balanced polymorphism

(redirected from stable polymorphism)
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balanced polymorphism

[′bal·ənst ¦päl·i′mȯr‚fiz·əm]
(genetics)
Maintenance in a population of two or more alleles in equilibrium at frequencies too high to be explained, particularly for the rarer of them, by mutation; commonly due to the selective advantage of a heterozygote over both homozygotes.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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In apparent violation of this simple model leading to competitive exclusion, the evolution of stable polymorphisms in laboratory populations of bacteria has been clearly demonstrated.
It will be interesting to examine those asexual populations more thoroughly to discern whether some stable polymorphisms might have been overlooked.
The same numerical simulations as described above have show that for each possible value of r (0 [is less than] r [is less than or equal to] 0.5) the system has a stable polymorphism. The volume of the attracting domain is [nearly equal to] 90% of the simplex for small r and it reduces to 80% with increasing r.
We have here: (a) a stable polymorphism (initial point {[x.sub.1] = 0.057, [x.sub.2] = 0.01, [x.sub.3] = 0.761, [x.sub.4] = 0.172} at [S.sub.1] belongs to the attracting domain of this polymorphism); (b an attracting "supercycle" with the length of about 1000 environmental periods (the point {[x.sub.1] = 0.4, [x.sub.2] = 0.0, [x.sub.3] = 0.41, [x.sub.4] = 0.19} at [S.sub.1] could be used as one of starts to obtain this limiting trajectory); (c) an unstable cycle dividing the attracting domains a and b (an example of a point which belongs to this cycle is [x.sub.1] [nearly equal to] 0.232201, [x.sub.2] [nearly equal to] 0.002704, [x.sub.3] [nearly equal to 0.741072, [x.sub.4] [nearly equal to] 0.024021).
Therefore, it does not necessarily produce sign-concordant environment, and we can not exclude the possibility of stable polymorphism even in case of additive genes with equal effects.
It was shown earlier that stabilizing selection in constant environment for a trait controlled by equal genes with dominant effect or nonequal additive genes may result in stable polymorphism (e.g., Lewontin 1964; Hastings and Hom 1990).
Let F(z) = 1/{1 + [(z - Z)/[Sigma]]2 Provided selection is intense enough, stable polymorphism is possible in this case even for [d.sub.a] = [d.sub.b] and [h.sub.a] = [h.sub.b] = 0, and, needless to say, for [d.sub.a] = [d.sub.b] or nonzero [h.sub.a] or [h.sub.b].
A stable polymorphism is possible because the type suffering a higher herbivory load, i.e., ND, benefits more from the neighboring D-plants' defenses as the proportion of the latter increases.