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Transitional form

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A transitional form is a species (living or fossil) intermediate between two other species, either in terms of ancestry or characteristics.

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Definition of transitional

Common descent of two forms implies a common ancestor with a chain of transitional forms, each varying only slightly from the previous and following forms in the chain, leading backward in time from one of the forms in question to the ancestor, and then forward in time from the ancestor to the other form. The chain will not consist of discrete species, but of a nearly continuous shift in characteristics from one variety to the next until the differences are great enough to warrant the definition of a new species. Common descent from one species implies common descent from all the ancestors of that species. The chains between species will generally not be singular, but will branch out to additional species.

The theory of evolution originally expounded by Darwin posits universal common descent, so that ultimately any two life forms which ever lived can be traced back to a common ancestor by a series of forms nearly indistinguishable from each other. This contrasts with the biblical account, which implies a number of created kinds, with common descent within each kind, but no ancestral forms connecting the kinds.

The evolutionary worldview also sees the geological column as representing millions of years in time, so that the fossils found in any strata (with few exceptions), are a record of forms that were alive at the time the strata were formed. If the fossil record were perfect, then it would contain all the transitional forms and allow the reconstruction of a complete ancestral tree, back to the earliest forms of life. Innumerable transitional forms connecting all life forms to each other by a chain of imperceptible differences are not found in the fossil record, implying that either universal common descent is not true or that the imperfection of the fossil record is too great to allow it to be directly observed.

The evolutionary view was illustrated by Charles Darwin in the epochal On the Origin of Species, as reproduced below. 1000px-Origin of Species.png

Here the forms n14 and y14, for example, can be traced through transitional forms back to the common ancestor z6. If the fossil record is imperfect, then many of these forms, and in particular the common ancester, might be missing. In some cases, the best that can be done may be to study a form that is is believed to be on a branch, and therefore not the direct ancester of either form, for example u5, or one of the branches from z7 other than z8. If the form studied is not too far removed from a true transitional form, then it should share enough characteristics with that form to allow useful deductions to be made.

Because strictly transitional forms are not generally available, paleontologists often use the term "transitional form" in a looser way. A transitional form in this sense is a species which possesses some characteristics in common with the one species (or higher taxonomical group) and some characteristics in common with the other species, and that is thought to possess those characteristics by virtue of being a close relative of a true transitional species. In the evolutionist view, common characteristics are usually the result of common descent, but can also be the result of convergent evolution, where two lineages independently acquire a particular feature because it is adaptive for survival in the common environment. The development of powered flight in both birds and bats is considered a classic example of convergent evolution.

Nor does a fossil need to be in the direct line of descent between two groups to be considered transitional. Archaeopteryx, for example, was doubtless not the direct ancestor of birds but rather one of that ancestor's cousins. Similarly the fishlike amphibian Ichthyostega was probably a dead end collateral branch of the fish-to-amphibian transition. The point is that a cousin of an ancestor is the more likely paleontological find, given the multiple splitting off of species and the general spottiness of the fossil record, and is evidence enough that a transition occurred.

—R.L.Ecker[1]

It is obvious that evolutionists have redefined “transition” to a lower standard for the word.

John Woodmorappe[2]

Expected number of transitional fossils

Creationists have highlighted what Stephen Jay Gould described as the "trade secret of paleontology", that transitional fossils are an "extreme rarity".[3] They go on to say that the relatively few claimants have either since been rejected or are disputed. These assertions are not accepted by many mainstream scientists. This question cannot be resolved without considering how many transitional fossils should be expected in an imperfect fossil record.

Charles Darwin himself recognized that transitional forms were absent.

Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.[4]

He proceded to discuss a large number of reasons to expect the geological record to be inherently imperfect, plus the fact that the number of fossils catalogued is "absolutely as nothing" compared to the species which must have existed in the past, concluding,

These causes, taken conjointly, will to a large extent explain why—though we do find many links—we do not find interminable varieties, connecting together all extinct and existing forms by the finest graduated steps.[5]

Vast numbers of fossils have been found since then, but the lack of transitional fossils remains. In 1979, David Raup wrote:

Well, we are now about 120 years after Darwin and the knowledge of the fossil record has been greatly expanded. We now have a quarter of a million fossils species but the situation hasn't changed much. The record of evolution is still surprisingly jerky and, ironically, we have even fewer examples of evolutionary transition than we had in Darwin’s time. By this I mean that the classic cases of darwinian change in the fossil record, such as the evolution of the horse in North America, have had to be modified or discarded as a result of more detailed information...[6]

The previous year, Colin Patterson, the senior palaeontologist at the British Museum of Natural History, had published a book about evolution[7] in which he had not included any illustrations of transitional forms. Asked about this, he replied

I fully agree with your comments on the lack of direct illustration of evolutionary transitions in my book. If I knew of any, fossil or living, I would certainly have included them. You suggest that an artist should be used to visualise such transformations, but where would he get the information from? I could not, honestly, provide it, and if I were to leave it to artistic licence, would that not mislead the reader?’ ... ‘Yet Gould and the American Museum people are hard to contradict when they say there are no transitional fossils. … You say that I should at least “show a photo of the fossil from which each type of organism was derived.” I will lay it on the line—there is not one such fossil for which one could make a watertight argument.’[8]

In the evolutionary interpretation of the geological column and the fossil record, any given species exists for typically 10 million years before becoming extinct, and the total time since the Cambrian explosion was some 500 million years. The total number of fossil species that have been catalogued is about 250,000, so for any given time in geological history fossils of about 5,000 species are known. If there have always been about 2 million species living, as there are today, then the chance of any one past species having been found in the fossil record is roughly 0.25%. The chance that any actual ancestor of a given species has been found is small. The chance that the last common ancestor of any two given species is known is very small. In the light of these estimates, or similar ones,[9] paleontologists do not in most cases expect to find actual transitional species or common ancesters, but try to learn what they can from the fossils of species closely related to these.

Regardless of whether the transitional forms are expected or not, it remains the case that they are not found, at least in useful numbers, so the evidence for evolution from transitional forms in the fossil record is largely absent. As Gould wrote in 1977:

… to preserve our favored account of evolution by natural selection we view our data as so bad that we never see the very process we profess to study.[3]

Difficulty of identification

One species evolving into another species is not going to go through merely one intermediate step, but numerous intermediate steps (Darwin's "finely graduated organic chain"). For this reason, a proponent of evolution proposing that species B is intermediate between species A and C has not really produced evidence of the transition. Furthermore, without that "finely graduated" sequence, the question remains as to whether species B is really an evolutionary intermediate between A and C or is simply an unrelated species with some features of each of the other species.

Proponents of evolution will sometimes object that when an intermediate form is found, creationists simply claim that there are now two gaps instead of one. However, this objection fails to recognise the size of the remaining gaps.[10] There is still no "finely graduated" sequence as Darwinian evolution would predict.

It was this rarity of transitional forms that led Niles Eldridge and Stephen Jay Gould to propose their theory of "punctuated equilibrium", which proposed that instead of a slow and gradual evolution as Darwin had envisaged, evolution had occurred in short bursts which left little evidence in the fossil record. In 1977 Gould wrote:

The extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology....The modern theory of evolution does not require gradual change. In fact, the operation of Darwinian processes should yield exactly what we see in the fossil record.[11]

He later clarified,

Transitional forms are generally lacking at the species level, but they are abundant between larger groups.[12]

Gould complained that creationists had "hijacked" his comments,[note 1] but that Gould acknowledged an extreme lack of transitional fossils is borne out by fellow proponent of evolution Colin Patterson, who wrote:

... Gould and the American Museum people are hard to contradict when they say there are no transitional fossils...The reason is that statements about ancestry and descent are not applicable in the fossil record.[13]

However, reaction to creationist publicity of Gould's earlier comments seemed to hit a nerve,[14] so Gould's later tried to demonise creationists while claiming that the transitional fossils do exist:

The anatomical transition from reptiles to mammals is particularly well documented in the key anatomical change of jaw articulation to hearing bones...We can trace, through a lovely sequence of intermediates, the reduction of these small reptilian bones, and their eventual disappearance or exclusion from the jaw, including the remarkable passage of the reptilian articulation bones into the mammalian middle ear...We have even found the transitional form that creationists often proclaim inconceivable in theory...Still, our creationist incubi, who would never let facts spoil a favorite argument, refuse to yield, and continue to assert the absence of all transitional forms by ignoring those that have been found, and continuing to taunt us with admittedly frequent examples of absence.[15]

Claimed transitional forms

The following are some examples of claimed transitional forms.[note 2]

Fishbians

The so-called “fishbian” sequence consists of fossils found in the Devonian system. The fossils are said to show a rather complete set of graduated characteristics from the distinctly fish-like Eusthenopteron to the four-legged amphibian Hynerpeton. Evolutionists now consider the fossil record of the fish-to-amphibian transition to be among the best documented of all.

Tiktaalik is is a fossil species showing some characteristics found in extant fish but not in extant amphibians, and other characteristics found in extant amphibians but not in extant fish.[16] It has a crocodile-like head which, unlike fish, it could move independently of its body. It also had fins that contain bones, claimed to be capable of supporting the animal on land[17], but not of walking.[18]

Tiktaalik is therefore claimed to be a transitional form between fish and amphibians.[19]

However, the discovery of fossil tetrapod tracks dated earlier than the tiktaalik show that this species cannot be the intermediate form it was publicly claimed to be. This doesn't bother evolutionists, given that their preferred definition of transitional form is one which encompasses claimed descendants of true intermediate forms.[20]

Rhinos

The fossil record of rhinoceroses is extensive. Starting with dog-sized creatures, such as the early Eocene Hyrachyus, that are barely distinguishable from early tapirs and horses, fossil rhinos gradually diversified into a wide variety of forms. Some of these forms became gigantic, culminating in the huge indricotheres, which were about 7 metres tall at the shoulder and weighed 20 tonnes. None of the early rhinos had horns. The living rhino family began in the middle Eocene with primitive creatures such as Teletaceras which looked much like the earlier “running rhino” type excepting for the development of a chisel-like upper tusk and pointed lower tusk.

Pinnipeds

The pinnipeds are seals, sea lions and walruses and are descended from primitive bears. A recently discovered fossil has provided a beautiful transitional form between the two groups[21]. Although superficially like a seal the fossil lacked most of the specialisations that modern pinnipeds require for their aquatic lifestyle. In addition, its “flippers” had long toes and claws, halfway between bear paw and pinniped flipper. There was subsequently a great “radiation” of seals and sea lions in the middle and late Miocene, all of which appear in the fossil record.

Archaeopteryx

Archaeopteryx is possibly the most famous and most-cited transitional form, except perhaps for some human transitional forms. In addition to distinctive bird-like features like feathers, an opposable big toe, a wishbone, and an avian "flow-through" lung,[22][23] it also has some features otherwise associated with reptiles, such as teeth, a bony tail, claws on its wings, and a flattish sternum.[24]

All fossils of Archaeopteryx were discovered in the same area of Germany and were the first fossils discovered with some reptilian and some bird-like features, but since then many more such fossil species have been found, mostly in northeastern China. A number of these have teeth[22] and feathers of various types. The feathers of Archaeopteryx are essentially of modern form, and this animal was apparently capable of flight.[22] Other fossils have simpler forms of feathers and are classified as feathered dinosaurs rather than birds.[25]

Others examples

There are many other claimed transitional forms including: velvet worms, lancelets, synapsids, ceratopsians, giraffes, ichthyosaurs and manatees. One paper reported at least 139 fine-grained species to species transitional sequences.[26]

Baraminological view

Young earth creationist and prominent baraminologist Todd C. Wood admits the existence of transitional fossils, but prefers to call them intermediate fossils, and further divides them into intrabaraminic and interbaraminic. Wood ascribes these forms as communication from God and not glorification of Him.[27]

However, it is unclear how intrabaraminic variation differs from evolutionary model. Creationists admitted in a recent paper, "The findings of this study can easily be explained by both the evolutionist and the creationist."[28] They also admit, "The arguments can be further honed by discussing varieties, or subspecies within the organization of a species. While this additional splitting of organisms is relatively closely tied to evolutionary phylogeny, it also works well with intrabaraminic phylogeny. The creationist can easily rectify different varieties with his beliefs."

Prominent young earth creationist Jean K. Lightner also admits the limitations of creation scientists' understanding of intrabaraminic variation: "Historically, many creationists have limited explanations of intrabaraminic diversity to initial variability, degenerative random mutations, and natural selection. Given that baraminologic investigation has found that holobaramins tend to include whole families or even several families of animals, there is serious question as to how well the above mechanisms account for what we observe in the world around us."[29]

Note

  1. Not necessarily this particular comment.
  2. The examples are taken from New Scientist magazine of 1 March 2008, What Missing Link? by Donald Prothero, professor of geology at Occidental College in Los Angeles and lecturer in geobiology at the California Institute of Technology in Pasadena.

References

  1. Ecker, R.L., Dictionary of Science and Creationism. Buffalo, NY: Prometheus (1990). Quoted in Prothero, Donald R., Evolution: What the Fossils Say and Why It Matters, New York; NY: Columbia University Press (2007).
  2. Woodmorappe, John, Evolutionary cladograms and malevolent, straw-men creationists: A review of Evolution: What the Fossils Say and Why it Matters by Donald R. Prothero, Journal of Creation 23(3):39–43, December 2009.
  3. 3.0 3.1 Gould, S.J., Evolution's erratic pace, Natural History 86(5):14, 1977, quoted by Batten, 2002.
  4. Darwin, Charles, On the imperfection of the geological record, Chapter IX of The Origin of Species, Edition 1, 1859, p.279.
  5. Darwin, Charles, On the imperfection of the geological record, Chapter XI of The Origin of Species
  6. Raup, D. M., Conflicts between Darwin and paleontology, Field Museum of Natural History Bulletin 50:22, 1979, quoted in Snelling, Andrew (Ed.), The Revised Quote Book, Creation Science Foundation, 1990, p. 7.
  7. Patterson, C., Evolution, The British Museum of Master Books, Natural History, London, 1978.
  8. Luther D. Sunderland, Darwin’s Enigma, Master Books, Arkansas, USA, p. 89, 1984. Patterson's letter was written to Sunderland on 10 April 1979.
  9. Wesley R. Elsberry, Relative Number of Transitional Sequences in the Fossil Record, Last updated: 980413.
  10. Woodmorappe, John, Does a ‘transitional form’ replace one gap with two gaps?, Journal of Creation 14(3):5–6, August 2000.
  11. Gould, S.J., Evolution's erratic pace, Natural History 86(5):14, 1977, reprinted in Gould, S.J., "The Episodic Nature of Evolutionary Change", The Panda's Thumb: Reflections in Natural History, New York: W. W. Norton & Company, 1980, pp. 182-184. The first part is quoted by Batten, 2002, and the last part is included in a list of Stephen J. Gould Quotations.
  12. Gould, Stephen Jay, Evolution as Fact and Theory. Quoted in Prothero, Donald R., Evolution: What the Fossils Say and Why It Matters, p.78, New York; NY: Columbia University Press (2007).
  13. Patterson, Colin, 1979, in a letter to Louis Sunderland, quoted in Sunderland, L., Darwin’s Enigma, Master Books, Arkansas, USA, pp. 101–102, 1998.
  14. Batten, Don, Gould grumbles about creationist ‘hijacking’, Journal of Creation 16(2):22–24, August 2002.
  15. Hooking Leviathan by Its Past, Dinosaur in a Haystack: Reflections in Natural History, New York: Crown Trade Paperbacks, 1997, pp. 360-361.
  16. Neil Shubin, Q: Would you describe the unique physical characteristics of Tiktaalik?, April 2006.
  17. Neil Shubin, Q: What do Tiktaalik's anatomical structures tell us about how it lived?, April 2006.
  18. Colin Mitchell, Is the fish really our ancestor?, Journal of Creation 23(1):29–32, April 2009.
  19. Neil Shubin, Fossil shows how fish made the leap to land, 5 April 2006
  20. What has the head of a crocodile and the gills of a fish? June 2010.
  21. The early Miocene fossil Enaliarctos. See Science vol. 244, page 60.
  22. 22.0 22.1 22.2 Menton, David, Bird evolution flies out the window, Creation, 16(4):16–19, September 1994.
  23. Gary Ritchison, Department of Biological Sciences, Eastern Kentucky University, Avian Respiration.
  24. University of California Museum of Paleontology, Archaeopteryx: An Early Bird
  25. Gary Ritchison, Department of Biological Sciences, Eastern Kentucky University, Feather evolution.
  26. Roger Cuffey, Paleontologic evidence and organic evolution, Journal of the American Scientific Affiliation 24(4). Cited in Wesley (1998).
  27. Wood, Todd C."What about Transitional Forms?" Todd's Blog, Dec. 17, 2009. Accessed Jan. 28, 2010.
  28. Fankhauser, Glen, and Kenneth B. Cumming "Snake Hybridization: A Case for Intrabaraminic Diversity" in A. A. Snelling (Ed.) (2008). Proceedings of the Sixth International Conference on Creationism (pp. 117–132). Pittsburgh, PA: Creation Science Fellowship and Dallas, TX: Institute for Creation Research.
  29. Lightner, Jean K. "Genetics of Coat Color, The Melanocortin 1 Receptor (MC1R)" Answers Research Journal 1 (2008): 109-116.
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