4.3: Stabilization Processes
(Continued from the previous page)
A stabilization process is any series of events in which selection for a stable reproductive cycle results in the production of one or more new, stable forms of life. Such processes usually involve some type of chromosomal mutation, although certain exceptions to this rule will be discussed.
Because (1) they usually require the occurrence of chromosomal mutations and (2) chromosomal mutations are ordinarily very rare events, one might suppose stabilization processes would be too rare to have any evolutionary significance. However, stabilization theory maintains that they are, if measured in terms of the number of new forms that they produce, in no way exceptional. In fact, it will be claimed that the typical new form is the product of such a process. It will be argued that such processes are rare events only in the sense that the vast majority of individuals composing any given natural population do not participate in a stabilization process that ends up producing a new type of organism. But this sort of rareness does not detract from the significance of such processes -- when they do occur, stabilization processes produce a new stable form with a new stable reproductive cycle. In other words, a rare, brief series of genetic events produces a permanent change.
Ordinary experience affords examples of events analogous to stabilization processes. For example, a man throwing a ball for a dog might complete ten thousand cycles of throwing and fetching, but on the ten-thousand-and-first throw the ball might lodge in the crotch of a tree. The dog would then be unable to return it. This single, extremely rare event breaks the cycle of throwing and fetching. It produces a permanent new state of affairs. In the same way, the cycle of meiosis and fertilization stably reproduces a particular karyotype until some rare disruptive event, such as a doubling of the chromosome number or the introduction of new chromosomes via hybridization, breaks the cycle and produces something new.
Each time meiosis produces a gamete, a chromosomal mutation can occur. Every time two gametes produced by two distinct chromotypes unite in fertilization, a chromosomal mutation does in fact occur. Each such chromosomal mutation can initiate the production of a new form via a stabilization process. But the number of such fertilizations taking place in a single year is vast. Likewise, the number of gametes produced on this planet even in a single day is astronomic. Either of these numbers must far and away exceed the number of forms of life that have existed in all the past eons of geological time. Therefore anyone wishing to assess the evolutionary significance of stabilization processes should ignore the fact that only a tiny fraction of meioses and fertilizations would be expected to initiate a stabilization process. Rather, they should consider whether that tiny fraction of such an enormous number might represent a significant number of forms. But first let's consider what different sorts of stabilization processes are known and look at some examples of each. NEXT PAGE >>
4.3: Stabilization Processes - © Macroevolution.net
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