How Transitional Forms are Identified, Pt. 1

Transitional forms are typically identified by their appearance. Take Archaeopteryx for instance. Because it has features that are similar to birds (feathers, wings) and features that are similar to reptiles (long bony tail, claws on forelimbs, teeth in beak), it is considered to be a transitional form between those two groups of creatures.

While transitional forms are typically identified by their appearance, it should be pointed out that a transitional form can only be positively identified by doing cross-breed experiments. Let’s review what a transitional form is supposed to be. A transitional form is a creature that is descended from a different creature and is ancestral to a third type. Using Archaeopteryx as an example again, Archaeopteryx is thought to be descended from reptiles (more specifically, dinosaurs) and ancestral to birds (Archaeopteryx is considered to be a bird itself, but it is considered to be ancestral to, or near the ancestry of, all other birds). Therefore, it would be theoretically possible to cross-breed Archaeopteryx with a bird and cross-breed Archaeopteryx with a dinosaur.

Maybe cross-breeding Archaeopteryx with a dinosaur or a bird would not work. While Archaeopteryx is considered to be a transitional form between these two groups, there may be additional transitional forms between Archaeopteryx and dinosaurs, and between Archaeopteryx and birds. In this case, Archaeopteryx would be able to cross-breed with the transitional forms nearest it, and then those transitional forms could cross-breed with birds and dinosaurs. The point is, if something truly is a transitional form, it should be able to cross-breed with and individual belonging to a different species, which could then cross-breed with still another species, which could cross-breed with still another species, and so on, until you finally reach a species that belongs to an entirely different group.

The problem with the cross-breeding test for transitional forms is that transitional forms are found in the fossil record: they are dead. You can not cross-breed animals that are dead. This is why appearance is often used for identifying transitional forms. Appearance is related to genetics, so the idea is that two animals that share many similar characteristics will have much of the same genes, and therefore they may very well be able to cross-breed. However, is it true that two animals that bear many similarities are closely related genetically?

Not in all cases. Evolutionists have a term for when two different animals that are not supposed to be related happen to look very much like each other. Such an unrelated similarity is said to be the result of convergent evolution. A classic example of convergent evolution is the similarities between the Tasmanian wolf and a true wolf. Both of these animals have the classic dog-type body. Both are about the same size. However, one is a marsupial, and the other is a placental.

Now based solely on outward appearances, it might be thought that the Tasmanian wolf represents a transitional form between marsupials and dogs. Of course, we are told that it is not. The similarities that exist between Tasmanian wolves and true wolves exist simply because they were subjected to similar selective pressures, and they adapted to those pressures by evolving similar traits. A legitimate question to ask is: why are the similarities between these two animals not representative of a genetic relationship while the similarities between Archaeopteryx and dinosaurs show that they are related?

The evolutionist’s answer to the previous question is: the differences between marsupials and placentals are primitive characteristics, that is, the features that distinguish those two groups arose very early in mammalian history, while the common features in both wolves and Tasmanian wolves arose much later. Therefore, the common features between Tasmanian wolves and true wolves must be convergent because they came long after the placental and marsupial groups arose. And how do they know that wolves and Tasmanian wolves evolved long after marsupials and placentals did? By the fossil record. The earliest placentals and marsupials are found in rocks that date back to the late Cretaceous while fossils of wolf-like animals and Tasmanian wolves come from rocks that date much later in the Tertiary.

Okay, so the first marsupials and placentals are supposedly found in older rocks that Tasmanian wolves and wolves. Could it be the case that wolves are an exception? Maybe wolves are directly descended from Tasmanian wolves and wolves are actually convergent with other placentals. The reason this latter idea is not accepted is because it is thought that ancestral conditions do not change much. There is a rule of thumb that a feature generally only arises once in evolutionary history. In cases where the same feature has arisen twice (such as in cases of convergent evolution) it is easier to accept that the ancestral conditions have remained unchanged while the later features are the ones that are convergent. Much of this thinking is codified in a study called cladistics.

Cladistics is the study of descent among living creatures. The end product of cladistics is a cladogram, a branching tree pattern with known species occupying the tips of the branches and the stems representing lines of descent. Nodes (points of branching on the tree) represent hypothetical common ancestors. A cladogram is constructed by creating a list of all of the features of the organisms being studied and scoring the features, usually with a zero or a one. Zero usually denotes the absence of a given feature, a one represents the presence of the feature. In addition to tabulating all the features, an outgroup also has to be selected. An outgroup is an organism that is known to more ancestral to, or derived from a more ancestral stock, than the creatures being studied. The outgroup is there so that the researcher may know what the primitive features are. After all of this data has been collected, the information is run through a program. The program looks through the data and creates branching tree (cladogram) out of it. The cladogram produced is the most parsimonious tree out of all the possible trees that could be constructed. This means that the organisms are ordered on the tree so that the development of the common ancestor to all of the descendants has the fewest number of changes and reversals (loss of a previously existing feature).

Evolutionary scientists like cladistics because it is rigorous and testable. If the exact same features are scored and the same programming is used each time, then the same results will be obtained. Being able to get the same results means that the method is repeatable, which means that the results of one scientist can be independently verified by another.

Now step back and examine what has been explained so far about organisms that look similar to one another, convergent evolution, and cladistics. Convergent evolution shows that not all similar features are the result of common ancestry, but there are ways (such as comparing ancestral features to more recently evolved features) that can distinguish convergent evolution from common ancestry. Cladistics is a rigorous, testable method used for tracing the ancestry of several organisms. So does this sufficiently answer the question, is it true that two organisms that bear similarities are genetically related? Yes and no. Yes, in the sense that it builds a model to explain how common features are the result of common ancestry except in some cases that can be identified and explained. No, in the sense that these explanations do nothing towards answering a larger question: do transitional forms exist at all?

Notice that to identify convergent evolution, more recently evolved features have to be compared to ancestral characteristics. Put another way, convergent evolution can be identified if it is first accepted as fact that the creatures in question have evolved from something. Similarly, cladistics is rigorous an testable if it is first accepted that the creatures in the study evolved from something and that most shared features are the result of common ancestry. In other words, convergent evolution and cladistics only make sense in the light of the theory of evolution, so they have no explanatory power in determining whether transitional forms (a necessary part of the theory of evolution) exist or not. So for the purposes of determining whether transitional forms exist, shared features can not be used to identify transitional forms.