On the Origins of New Forms of Life
7.5: From One Karyotype to Another
EUGENE M. MCCARTHY, PHD GENETICS
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Any acceptable theory of evolution should account for the genesis of the karyotypic differences that so often distinguish distinct types of organisms. But neo-Darwinian theory falls short in this respect. Most biologists, population geneticists in particular, weren't convinced by Goldschmidt's explanation of the fact that distinct chromosets are often treated as distinct species. They dismissed the data on karyotypic differences along with Goldschmidt's explanation of that data, and chose instead to focus on mutations in genes and the long-term effects of meiotic recombination. Attributing all evolutionary change to such phenomena, they neglected stabilization processes and chromosomal mutations. To this very day, karyotypic differences tend to be disregarded by many evolutionary biologists, as is the fact that stabilization processes can disrupt karyotypes and restabilize them in new forms.
Stabilization theory provides a clear and simple explanation of this phenomenon. It assumes new stable forms are typically produced by stabilization processes. Therefore, under stabilization theory it is to be expected that distinct, related forms would commonly have distinct karyotypes. New types of organisms produced by such processes would usually have new karyotypes because such processes typically do reassort and rearrange chromosomes. As we have seen, the effect of stabilization processes on karyotypes is known from direct observation. By definition, new polyploids always have different karyotypes from their parents. In general, new agamosperms do too. In those cases where their parents' karyotypes differ, so do forms that stabilize as new recombinant derivatives. The same is often true of a vegetatively-reproducing form produced by hybridization. As we have seen (Chapter 6), the geological record indicates new somatypes typically arise abruptly and remain recognizably the same for many millions of years. Such observations would be expected whenever a somatype arose via a chromosomal mutation producing a new chromoset. Under such circumstances, the members of a chromoset would be expected to be relatively homogeneous in form and comparatively distinct from those of other, preexisting chromosets because the new karyotype defining the chromoset would lock in a particular set of loci. A particular gene might vary from one individual to another (in other words, at a given locus there would be allelic variation of genes), but the set of loci present in the karyotype would not, which in such cases would place a limit on genetic variation. By defining these limits, it would therefore, to a great extent, stabilize the morphology of the new form.
The new karyotype would also restrict the scope of morphological variation over time because the karyotype of parent and offspring would contain the same set of loci, generation after generation. Any variation present in the population subsequent to stabilization would be restricted to allelic variation at the various stable loci. Such allelic variation would be the result of intrachromoset meiotic recombination or point mutation. Recall from Section Three that any ongoing production of variation from either of these two sources would be minor in comparison with the initial leap that occurs when a new stable form is produced by a chromosomal mutation. For those organisms that reproduce sexually, the stability of meiosis in large groups of individuals with identical, fully paired karyotypes would permit new chromosets to maintain themselves largely unchanged, indefinitely (the exact genetic basis of the stability of sexual chromosets is spelled out in Appendix I). Organisms whose life cycles do not involve meiosis reproduce clonally. So they, too, would be expected to vary little with time. Even those exceptional organisms whose peculiar meiotic mechanisms permit a certain amount of karyotypic variation (e.g., the fruit flies of the genus Drosophila) are constrained to a degree by the adverse affects of structural heterozygosity. So that they, too, are bounded with respect to morphological variability by karyotypic stability.
These considerations provide a mechanistic explanation for the long-term evolutionary stasis that paleontologists have observed in fossil forms. It would be a simple consequence of the fact that the typical form treated as a species has a stable reproductive cycle. Once a stabilization process produces a new form with a particular karyotype and that form establishes a stable reproductive cycle, a particular set of chromosomes is repeatedly passed, unaltered, from parent to offspring in the same way, generation after generation. Any ongoing minor variation observed between major chromosomal mutational events producing new stable forms can be attributed to ongoing intrachromoset recombination and occasional point mutation.
Stabilization theory assumes that stabilization processes are the main source of new types of organisms. It also assumes that ordinary reproductive processes are very accurate and that each form persists largely unchanged until some stabilization process, usually triggered by hybridization, disrupts the ordinary life cycle and gives rise to a new form. These assumptions account for, and are consistent with, the observed stasis of fossil forms. Stabilization theory says the morphological stability seen in most fossil forms is simply the result of having a stable reproductive cycle. It also holds that ordinary interchromoset recombination produces ongoing minor variation, but that it does not usually accumulate to such a degree that a new form is taxonomically recognized. Instead, it equates the differences arising from such processes with what has been variously called accidental, individual, or fluctuating variation.
Point mutations, under stabilization theory, can accumulate over time within a single non-hybridizing population. Nevertheless, the theory claims that this process does not usually produce the sort of new forms normally treated as species. That role is assigned to chromosomal mutations, which, according to stabilization theory, cause the large abrupt steps seen in the fossil record. The theory portrays point mutations as significant only in the production of new traits. These claims are made because most fossil forms, especially those treated as distinct species, arise abruptly with numerous distinctive traits from the outset. The slow accumulation of point mutations cannot produce this effect. Therefore, it would simply be illogical to claim such a process does in fact produce such forms.
Just as Cuvier could not conceive that huge boulders had been transported far from their origins by the glaciers of past ice ages, Goldschmidt failed to consider the possibility that new types of organisms might be produced by stabilization processes, in which the new karyotype is frequently derived from not one, but two parents. In fact, such processes were poorly known when he presented his theories. He rejected gradualism, but retained a basic component of the gradualistic paradigm: the notion that a new form must be descended from one and only one form immediately ancestral to itself. In speaking of how new chromosomal patterns arise, he clearly stated his assumption that "one pattern is evolved from another one, which can hardly be doubted." With such an assumption, it is difficult to see how one karyotype could be changed into another, even with time, let alone abruptly as Goldschmidt suggested. Such changes would not be expected to spread through a population under the influence of natural selection — Neo-Darwinian theory says structural changes are unlikely to accumulate over time, because structural heterozygosity is usually associated with reduced fertility.
On the other hand, stabilization processes are known typically to result in the establishment of new chromosets, as has already been explained. It isn't necessary to suppose such processes produce viable organisms. It is a well-known, documented fact that they can. Indeed, many different hybrid crosses produce not only viable, but even partially fertile offspring. Such processes can combine the previously tested genes of two existing, viable types. There is no need to posit a lonely "hopeful monster" looking for a mate — such processes normally produce multiple individuals on an ongoing basis (as de Vries said long ago), which can mate among themselves or can backcross. In fact, in many cases, no partner is required to establish a new stable type (e.g., a new type of agamosperm, or an organism capable of self-fertilization or effective vegetative reproduction).
In organisms derived from stabilization processes, chromosomes are not simply rearranged, as Goldschmidt supposed. Such organisms are genuinely new: they have a new complement of genes. These genes can interact in new ways during the course of development and produce novel effects. The extreme variation generated by such processes would allow natural selection rapidly to create new types. As we have repeatedly seen (Chapter 4), new, stable forms can come into being via stabilization processes in as little as a single generation. Even the few generations required to generate a new stable recombinant derivative are an instantaneous blip in the context of geologic time.
On the basis of available evidence it does indeed seem probable that such processes are frequently, or even typically, the source of new types of organisms. If such is the case, the saltationist tradition, which has long sought an unknown force, will finally be vindicated — the mechanisms underlying the origins of new forms would, in fact, be distinct. They would be qualitatively different from those involved in ordinary reproduction. When a population geneticist asserts that the mechanisms of inheritance underlying "speciation" are the same as those involved in everyday reproduction, she means that they are governed by the same rules — those formulated by Gregor Mendel. But Mendel's Laws do not apply to the sorts of chromosomal processes typical of stabilization processes.