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. 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, and we don’t know what specific fossils are going to be found.

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.”

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/.

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.

We have seen species split, and we continue to see species diverging every day. For example, there were the two new species of American goatsbeards (or salsifies, genus Tragopogon) that sprung into existence in the past century. In the early 1900s, three species of these wildflowers – the western salsify (T. dubius), the meadow salsify (T. pratensis), and the oyster plant (T. porrifolius) – were introduced to the United States from Europe. As their populations expanded, the species interacted, often producing sterile hybrids. But by the 1950s, scientists realized that there were two new variations of goatsbeard growing. While they looked like hybrids, they weren’t sterile. They were perfectly capable of reproducing with their own kind but not with any of the original three species – the classic definition of a new species.

But just because we can’t see all speciation events from start to finish doesn’t mean we can’t see species splitting. If the theory of evolution is true, we would expect to find species in various stages of separation all over the globe. There would be ones that have just begun to split, showing reproductive isolation, and those that might still look like one species but haven’t interbred for thousands of years. Indeed, that is exactly what we find.

The apple maggot fly, Rhagoletis pomonella is a prime example of a species just beginning to diverge. These flies are native to the United States, and up until the discovery of the Americas by Europeans, fed solely on hawthorns. But with the arrival of new people came a new potential food source to its habitat: apples. At first, the flies ignored the tasty treats. But over time, some flies realized they could eat the apples, too, and began switching trees. While alone this doesn’t explain why the flies would speciate, a curious quirk of their biology does: apple maggot flies mate on the tree they’re born on. As a few flies jumped trees, they cut themselves off from the rest of their species, even though they were but a few feet away. When geneticists took a closer look in the late 20th century, they found that the two types – those that feed on apples and those that feed on hawthorns – have different allele frequencies. Indeed, right under our noses, Rhagoletis pomonella began the long journey of speciation.

The point is that all kinds of creatures, from the smallest insects to the largest mammals, are undergoing speciation right now. We have watched species split, and we continue to see them diverge. Speciation is occurring all around us. Evolution didn’t just happen in the past; it’s happening right now, and will continue on long after we stop looking for it.