Clearly many different alleles exist that control color and shape. This is an example of a polymorphic or variabile population. The cheetah is a species that suffers from low genetic variability. Because of hunting and the narrowing of its habitat, fewer and fewer cheetahs are found. Those that are found are often related. A population of closely related individuals exhibits low variability.
This is especially critical and dangerous if the environmental conditions change and the population does not have the variability to cope with the change.
That population could rapidly move toward extinction. One method of expressing variability is by analyzing the genetic data and expressing the data in terms of gene or allelic frequencies. Any gene will have at least two alleles. The summation of all the allelic frequencies, for all the genes that are analyzed in a specific population, can be considered a characterization of that population.
Any population can have a wide range of allelic frequencies for each of the genes that are being considered.
Furthermore, two populations do not necessarily have the same set of frequencies even though they are the same species. This is derived more thoroughly in table 5. Example: consider a sample of individuals with the following genotype frequencies:. Observed are different from expected, thus some force must be at work to change frequencies.
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If we consider a continuously distributed trait e. If after selection one extreme is most fit this is directional selection ; if the intermediate phenotypes are the most fit this is stabilizing selection ; if both extremes are the most fit this is disruptive selection. Fisher proposed a simple bookkeeping, or population genetics, approach for one locus with two alleles: we have AA, Aa and aa in frequencies p 2 , 2pq, q 2.
Define l ii as the genotype-specific probability of survivorship, mii as the genotype-specific fecundity. We build a model that will predict the frequencies of alleles that will be put into the gamete pool given some starting frequencies at the preceding zygote stage;. The gamete column is what determines the frequencies of A and a that will be put into the gamete pool for mating to build the next generation's genotypes. These fitness values will determine the contribution of that genotype to the next generation. Or for the a allele,. Note that the heterozygotes are not 2pq but pq because in each case they are only being considered for the one allele in question.
Note both allele frequencies and genotype frequencies have changed compare to what we saw with inbreeding. This can be continued with the new allele frequencies and so on.
When will the selection process stop? First three are directional in that selection stops only when allele is eliminated. In I the elimination process slows down because as q becomes small the a alleles are usually in heterozygote state and there is no phenotypic variance. In II selection is slow at first because with q small most genotypes are AA so there is low phenotypic variance; as selection eliminates A alleles q increases and the frequency of the favored genotype aa increases so selection accelerates.
IV is known as balancing selection due to overdominance heterozygotes are "more" than either homozygote.erp.legacyrealties.com/fushigi-ygi-genbu-kaiden-vol-1.php
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Both alleles maintained in population by selection. The frequencies of the alleles at equilibrium will be:. S should be eliminated because sickle cell anemia lowers fitness. S is maintained where malarial agent Plasmodium falciparum exists because AS heterozygotes are resistant to malaria. Note that S allele is very low frequency where there is no malaria the selective coefficient of S is different because the environment is different.
See figure 5. Another way that genetic variation can be maintained is through multiple niche polymorphism polymorphism maintained by environmental heterogeneity in selection coefficients. If different genotypes are favored in different niches, patches or habitats, both alleles can be maintained. Heterozygotes will have the highest average fitness although they are not the most fit in either habitat see figure 5.
The same dynamics would apply to temporal heterogeneity spring and fall; winter and summer assuming that selection did not eliminate one allele during the first period of selection. Classic example of temporal heterogeneity: third chromosome inversions of Drosophila pseudoobscura studied by T.
Natural Selection, Genetic Drift, and Gene Flow Do Not Act in Isolation in Natural Populations
Different chromosomal arrangements "Standard" and "Chiricahua" show reciprocal frequency changes during the year. Yet another way to maintain variation by selection is through frequency dependent selection. If an allele's fitness is not constant but increases as it gets rare this will drive the allele back to higher frequency. Example: allele may give a new or distinct phenotype that predators ignore because they search for food using a "search image" e. Most by no means all evolutionary biologists believe that selection plays a major role in shaping organic diversity, but it is often difficult to "see" selection.
When selection coefficients are large D p can be large, but the problem here is that with directional selection fixation is reached in a few generations and we still can't "see" selection unless we are lucky enough to catch a population in the middle of the period of rapid change. What affects the rate of change under selection? With some simple algebra we can rearrange this.
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