A common argument of creationism is that no transitional forms between major organismal groups (e.g. the classes of the vertebrates: fish, birds, mammals etc.) are seen in the fossil record. Evolution posits that such intermediates do exist, but may not be recognized as such. In the end, it often boils down to an argument of semantics.
Science prides itself in its open framework. Science is considered to be at its best when authors freely disclose any and all assumptions that were made in their research and also present all the raw data before giving their own interpretation of it. This way, as new understandings arise and assumptions are shown to be invalid, scientists coming later can re-interpret the raw data in this new light. While laying out all the information that goes into a conclusion leaves the first scientist open to be disproved, it is GOOD science nevertheless.
In the spirit of this open framework, I wish to explain here some basic principles of biology and paleontology that, if improperly understood, can strongly affect the answer to the question: Are transitional forms found in the fossil record?
The biological species concept
A biological species (biospecies) is a population of organisms that, if left alone in their natural environment, are capable of. and DO reproduce to produce viable FERTILE offspring. So while horses and donkeys are capable of mating and producing offspring, they would not do so naturally, and their offspring (the mule) is almost never fertile. It is a similar story with lions and tigers. Which ligers and tigons do exist, the two species (lions and tigers) would never interact in the wild. This is the common definition of species used in modern biology and by all those who toss about the term ‘species’ loosely.
The morphological species concept
When all you’ve got to look at are fossils, it is not possible to tell whether two populations interbred, or if the offspring were fertile. All we have, generally, are the bones to look at. In paleontology, the term species is used to describe a population of organisms, which are distinguishable by their shape and structure from all other populations. If it looks sufficiently different, then it’s a new species.
This morphological species concept is really the only option for paleontologists. The trouble is, and this is no secret, that sometimes what in reality is one biological species may be comprised of many morphological species. In the case of mammals, a fossil morphological species (or morphospecies) may be defined based upon one or two teeth in, say, the upper jaw. A lower jaw may later be found by some other worker in some other quarry and defined itself as a new morphospecies. Only later, through careful review, might a third worker find that the two morphospecies are really only one species.
Similar problems have also arisen where there is strong sexual dimorphism in a single biological species. It is not uncommon in nature for males of a biological species to be physically quite different from females, and unless complete skeletons are found, the two sexes of one biospecies may be defined as two different morphospecies. Additionally, ontogenetic changes can cause adults of a biospecies to be distinct morphologically from the young of the same biospecies, resulting again in more than one morphospecies comprising a biospecies.
Taxonomic nomenclature
Perhaps one of the most difficult concepts to get across is the fundamental understanding that, even though our scientific naming scheme used globally for organisms (binomial nomenclature) superficially is divided along “evolutionary” lines, the naming scheme itself is truly arbitrary. The only level of nomenclature that has any physical meaning is that of “species” as discussed above. All other levels, from Kingdom to Genus, are entirely human constructs, used for convenience of discussion. In the stricter sense, many of the larger groupings (Kingdom, Phylum, Class) really apply best only to modern organisms.
Taxonomic classification of the domestic dog:
Furthermore, there are organisms in the fossil record that do not fit well into the modern taxonomic scheme. But, because we use this scheme, we essentially must force them to fit the mold. A transitional species between two classes is difficult to classify as it frequently could equally be placed into either class, or perhaps belongs in a third, now extinct class. But once classified in one class or another, it appears to cease to be transitional.
For example, we can consider the transition species from Reptilia to Aves to be called a “repti-bird.” It is at one time a ‘bird’ and a ‘reptile’ but also neither a ‘bird’ nor a ‘reptile.’ The most often cited example of the “repti-bird” is the much-maligned Archaeopteryx. It is arguably a bird, equally arguably a reptile, or potentially not really either, but instead a dinosaur. (I hasten to add here that paleontology has since discovered many more species of “repti-birds” which document the transition from dinosaur to bird far better than poor Archaeopteryx. In paleontology, Archaeopteryx is very seldom if ever touted as a direct transitional species between reptiles and birds, and is more often considered a very early bird.)
Once Archaeopteryx was ‘officially’ classified as a bird, its status as a transitional species was cast to doubt. If it’s a bird, creationism says, how can it be a transitional species leading to birds? Classification schemes are arbitrary! We use them as a matter of convenience. The classification scheme was never intended to directly imply something about phylogenetic (or evolutionary) relationships.
Primitive versus Advanced
The terms “primitive” or “advanced” must be used with caution, yet they are frequently used freely ignoring the potential confusion that could result. These terms suggest some amount of “improvement” for animals considered advanced over those considered primitive. What is really meant by “primitive” is that an animal is more similar to the original forms from which they evolved. A modern fish is equally evolved as a modern bird. We know that birds evolved from animals which were fish. However, that ancestral fish is long extinct. That same ancestral fish may have been the ancestor of modern fishes as well. So when we call a fish ‘primitive’ compared to birds, it is only with reference to the fact that fish-like vertebrates swam the oceans well before any bird soared in the sky. The term “derived” is also often used interchangeably with “advanced.”
Taxonomic relationships vs Phylogenetic relationships
Taxonomic nomenclature is a convenient way with which biologists and paleontologists group ancient organisms in order to be able to think about them. The modern nomenclature was devised by Carolus Linnaeus in 1758, based upon modern organisms, especially plants. As noted above, all taxonomic names above the species level are arbitrary. Organisms were classified in various groups based on similarities, but could not be in more than one group at a time (e.g. it is either a reptile or a bird, but not both).
From this has been derived the typical text-book image of evolution of vertebrates, from the most primitive (fishes) to the most advanced (mammals), and the notion that there ought to be intermediates like fish-ibian, repti-birds, and repti-mammals. This also suggests that from modern fish evolved modern amphibians evolved modern reptiles, evolved modern birds and mammals.
Phylogeny is the actual evolutionary relationships among different groups of organisms. There are no arbitrary divisions and classifications. The classic image of the “tree of life” is a very simple version of phylogeny. Today, biologists, botanists, and paleontologists use mathematical analysis of characteristics of organisms in order to work out phylogenies. The phylogeny appears very different from the taxonomic vision of organismal relationships.
Importantly, groups are united based on shared “derived” characteristics. That is, groups of organisms are distinguished from each other on the basis of new traits never before seen. Sometimes, the result is that groups considered distinct by Linnaen taxonomy are nested within one another.
Above is a simple phylogeny for vertebrates. All vertebrates are united by having vertebrae. The red lines are the evolutionary relationships among various groups of vertebrates. As one moves upward along any of these evolutionary lines, one moves toward more advanced species. The splits between groups occur among the most primitive members of the groups. For example, the first amphibians lie on the phylogeny at the point labeled “2” which represents the origin of terrestrial locomotion. These first amphibians, while by definition are amphibians, are not the modern, advanced amphibians that we see today. These first amphibians evolved from primitive fishes, which are also unlike any fish we have today.
Notice that birds and mammals and dinosaurs lie nested within other animals labeled reptiles. So birds and mammals are “reptiles,” but are distinguished from other reptiles based on characteristics of the skull (mammals) and by the ability to fly (birds). There is overlap among dinosaurs, birds, and reptiles. Phylogenies allow for one species to belong to more than one group.
Shown above are the positions on the phylogeny where the intermediated forms between the major classes would exist. Note that these are not at the tips of the evolutionary lines (modern forms) but lower on the phylogeny. The transitions occur from primitive members of one group into primitive members of another group. We do not draw frog’s legs on a trout and say this is what the transitional species between fish and amphibians looked like. We do not imagine a turtle with wings and call that the transition between reptiles and birds. (And we most assuredly do not look at a duck-billed platypus and call it a transition between birds and mammals, despite what I’ve seen published at times in the creationist literature.)