5.8: Known origins
(Continued from the previous page)
We have proof of evolution occurring via stabilization processes. A wide variety of forms treated as species are known to be derived from stabilization processes. Many have actually been reproduced from their natural progenitors. Their origins are known, and not a matter of mere theoretical conjecture. Moreover, there is reason to suppose forms derived from such processes are actually underreported because hybridization itself is underreported (read a discussion of this topic).
In contrast, claims that new types of organisms come into being gradually in isolation are weakly supported by observation. Since scientists cannot hope to observe gradual evolutionary processes in the direct way stabilization processes can be observed, it may well be asked: Has reproductive isolation brought about the gradual emergence of even a single form treated as a species? Many biologists would say that the typical organism comes into being in isolation in a gradual manner. But does this belief reflect empirical findings? Or is it a mere habit of thought, prompted by theory, but unsupported by evidence?
Given the briefness of human existence, proof concerning the gradual emergence of new types can only be based on fossil evidence. But the fossil record does not support the claim that new forms typically arise gradually (see Section 6). Of course, scientists look to nature and claim that certain natural populations seem to represent the various stages that might be expected to occur during gradual "speciation" in the past. Various authors offer plausible examples of this type. But the evidence they offer never seems actually to prove that the case in question is one of gradual origin. In other words, forms alleged to be of gradual origin, never seem to be of known origin. Certainly their origins are not known in the same sense that the origins of forms derived from stabilization processes are known. In point of fact, it seems that whenever a claim is made that a particular type of organism is of gradual origin it is always possible to construct some other, equally plausible scenario accounting for its origin in terms of some stabilization process.
The Guam Rail. For example, a colleague once claimed that the Guam Rail (Gallirallus owstoni) must be of gradual origin. His argument was that G. owstoni is flightless, occurs only on Guam, and that there are no other rails on that island, even in the fossil record, that might have hybridized to produce G. owstoni. Moreover, polyploidy is very rare among birds and vegetative reproduction unknown. On this basis he concluded that this bird must be of gradual origin.
However, his explanation of the origin of G. owstoni. is merely hypothetical and it's possible to construct alternative (equally hypothetical) scenarios that are just as plausible as his: First note that
Given these facts, we can equally well suppose, again, entirely hypothetically, that
It would be possible to continue fabricating such unsubstantiated histories indefinitely. For example, a second scenario consistent with stabilization theory might suppose multiple types of rail once existed on Guam and hybridized to produce the Guam Rail, and that these parental types were not preserved, or have not been detected, in the fossil record there. The mere construction of these two scenarios, of course, proves nothing. The important point is that the scenario constructed by my colleague doesn't prove anything either.
The Evolution of Horses. The evolution of horses is the example of gradualistic evolution that springs to many people's minds. They usually picture a series of fossil horses assembled in the 1870's by paleontologist Othniel C. Marsh. But even this well-known case is no longer considered valid. In his Encyclopedia of Evolution, Milner (1993: 222) notes that
Marsh's classic unilineal (straight-line) development of the horse became enshrined in every biology textbook and in a famous exhibit at the American Museum of Natural History. It showed a sequence of mounted skeletons, each one larger and with a more well-developed hoof than the last. (The exhibit is now hidden from public view as an outdated embarrassment.)" [parenthetic comment is Milner's]
These fossils were held up as a shining example of gradualism until Simpson (1951) finally showed Marsh's specimens were not even successive members of a single line of descent, let alone stages in an unbroken, gradually evolving lineage. Indeed, for all that is known, they might have been the products of a series of complex hybrid crosses. There is not enough information to decide.
At least Marsh tried to provide actual evidence of gradual evolution. In the writer's experience it seems biologists rarely even try to document claims of gradual origin (at least not in the rigorous sense that forms derived from stabilization processes have been documented). Instead the tendency is to offer unsubstantiated claims, such as my colleague's concerning the Guam Rail, as "proven" examples. Certainly, no one has documented the gradual origin of any type of organism in the absolutely unambiguous way that Rieseberg and his co-workers have demonstrated the hybrid origin of the sunflowers Helianthus anomalus and H. paradoxus (Rieseberg 1991; Rieseberg et al. 1990, 1991, 1993, 1995, 1996; Ungerer et al. 1998). There seems to be a double standard in the evidentiary requirements.
When they think about the origin of new forms of life, especially of new animal forms, most biologists still do not think of stabilization processes as typical. This attitude is counter-inductive: knowledge of the better researched cases should be applied in inferring the origins of those forms whose histories are more poorly known. At the very least, since those organisms whose origins are well-known to us typically do arise via stabilization processes, there is absolutely no reason to suppose those of unknown origin typically arise via a different and poorly documented process (the gradual accumulation of favorable mutations in reproductive isolation). Likewise, since most organisms of known origin are derived from processes involving hybridization, there is no reason to suppose ones of unknown origin are mostly derived from processes not involving hybridization. NEXT PAGE >>
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