5.3: Natural recombinant derivatives: Prevalence
(Continued from the previous page)
The Prevalence of Natural Recombinant Derivatives. Biologists tend to assume any organism of unknown origin is not of hybrid origin. Since the vast majority of forms treated as species are of unknown origin, this tendency poses a difficulty for anyone who attempts to argue that stabilization processes are probably far more prevalent than has been heretofore supposed. From a technical standpoint, it is far more difficult to determine the origin of a recombinant derivative than to determine that of a polyploid or agamosperm, which have complete sets of unaltered chromosomes that can easily be matched with those of putative parental organisms. Typically, in a derivative of interchromoset matings, only some of the chromosomes are present that were found in its parents. It is also more difficult to reproduce a natural recombinant derivative by artificially crossing its parents, than to reproduce a polyploid or an agamosperm by similar means. Stabilizing such a derivative may take many generations. Moreover, in a hybrid population derived from interchromoset matings, meiotic recombination breaks up and recombines any mismatched chromosomes in hybrids. Such breakups and recombinations occur repeatedly in every generation until the derivative is stabilized. As has already been mentioned, this extensive recombination makes it far more difficult to equate the chromosomes of a recombinant derivative's karyotype with the chromosomes of its parents. In many cases, only pieces of chromosomes can be identified as equivalent. In contrast, the chromosomes of organisms that reproduce vegetatively and/or agamospermously are easily equated with those of any potential parental organism; in such organisms the structure of the chromosomes normally remains unaltered because the stabilization processes producing organisms of this type do not ordinarily involve meiotic recombination. Nor do the processes producing new polyploids. The stabilization processes creating such forms therefore do not restructure the chromosomes. Due to the difficulties involved with documenting forms derived from recombinational stabilization, one expects the number of verified cases to be smaller than in the case of polyploids and agamosperms. And such is in fact the case. As Coyne and Orr (2004: 351) note, this may mean only that this mode of producing new forms "is difficult to detect and document." But well-documented examples do exist. Such forms have been treated as species or subspecies.
On the other hand, if one estimated the prevalence of the gradualistic processes described in orthodox theory on such a basis, one would be forced to conclude that such processes were either extremely rare or that they did not occur at all. There seem to be no documented cases in which an existing form treated as a species arose gradually from a preexisting one under natural circumstances. It is widely believed that many forms of unknown origin are the result of a gradual accumulation of favorable mutations in isolation, as neo-Darwinian theory claims. Such may indeed be the case. But well-documented examples are sparse. Certainly, even in the case of recombinant derivatives, the evidence is far more substantial than even the best-documented case of a natural somaset treated as species arising from a preexisting one by the gradual accumulation of favorable mutations in isolation. The question of the relative prevalence of stabilization processes versus the processes posited by neo-Darwinian theory must be evaluated by other criteria. The most reliable criterion is the fossil record, which will be discussed in Section 6.
One thing, however, does suggest interchromoset recombinational stabilization occurs frequently -- many closely related forms differ not only with respect to karyotype, but also with respect to the structure of individual chromosomes. Most of the stabilization processes described in Section 4 merely take intact chromosomes and recombine them into a new karyotype. They do not change the structure of individual chromosomes. For example, polyploidization multiplies the number of sets of chromosomes. Aneuploidization adds and subtracts chromosomes. But in both cases the chromosomes remain intact. However, as we have seen, interchromoset recombinational stabilization can rip chromosomes apart and reassemble the severed blocks into new, restructured chromosomes. Genes retain their relative order within those blocks, but the blocks themselves are rearranged into a new order. They may also be broken up and joined to blocks from other chromosomes.
Over the last three decades, studies comparing the chromosomes of a broad range of organisms have clearly demonstrated that such chromosomal blocks commonly occur in a rearranged order in different types of organisms (or broken up onto separate chromosomes). Such blocks are known as "syntenic groups." They are chromosome segments in which the same genes occur in the same order in different types of organisms. Typically, closely related organisms differ with respect to fewer such rearrangements. Those more distantly related differ with respect to more. For example, Ruvinsky and Graves (2005: 352) say that when human, mouse, cat, and cattle are compared, the chromosomes are "scrambled almost beyond recognition," but the same authors say the chromosomes of cattle differ from those of sheep with respect to only a few rearrangements. If many forms of life come into being via recombinational stabilization, one expects to find such a pattern. Closely related organisms would differ with respect to fewer rearrangements (they would be separated by only one or a few recombinational stabilization events). On the other hand, distantly related organisms would differ with respect to more (they would be separated by many such events). If relatively few forms came into being via interchromoset recombinational stabilization, one would not expect the phenomenon of chromosomal rearrangement to be so prevalent; there is no other well-characterized mechanism that would allow new chromosomal rearrangements to get established (since they are deleterious when rare and unlikely to spread in a non-hybridizing population).
Verne Grant was one of the primary proponents of the idea that recombinant derivatives of interchromoset mating can get established in a natural setting. He called this process "recombinational speciation" and called such strains "homoploid derivatives" to distinguish them from polyploid derivatives of hybridization. At one time, Grant thought the production of such derivatives was a common process in a natural setting. However, the large amount of work involved in his artificial extraction of a recombinant derivative from the cross Gilia malior x G. modocensis convinced him otherwise. The experiment was successful, but difficult. He concluded that the production of stable recombinant derivatives from interchromoset matings "is a far less common mode of speciation in plants than is amphiploidy" (Grant 1981: 270). But Grant reached this conclusion before reports of naturally occurring derivatives of this type became available. The necessary technology for positively identifying such forms was lacking at that time. Nor was there the widespread interest in identifying such populations there is today. Moreover, plants do not labor. If the same series of matings occurred in the wild that Grant carried out in the greenhouse, then a new form of life would appear. The amount of human labor required to reproduce such an event is irrelevant. Speaking of the production of fertile breeds derived from avian crosses, Buffon said long ago that "all that we can do by art, Nature, too, can do, and has done, thousands and thousands of times over." Computer simulations indicate the production of such recombinant derivatives is entirely feasible process. Indeed, judging from the simulations, the process seems almost inexorable when some of the potential derivatives of a cross are fitter than their parents (McCarthy et al. 1995). Both of the two most cited theoretical papers on the topic (Buerkle 2000; McCarthy et al. 1995) predict the production of new forms by this means is an entirely workable process (see Appendix F). Indeed, it is well known that the products of a wide variety of crosses exhibit hybrid vigor. So it is not at all surprising some hybrid forms are able to get established as new types since many are also partially fertile. It is for this reason, for example, that Slack (1979: 79) says hybrid carnivorous plants "are generally relatively easily grown as compared with their parents."
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