What are Transitional Species?

This week I’d like to take a look at an important concept to evolutionary theory- that of the transitional species. This concept is also often referred to by a less-meaningful term, “missing link.” This concept is packed with mistaken assumptions and used unscientifically far too often, which is partially the fault of uninformed journalists and partially the fault of creationists.

The problem is that there is really no such thing as a transitional species. The reason for this is that all species are transitional species. Now, obviously that sounds like I just contradicted myself, but let me explain. Dedicated listeners will remember from the episode about species that I never really defined what a species is. I gave a number of different methods that can be used to describe a species, but I also said that there’s no clear-cut definition, because every system has exceptions. So, if there’s no way to absolutely define species, then there’s no way to absolutely define transitional species. However, our brains don’t like ambiguity. Humans like to classify things, and so we come up with systems of organization and classification, such as the Linnean taxonomy that I’ve mentioned already. And usually classification makes sense- dogs are different species from cats, for example, by any objective measurement. But what about the ring species that I mentioned before, like the salamanders of which all subspecies but two can interbreed? There, the concept of species is not so clear.

In the same way that the concept of species can be provisionally meaningful to describe organisms at a single point in time, the concept of transitional species can be provisionally meaningful to describe organisms over a length of time, usually quite a long time, like hundreds of thousands or millions of years.

The concept, in essence, is fairly straightforward. Let’s say that you have a Species A that existed some time in the past, say, 10 million years ago. Currently, we observe Species C that exists now and shares a lot of the anatomical characters that are seen in fossils of Species A, but which also has several characters that are not seen in Species A. Evolutionary theory predicts that if Species C is descended from Species A, then there is likely a Species B which has more characteristics in common with Species C than Species A. We refer to Species B as a transitional species, but this is only in the context of the difference between Species A and C. These transitional species are often referred to as “missing links” because they are hypothesized to exist, given the fact that fossils are not found one after the other in a continuous line into the past, but are found corresponding to various points in prehistory, which is the reason that gaps exist in the fossil record.

The fact that these gaps exist is not a failing of evolutionary theory, however- it is a limitation of human investigation. We have no way of knowing where fossils are going to be exactly (although we can make some pretty good guesses), and we don’t know what specific fossils are going to be found (although we have some pretty good guesses on those too).

I also want to point out again that the concept of a species being “transitional” is only relative to the species that existed before and after it. And the concept of “species” is a classification that is made by humans strictly for organizational purposes. So a “transitional species” is a contextual classification, nothing more. This is what I meant when I said that there’s really no such thing as a transitional species. But since, given evolutionary theory, all species are in the process of evolutionary change (assuming they don’t become extinct), all species are themselves giving rise to new species eventually, and thus we can say equally that all species are “transitional.”

Now, since I’ve just gone to the trouble of confusing you at length by telling you that transitional species don’t really exist, let me confuse you further by giving you some examples of some. What I mean here is that, since the concept of a transitional species is contextual and relative to a specific classification, it can be meaningful if we view it in that restricted way- that is, if we assume a specific context relative to specific classifications.

For example, if you assume the classifications Fish and Amphibians, there are a number of excellent transitional species, including one amazing species discovered in the past year, called Tiktaalik. Tiktaalik lived about 375 million years ago, and belonged to the group of fish called “lobe-finned,” which are ancestral to all tetrapods, that is, all animals with four limbs. Tiktaalik had characteristics of both fish and tetrapods, including the scales and gills of a fish, limbs that are intermediate between fish and tetrapods, and the mobile neck and lungs of a tetrapod. That’s right- it had both gills and lungs. You can learn more about Tiktaalik at its very own website, http://tiktaalik.uchicago.edu/.

If, however, you assume the classifications Reptiles and Birds, there are also a number of transitional species, most notably Archaeopteryx. Archaeopteryx lived about 150 million years ago, and would technically be classified among dinosaurs, which are a subset of the reptile group. Archaeopteryx had characteristics of both reptiles and birds, including a long bony tail, and a bones structure that is very similar to a reptile. It also had fully-formed, flight-capable feathers, which makes it distinctly similar to birds. It’s unknown whether Archaeopteryx was able to fly the same way that modern birds do- it may have only been able to glide, or perhaps to take wing-powered hops, but the feathers are there, and they show it to be distinctly transitional between reptiles and birds.

There are also a couple well-characterized transitional species in the mammal lineage, especially in the evolution of the whale and the evolution of the horse. Ambulocetus was an amphibious mammal and ancestral to modern whales- it lived about 50 million years ago and has many characteristics of modern whales and many characteristics of the artiodactyla family, the cloven-hoofed mammals, which it is transitional between. In the evolution of the horse, clear transitions can be seen between Eohippus, which is recognized as the first horse, and all the later species such as Mesohippus, Parahippus, Merychippus, all of which used fewer and fewer digits on the foot until our modern horses, which use only one.

More recently, and of more personal interest, is the evolution of humans. Although the specific relationships between fossil species are still somewhat controversial, it is clear that transitional species exist between Australopithecus and modern Homo sapiens, including Homo habilis, and Homo erectus. Homo neanderthalensis, also known as Neandethal Man, is not our direct ancestor, as has been shown by mitochondrial DNA analysis, but is a related ancestral human species, sort of like an uncle.

To review- a transitional species is a classification based on a specific context- a species that exhibits characteristics of species that existed prior to and following it. Gaps in our knowledge of specific transitional species is a function of limited detection, not a failing of evolutionary theory. And many excellent examples of transitional species exist between any number of biological classifications, and more are being discovered every year.