On the Origins of New Forms of Life

7.8: Stability of Form



(Continued from the previous page)

It is often asserted that a given type of organism has a particular form and particular habits because it is constrained by the demands of its habitat. Many biologists consider natural selection to be a force that shapes every aspect of an organism to fit its environment. But this idea seems only to be a retention from an older system of thought. In early nineteenth century England the conventional idea was that "species" had been designed by God, each unique and perfect for the circumstances under which it lived. Darwin's contemporaries saw "species" as "complex mechanisms from the divine workshop, and exquisitely fitted to their places in the world," so obviously designed that their very existence seemed to imply the existence of a Designer. This doctrine (“physico-theology”) had a major proponent in English philosopher and theologian William Paley, whose arguments were, at one time, accepted by all "gentlemen of respectability." As a young man, Darwin himself seems to have embraced this perspective. While studying at Cambridge, Darwin had been required to read Paley. Late in life (1876), Darwin recalled his college experiences in his autobiography. There he writes

In order to pass the B.A. examination, it was also necessary to get up Paley's 'Evidences of Christianity,' and his 'Moral Philosophy.' This was done in a thorough manner, and I am convinced that I could have written out the whole of the 'Evidences' with perfect correctness, but not of course in the clear language of Paley. The logic of this book and, as I may add, of his 'Natural Theology,' gave me as much delight as did Euclid. The careful study of these works, without attempting to learn any part by rote, was the only part of the academical course which, as I then felt and as I still believe, was of the least use to me in the education of my mind.

From the study of Paley's works, Darwin retained in later life the idea that "species" are wondrously suited to "their places in the world." Thus, as an old man, he said that he had wanted his theory to "account for the innumerable cases in which organisms are beautifully adapted to their habits of life — for instance a woodpecker or a tree-frog to climb trees, or a seed for dispersal by hooks or plumes." Paley believed a sentient deity accommodated the structure of organic beings to particular ends. Darwin's faith was in an unconscious god, natural selection, which he believed to be omnipotent when allowed to act over geologic time. It, too, shaped organisms to particular ends. A tendency to describe the attributes of "species" with quasi-religious awe persists even today in biology. But it seems fair to say that the decision to describe animals as "wonderfully suited" or "exquisitely fitted" or "beautifully adapted" is a subjective judgment. On the objective level, one can say only that any organism not as yet extinct is "adequately suited" to the circumstances under which it finds itself.

Only when the traits in question can be construed as useful does neo-Darwinian theory satisfactorily explain why all individuals of a given type of organism are characterized by certain attributes. It is perhaps for this reason Darwin wished to see utility in every aspect of every organism. But even his allies often disagreed with him on this point. According to Desmond and Moore (1991: 246), “His relentless utilitarianism — his demand that every curlicue and hue of the oddest orchid must function in order to be selected — made Huxley wince.” For example, Darwin claimed in the Origin (1859: 200) that

every detail of structure in every living creature (making some little allowance for the direct action of physical conditions) may be viewed, either as having been of special use to some ancestral form, or as being now of special use to the descendants of this form either directly, or indirectly through the complex laws of growth. [italics added]

In fact, the traits that consistently distinguish one form treated as a species from another often seem to lack any utility, and yet remain unchanged, generation after generation. As Bateson puts it,

more often they are just those features which seem to us useless and trivial, such as the patterns of scales, the details of sculpture on chitin or shells, differences in number of hairs or spines, differences between the sexual prehensile organs, and so forth. These differences are often complex and are strikingly constant; but their utility is in almost every case problematical.

Many physical, chemical, and mathematical systems tend toward particular states. When perturbed, a pendulum will eventually return to a stationary, plumb position. When the square root of any positive number is repeatedly taken, the series of results will converge on 1. When hydrogen and oxygen are combined, they burn to form water. These stable states towards which processes tend are called absorptive. Thus, if a group of mating organisms is thought of as a process, what would constitute an absorptive state of that process? Clearly, one such state would be that in which all individuals in the group share an identical fully paired karyotype because any individual that deviates from this state either will be of reduced fertility or produce progeny of reduced fertility. Typically, in an unstable hybrid population, most individuals have karyotypes that are not fully paired and, consequently, are of low fertility. If a fully paired karyotype does appear in any generation, in any small group of individuals engaging in sexual reproduction, then that group will have a tremendous reproductive advantage over the other hybrids in the population. If members of that group are well enough suited to the environment in which they happen to find themselves, they will rapidly increase and become a stable chromoset (McCarthy 1995; McCarthy et al. 1995). The process producing such a recombinant derivative leads to a new stable genetic state.

Note that stabilization processes are not only stabilizing, but also formative. For example, in the case of the production of a recombinant derivative from interchromoset matings, a given hybrid variant with poor pairing might be extremely well suited to the environment in which it found itself, but be at an extreme disadvantage relative to a fully paired type when it came to producing gametes. Thus, if the paired type were even reasonably suited to its environment, it would win out. The process is formative because the genes contained in the winning, fully paired karyotype specify a particular genetic program and, thus, determine the development of a new type of organism.

More broadly, any stabilization process involving chromosomal mutation leads to an absorptive state when it gives rise to a new life cycle in which a particular karyotype regularly gives rise to an identical offspring karyotype. For example, when two lizards hybridize to create a female parthenogen, they create a new form of life with a stable life cycle in which the karyotype is regularly replicated. Likewise a new type of grass produced by the crossing of two other grasses might be sterile but be capable of stable vegetative reproduction. In this case, too, the karyotype is stably replicated. The life cycles of such forms are perfect examples of an absorptive state — both reproduce clonally and do not change thereafter. The only possibility for change in such clonal lineages is point mutation. But point mutations are very rare, so rare in fact that distinct clones produced anciently by separate occurrences of the same type of hybrid cross are usually morphologically identical and extremely similar at the genetic level even today.

Thus, it isn't surprising that related forms treated as distinct species often differ with respect to useless traits. Under stabilization theory such traits can be viewed as by-products of the karyotypic restructuring characteristic of stabilization processes. This is true especially if such processes are assumed to be the typical source of new forms of life (as is the case under stabilization theory). Not all genes contained in the stable karyotype of a new type of organism produced by such a process would be advantageous. Some genes would be present only because they happened to be on a chromosome that happened to be present when the new karyotype first stabilized and the new chromoset became established. The total complement of genetic material defined by the stabilized karyotype would program the development of a particular stable somatype. Therefore, traits would tend to persist even if they were not beneficial, and would come to characterize the chromoset. Even detrimental traits, such as hereditary diseases, might persist for many generations if the genes specifying them were present in the initial karyotype that originally conferred reproductive stability (and no alternative, more favorable alleles existed in the chromoset). NEXT PAGE >>


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