(Continued from the previous page)
Although stabilization theory posits that saltation is generally the result of chromosomal mutations, it does include the supposition that some point mutations have a more or less marked effect, producing what amounts to an abrupt, permanent change. Thus, a mutation in a single gene will sometimes affect a prominent trait, such as flower color. However, a point mutation typically affects only a single character, or a set of related characters. For example, a mutation in a single pigmentation gene might affect one trait (e.g., fur color) or several different traits that all depend on that same gene (e.g., fur, skin, and eye color). Indeed, the decision to treat two forms as separate species is occasionally based on a distinction involving a single trait. Thus, Heiser (1966: 32) notes that when systematists make this decision, "Sometimes a single character is used; this may admittedly result in quite artificial groups but can sometimes be defended on the grounds of convenience." However, most biologists frown on such practice.
Suppose, then, that such a point mutation affected a prominent characteristic recognizable in a fossil specimen. If such a mutation occurred in some ancient population, then the mutated and unmutated types would today be observed as distinct fossil forms. Such would be the case even if the two types interbred on an ongoing basis. Under such circumstances a point mutation would have produced a change that would be perceived today as saltational. Likewise, even ordinary meiotic recombination without point mutation, when it involves genes affecting prominent characters, can produce a change that seems saltational. De Vries' O. rubrinervis and O. nanella are examples already discussed. But, again, under such circumstances multiple traits are not usually affected. Moreover, such recombination produces a permanent, stable change, only in the case of certain special genetic systems, such as de Vries' permanent translocation heterozygotes. In permanent translocation heterozygotes, trait changes due to recombination are stable because they occur so rarely that they do not fluctuate from one generation to the next. In most types of genetic systems, however, meiotic recombination results only in ongoing, fluctuating change.
Moreover, most point mutations do not produce obvious changes in the traits of organisms. And those that do produce such changes typically affect only one, or a few related, traits. There seem to be no examples of a point mutation affecting a wide variety of traits, as is typically the case with a chromosomal mutation. Chromosomal mutations, though, are indeed expected to affect many different traits because they typically involve many different genes. Indeed, a single chromosomal mutation often encompasses thousands of genes. And most cases of saltation do involve changes in many traits, and not an isolated change in a single salient character. So for these typical cases of saltation, point mutation is not a satisfactory explanation.
According to one proposed scenario, point mutation can account for saltations involving multiple character changes. This explanation supposes that a point mutation occurs in a gene affecting early development and that as a result many traits are altered by a change in a single gene. However, this explanation has several shortcomings. First, there is the fact that such a developmental mutation would not produce a change in karyotype. Organisms differing with respect to many traits and treated as distinct species commonly differ also with respect to karyotype. The idea of a point mutation in a gene governing development does nothing to explain the origin of such karyotypic differences. Second, many organisms treated as distinct species produce hybrids of reduced fertility. There is no reason to expect that a mutation in a gene affecting early development would result in the production of a new type producing infertile hybrids in matings with the preexisting type. Third, and most important, there seem to be no documented examples of such a developmental mutation producing a new type treated distinct species, while there are many documented examples of such forms being produced by chromosomal mutations.
It should also be noted that minor chromosomal mutations involving small blocks of DNA would be expected to affect far fewer traits than major ones involving multiple entire chromosomes. Because they would change fewer traits, lesser chromosomal mutations would have effects that would be expected to be more like those of a point mutation. However, despite this contingence of the lower end of the chromosomal mutation continuum with the upper end of the point mutation continuum, the effects of point mutations and chromosomal mutations are for the most part qualitatively distinct. Most chromosomal mutations involve whole chromosomes or large portions of chromosomes and affect many different traits. They are therefore on a different scale. Point mutations affect a single gene if they affect any gene at all. So they can be expected to have major effects only if they are allowed to accumulate. But most fossil forms arise abruptly with numerous distinctive traits. Therefore, even though it is true that some few forms have been treated as species on the basis of traits that could plausibly arise via a single point mutation, we certainly have no reason to suppose most forms so treated had such an origin, or even that many did. NEXT PAGE >>
Most shared on Macroevolution.net: