A definition of natural selection can be given either for individuals or for forms of life. Conventional evolutionary theory describes evolution primarily as a matter of selection among individuals (and, sometimes, among groups) within reproductively isolated populations. In contrast, stabilization theory argues for the opposite view, that evolution is primarily a process that selects between distinct, stable forms or types of organisms and that selection between individuals has a negligible evolutionary effect. Under this view, the various stable forms are produced by non-Mendelian stabilization processes.
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Natural selection among individuals
Image: Wikimedia
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A definition of natural selection with regard to individuals (the definition given under conventional theory):
A natural process taking place within a population. During this process individuals with certain heritable traits produce more offspring than individuals lacking those traits. Since the traits are heritable, individuals with those traits become more common in the population.
A definition of natural selection with regard to forms of life (the definition given under stabilization theory):
A natural process taking place within successive similarity sets. A similarity set is a set of forms all of which can hybridize with at least one other form in the set. Some forms within such a set will hybridize to produce new types of organisms (or produce them by any other stabilization process) at greater rates than other forms in the set. The heritable traits of those forms producing more offspring forms will, obviously, be present in a higher proportion of offspring forms in subsequent similarity sets, than will the traits of those forms that do not succeed in producing so many offspring forms. With the passage of time, a higher proportion of forms in the similarity set will possess those traits that have assisted in the production of new offspring types (see figure at right).
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Natural selection among forms: A series of similarity sets showing
the spread over time of an advantageous trait (forms with this
trait shown in bold italic); (1) Initial similarity chain. A series of
five different types of organisms, A, B, C, D, E (adjacent forms in
the series are assumed to be more similar and hence more
hybridizable). At first, the only form with the beneficial trait is B;
(2) B has hybridized with C to produce offspring form OBC (in the
figure "O" stands for "offspring form"); (3) OBC has hybridized with
D to produce offspring form OBCD; (4) A has hybridized
with B to
create offspring form OAB; C has become
extinct; (5) A and D have
become extinct; (6) OBCD has hybridized with E to create
offspring
form OBCDE; (7) B has produced a tetraploid OT; E is now extinct.
From the set of forms in the original chain only B has survived; all
forms in the similarity chain now have the trait.
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