On the Origins of New Forms of Life
7.15: Hybrids versus stable types
EUGENE M. MCCARTHY, PHD GENETICS
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New forms arising as polyploids, agamosperms, or vegetatively reproducing organisms are stable and uniform as soon as they come into existence. Although many of these are of hybrid origin, they do not have the characteristics often associated with the name hybrid (inviability, infertility, variability). However, all recombinant derivatives, in getting established, pass through a stage where they are at least variable. In the case of those produced by interchromoset hybridization, there are generations in which many individuals are infertile and/or inviable as well. These unstable hybrid populations from which recombinant derivatives emerge are typeless because they are composed of a broad variety of genetically distinct individuals. They tend to be infertile and inviable, and to have a localized occurrence that's typically limited to the geographic region intervening between their parents' ranges. All these traits mean that these variable hybrids are relatively rarely seen in comparison with the fully fertile, viable, and widespread parental forms that produce them. They are the rare inhabitants of Goldschmidt's bridgeless gaps. The parents are numerous because they are chromosets composed of fully fertile and viable individuals sharing the same fully paired karyotype, which confers a stable reproductive cycle.
If we assume the typical form treated as a species arises by one of the various known types of stabilization processes, it becomes much easier to understand the observed discreteness between, and uniformity within, most populations treated as species. The karyotype corresponding to a particular chromoset specifies a particular invariant set of loci. Allelic variation can occur at each of these loci (and indeed, new alleles can arise through mutation at each those loci), but the set of loci present is stable. The fact that all individuals with the same karyotype share the same set of loci limits their development within a certain scope. In other words, the fact that a chromoset's members share a single karyotype makes them relatively uniform in morphology, as well as in other respects. For the same reason, distinct chromosets are morphologically discrete (so long as the distinct karyotypes defining them are distinct with regard to the genetic information they contain). There is no existing intermediate form between a polyploid and its parents because the process that produces a polyploid does not produce intermediate individuals with intermediate karyotypes. A single new karyotype is produced. Nor are there such intermediates in the case of new types of organisms that reproduce agamospermously or vegetatively. Even in the case of new recombinant derivatives, where intermediates do exist, they are relatively rare compared to the stabilized derivative or its parental forms -- the casual observer might not notice intermediates at all. So even here, the perception is one of discreteness. Only certain karyotypes both
- program the development of a viable organism
- and are reproductively stable.
These are stable points in the universe of all conceivable chromosome sets. They expand and preponderate. Karyotypic intermediates between such points correspond to organisms that are either entirely inviable, and so that do not exist, or that are so inviable that they are rarely seen. Thus, the prediction of stabilization theory is, in fact, the morphological discreteness commonly observed between the various named types.
The idea of the production of new forms as a process of rapid transition between points of long-term stability fits better with the saltationist ideas of Darwin's cousin, Francis Galton, than with those of Darwin himself. Galton (1869) compared the evolution of new types of organisms to a stone with many flat sides (“Galton's Polyhedron”). When such a stone is rolled across a table, it is unstable until it comes to rest on one of its facets. Once at rest, it becomes stable once more and requires considerable force to set it rolling again. Galton did not base this analogy on any known genetic mechanism—the science of genetics did not yet exist—but the analogy does extend to mechanisms known today. Each flat side can be seen as analogous to a stable karyotype with its corresponding chromoset and somaset. The rolling, unstable state is hybridization (with its disruption of stable reproductive cycles and resulting chromosomal reassortment and recombination). The force that pulls the unstable polyhedron down to rest on a new facet is selection for a new form that is both reproductively stable and adequately suited to its environment.
Goldschmidt's observation that populations treated as distinct species commonly have distinct karyotypes, separated by karyotypic gaps (intermediate karyotypes that do not actually occur), can also be explained by considering the implications of observed hybrid variation in light of what Cuvier called the Principle of the Conditions of Existence. Consider the innumerable different karyotypes produced by chromosomal mutations. Some will contain the necessary information to produce a viable individual and some will not. Only those with all of the genes necessary for survival will be reproduced and continue to exist. As Cuvier observed long ago,
Seen in this light, the ability to survive under a given set of conditions is not a consequence of environmental influences. It is a necessary attribute, demanded of each new form of life from the moment of its inception. This idea is by no means new. In speaking of the origin of the functional traits of organisms, Aristotle (Physics, Book II, Ch. 8) said it had probably been a matter of trial and error:
Long ago, Lucretius observed that
When a stabilization process creates a new stable chromotype, “the first beginnings” are in the initial cell in which a chromosomal mutation occurs. Often, that founding cell is a fertilized egg (i.e., a zygote). It may also be an unfertilized egg in which chromosome doubling occurs, or a single mutated somatic cell that amplifies by cell division and separates from the parent organism to live on as a new form of life. In all such cases a new karyotype arises. The organisms specified by such karyotypes may or may not survive to reproduce because, when such changes occur, it is uncertain what can and what cannot “rise into being.” Many will fail to meet the conditions of existence.
Some of these new forms are immediately stable. They reproduce clonally, by self-fertilization, or arise repeatedly in sufficient initial numbers to allow sexual reproduction. Other forms, derived from hybridization, are not immediately stable. They require more than a single generation to establish themselves and are initially far more variable than the parents that crossed to produce them. In these sexual hybrids, meiosis gives rise to a hypervariable array of gametes. Even one such hybrid may produce vast myriads of gametes, each with a distinct genetic content. Some of these germ cells may contain the proper genetic information to permit fertilization, the first step in the cycle of life. Those gametes lacking the requisite genes will degenerate and cease to exist. They do not fulfill the necessary conditions of existence. Again, the union of those gametes that do survive will form a variety of zygotes. Some of these will go on to develop into mature organisms. Those that do not are, again, those that fail to the meet the conditions of existence. They die as embryos, fetuses, infants, or juveniles—and their karyotypes pass out of existence with them. They are among the myriad non-occupants of the bridgeless gaps. Of all the populations produced by hybridization, only a few will have reproductive traits permitting continued existence in the absence of ongoing hybridization. Among these, those derived from hybridization among somatypes of the same chromoset will be maintained by the same sorts of forces described in neo-Darwinian theory. Those derived from hybridization between chromosets will maintain themselves only if they have a stable reproductive cycle. Each such population of the latter type will have a specific, new, stable karyotype common to all its members—it will be a new chromoset. With the passing generations, each such novel sexual chromoset will become increasingly stable and uniform as selection increases fertility and eliminates unfavorable variants. Of all these stabilized chromosets, some will continue to deal effectively with environmental demands. Those that do not will decline in number and cease to exist.
Conclusion. Under neo-Darwinian theory, at least eight separate phenomena remain opaque:
- The saltational pattern of the fossil record;
- The source of the extreme variation that natural selection would require for the rapid production of new types of organisms (which is mysterious when one thinks only in terms of normalizing selection for adaptive traits);
- How distinct karyotypes might arise during the production of new somatypes;
- Why closely related somasets often differ consistently with respect to seemingly useless traits;
- The prevalence of sexual, as opposed to agamospermous, reproduction;
- The existence of altruism;
- The occurrence of neuter forms among social insects;
- The origin of complex traits that seemingly have no function in an imperfect state.
Explanations in terms of genes (instead of chromosomes) are insufficiently comprehensive. They provide no adequate explanation of why populations treated as separate species commonly produce hybrids of reduced fertility. They also fail to explain why somasets and chromosets are typically stable, even when they interbreed with other somasets and chromosets. They don't explain why fossil forms typically remain unchanged for millions of years. Stabilization theory does account for all these phenomena and is thus a better explanation. NEXT PAGE >>