On the Origins of New Forms of Life
7.11: The Nature of Natural Selection
EUGENE M. MCCARTHY, PHD GENETICS
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Under stabilization theory, the primary factor governing the emergence of new types of organisms is natural selection for a stable reproductive cycle. Among the many different types of hybrids produced in a hybrid zone, a variety of types might survive to reproductive age and thrive as individuals. But no single type could emerge and establish itself as a new, stable form if it were incapable of stable reproduction. In sexual organisms, the major determinant of reproductive ability is chromosome pairing.† If the karyotype of a sexual organism is fully paired so that meiosis can proceed normally, then that organism will be much more fertile than one having a karyotype that is not fully paired. Thus, during the early stages of its emergence as a new form, a recombinant derivative derived from interchromoset matings would be subject to at least one strict constraint. It would need to stabilize its reproductive cycle via the reestablishment of chromosome pairing. For a recombinant derivative of intrachromoset mating, the factors contributing to stabilization would be the same as those acting to stabilize populations in neo-Darwinian theory (isolation, selection, random loss of alleles, etc.).
Of course, stabilization processes can equally as well produce forms that lack fully paired karyotypes. For example, many agamosperms are triploid, but have stable, clonal reproductive cycles. Forms that reproduce vegetatively, too, have no need of a fully paired karyotype. Nevertheless, any form lacking a stable reproductive cycle of some kind, resulting in the stable reproduction of a particular karyotype, will be ephemeral. It cannot persist as a stable type. Obviously, in order to get established, even a form with a stable reproductive cycle would need access to a suitable environment. Once established, the form could spread to a new environment only if that environment suited its nature.
But in most cases some degree of allelic variability would remain after the karyotype had stabilized. The exact range of that variability would be determined by the range of allelic variation occurring in the set of loci defined by the karyotype. This variation might be quite high in the case of a new recombinant derivative. Advantageous allelic variants would tend to persist, and deleterious ones, to die out. Under such circumstances, the long-term effect of selection would be the gradual diminution of variability.†† Nevertheless, in some cases even detrimental traits might linger for many generations (for example, if they were recessive, or if no more favorable allele existed). Thus, the sort of selection that occurs after karyotypic stabilization would be similar to that described in standard neo-Darwinian models (individual selection among variant alleles).
But the kind of selection that creates a new chromoset would, obviously, choose between karyotypes -- it would select a reproductively stable karyotype. That karyotype would contain a particular set of genes (and hence specify a particular set of traits). Those genes might vary to some degree between different members of the chromoset (there might be allelic variants at any given locus). But from the moment of its inception, the chromoset defined by the karyotype would have to (1) have a sufficiently stable reproductive cycle to get established, and (2) specify the development of an organism that was sufficiently suited to the environment in which it first found itself (otherwise it would not survive). Thus, under stabilization theory there is a broad brushstroke that initially creates a chromoset. It chooses not between individuals, but between karyotypes, and, consequently, between the somatypes corresponding to those karyotypes.
Under this view, phenomena are easily explained that are hard to understand under neo-Darwinism's perspective. For example, there has been much discussion of the question of why agamospermy is not more widespread (Maynard Smith 1971, 1978, 1988; Mogie 1992; Williams 1975). Such a trait would appear to be highly advantageous to any individual that possessed it. So orthodox theory predicts that far more different types of organisms should have it than the number that actually do. However, stabilization theory assumes the typical form treated as a species arises via a stabilization process and that most such processes involve hybridization. Hence, since hybridization requires the parents be capable of sexual reproduction, and since agamospermy only arises de novo from purely sexual parents in a small minority of crosses, most forms produced by stabilization processes will not be agamospermous.
Altruism: Male baboons will sacrifice their lives to kill leopards preying on their group. Credit: Charles J. Sharp
Similarly, neo-Darwinian theory fails to adequately account for the existence of altruism, since everything is there explained by the selfish needs of the individual. For example, at the approach of a predator some birds will give a warning call, even though that call endangers the individual making it. If the typical organism were shaped by selection for traits advantageous for the individual that possessed them, as neo-Darwinian theory claims, then the trait of being unselfish would be selected against. An identical difficulty pertains to the existence of social insects with distinct neuter forms. How do such forms arise gradually under the influence of selection if they do not produce offspring? Such phenomena have been explained in terms of "kin selection," in which the benefit of a trait to genetic relatives supposedly outweighs the detriment to the individual that possesses it. But kin selection is a notion many biologists don't accept. However, according to stabilization theory, the typical form treated as a species already has all of its characteristic traits at the time it first arises. Individual competition is not an important factor in stabilization theory. Therefore, such forms can be successful and yet be composed of individuals that cooperate and make sacrifices for each other or for the benefit of the whole (e.g., bees, baboons, humans) or they can be composed of selfish individuals that do not cooperate and make few sacrifices of any kind (e.g., sharks). Likewise, a recognized form can be composed of several distinct types from the time of its inception. For example, a stabilization process could give rise to a karyotype capable of reproducing itself via a cycle that also produced alternative karyotypes corresponding to neuter forms incapable of reproduction.
Critics of neo-Darwinian theory have objected that certain traits are useless except when perfectly formed. They say this fact proves natural selection could not form such traits in a gradual manner. But, as we have seen, gradual natural selection is not the only theoretical option. Every day, millions of highly variable gametes are randomly produced in the gonads of millions of hybrids in thousands of different hybrid zones worldwide. These gametes, and the offspring produced by them, are constantly winnowed because, if they are to continue to exist, they must have the necessary characteristics for survival. Extended over geological time, this process would generate an astronomical number of karyotypic variants and, thus, seems easily capable of creating a vast array of complex organic structures corresponding to those variants. Moreover, the number of ancestral point mutations would be far higher with hybrid evolution (see discussion of this issue). Also, the various stabilization processes not connected with hybridization (e.g., autopolyploidy) can produce forms with novel traits. Finally, it should be considered that under stabilization theory a trait need not be useful to be characteristic of an organism. So there is no need to assume an imperfectly formed trait has to be useful. For example, a form incapable of flight might have rudimentary wings and yet give rise to an offspring form with better developed wings and capable of flight. All these factors greatly increase the probability of the production of viable complex variants.
† Of course, other factors (e.g., viral and bacterial infection) can affect fertility, but such factors affect only certain types of organisms or individuals. In contrast, the effect of improper chromosome pairing on meiosis is a general phenomenon affecting the fertility of hybrids produced by a broad range of eukaryotes.
†† Therefore high levels of variability in an extant population would suggest a recent origin, and low levels, an ancient one.