belonged to the same species.
But what if you couldn’t observe behavior? That is a core problem in paleontology and it shows itself in two different ways: chronospecies and lumpers v. splitters. When scientists study fossil organisms, we are limited to morphology because bones do not preserve behavior. We have no way of knowing whether or not organisms could still interbreed after millions of years. Could a theropod dinosaur from the Early Cretaceous breed with theropod from the Late Cretaceous? Another challenge is the scarcity of fossils. Paleontologists might only find a handful of bones and from them extrapolate and describe an entire individual. However, that comes with consequences. When is variation too insignificant to justify naming a whole new species? Are the differences just normal variation? There are two primary schools of thought: lumping and splitting. Lumpers tend to allow more variation within their species, while splitters allow very little. Dinosaur paleontologists tend to be the most prolific splitters.
It can be difficult to describe what makes a species and some organisms are more complicated than others. It is vital to remember that even though classification systems make biology more manageable, everything has been devised by humans. Science is constantly changing and adapting to new observations, but it is simply attempting to describe what we know. Nature does not fit into little boxes.