In the broadest sense, a stabilization process is any series of events in which selection for a stable reproductive cycle results in the production of one or more stable new forms or life. Most, but not all, such processes involve some form of chromosomal mutation.
|The Quebec hawthorn (Crataegus submollis), a tetraploid that differs from it its diploid relatives in many respects. For example, it has 10 stamens, while they have 20.|
For example, a parent form may produce an offspring form in which the chromosome number has been increased relative to the parent. A well-known, commonly occurring stabilization process of this sort is the production of a tetraploid offspring form by diploid parents (in the former the chromosome number is twice as high as in the later). To establish a distinctive new tetraploid form, then, different tetraploid individuals (themselves produced, perhaps, by matings between diploid parents) only need to mate with each other and produce more tetraploids. With tetraploids regularly producing tetraploid offspring like themselves, a new form has established a stable reproductive cycle—many genetic processes produce offspring unlike their parents, but only a small subset of those processes produce offspring that can go on to stably reproduce themselves. Many diploid organisms produce tetraploid offspring on an ongoing basis. As a rule, tetraploids differ from their diploid parents with respect to a variety of traits. Note that the genetic change that produces a tetraploid normally takes one or a few generations and that they are genetically stable in subsequent generations.
Another example of a stabilization process is that of a hybrid cross producing offspring that are capable of reproducing themselves. True, most hybrids are sterile—but many are not (this is discussed at length this website's discussion of hybridization). Moreover, it is now known that hybridization occurs frequently in a natural setting, and that fertility is a heritable trait that is variable in degree. So the relatively sterile hybrids produced by an initial cross can give rise to later generations in which fertility steadily improves under the influence of artificial or natural selection (breeders of animals and plants often take advantage of this fact). When a hybrid is capable of apomixis or vegetative reproduction—both very common circumstances—sterility is not even an issue.
A stabilization process can be likened to certain events in common experience. 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 type of organism 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 a new stable type.
A variety of stabilization processes are known. Their prevalence is discussed at length in a separate section of this website (there is a table of contents that provides an overview of the entire discussion of stabilization theory presented on this website). But now, let's take a look at the very solid and well-documented reasons underlying stabilization theory's claim that the typical life form is the product of a stabilization process (presented in Section 6 of this website).
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