Transitional form

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A transitional form is a fossil or living organism that is intermediate between two other organism in an evolutionary sequence. A transitional form that has not yet been found is colloquially called a 'missing link'.

Creationists claim that transitional forms between different kinds of creatures are systematically absent. Other scientists typically reject this claim. One such attempted rebuttal is that by Kathleen Hunt who created a Transitional Vertebrate Fossil FAQ.

Identification problem

A major problem with identifying transitional forms is in determining whether a particular fossil or living thing is truly intermediate between two others, or is merely an unrelated form which has characteristics common to both other forms.

Archaeopteryx, for example, has some features in common with both living birds and reptiles, and is often quoted as an intermediate form. It was not covered with part-scales/part-feathers, but had fully-functioning feathers and wings, and was capable of flight. There is therefore no reason to think that it is not simply a bird with some features in common with reptiles.

Evolutionists have criticised creationists for claiming that if an intermediate form is discovered, it merely creates two gaps where there was previously one. This criticism ignores the size of the gaps. If the remaining two gaps are large, then the creationist claim is justified. Rather than simply producing the occasional form which appears to be intermediate between two different forms, evolutionists need to demonstrate what Charles Darwin called a "finely-graduated organic chain".

Despite many more fossils being discovered since Darwin's time, the continued lack of a series of fossils showing a smooth transition led Stephen Jay Gould and Niles Eldredge to propose Punctuated Equilibrium, which says that the change from one form to another took place is short bursts which generally would not leave a fossil trace. Evolutionists continue to claim that such intermediate forms exist or existed, but have failed to adequately demonstrate any such smooth sequence, as opposed to the occasional form with features common to two other forms.

Claims of transitional forms

In The Origin of Species Charles Darwin wrote that the fossil record (as it was known then) was lacking in transitional forms. He predicted, however, that transitional forms would be found. Nevertheless, 120 years later, David Raup said that the situation had not improved.

Nevertheless, evolutionists continue to make claims of transitional forms. One such set of claims was in the 1st March 2008 issue of New Scientist The article claimed that the so-called “fishbian” sequence of fossils shows the steps by which fish crawled out of the water and onto the land during the Devonian period. New Scientist claims that fossils have been "found spanning the entire transition from the distinctly fish-like Eusthenopteron to the four-legged amphibian Hynerpeton".

However, the accompanying "family tree" showing a mere eight fossils in this sequence does not show any of them as being intermediate to any others, but all being on separate branches. Furthermore, it has many of the forms in the sequence living contemporaneously with others in the supposed sequence, including some "later" ones living earlier then their supposed ancestors!

Nevertheless, New Scientist claims that the "fossil record of the fish-to-amphibian transition is now among the best documented of all"!

Claims of a lack of transitional forms

It is a commonly claimed misconception by critics of evolution that there are no transitional fossils. Such claims may be based on a misunderstanding of the nature of what represents a transitional feature, or according to Donald Prothero, may be a tactic actively employed by creationists seeking to distort or discredit evolutionary theory. Prothero has called that claim the "favourite lie" of creationists and further said that it was "manifestly untrue".

A common, though fallacious, creationist argument is that no fossils are found with partially functional features. Vestigial organs are common in whales for example.. Also, there is evidence that a complex feature with one function can adapt to a wholly different function through evolution in a process known as exaptation. The precursor to, for example, a wing, might originally have only been used for gliding, trapping flying prey, and/or mating display. Nowadays, wings may still have all of these functions, while also being used for active flight.

Although transitional fossils elucidate the evolutionary transition of one life-form to another, they only exemplify snapshots of this process. Due to the special circumstances required for preservation of remains, only a very small percentage of all life-forms that ever have existed can be expected to be represented in discoveries. Thus, the transition itself can only be illustrated and corroborated by transitional fossils, but it will never be "caught in the act" as it were. Creationists often argue against this, claiming it is merely a convenient way to explain the lack of 'snapshot' fossils that show crucial steps between species. However, progress in research and new discoveries continue to fill in such gaps, and in modern thinking evolution is pictured as a bush of lines of development, not the simplistic ladder of progress that was common before Darwin published his theory and still influences popular opinion.

The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in 1972 is often mistakenly drawn into the discussion of transitional fossils. This theory, however, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution.

See also

• Missing link
• Transitional fossil
• Theory of Punctuated Equilibrium