For full credit, write four small paragraphs explaining those four forms of sele
ID: 36413 • Letter: F
Question
For full credit, write four small paragraphs explaining those four forms of selection. Each paragraph should:
a. Explain what fitness values of the three genotypes define that particular form of selection (complete dominance, incomplete dominance, overdominance or underdominance).
b. Explain the evolutionary fate of a gene pool for that model (= the equilibria for the model). Explain how many equilibria there are. Explain the starting conditions that would lead to the stable equilibria. Explain the starting conditions that would lead to the unstable equilibria.
c. Based on the equilibria, state whether the selection model is likely to explain polymorphism in natural populations.
Explanation / Answer
The forms of selection come under Natural Selection. The four types of selection are:
Stabilizing selection
Directional selection
Disruptive Selection
Balancing selection
1) Stabilizing selection- In stabilizing selection, both ends of the frequency distribution are eliminated. it operates in constant or unchanging environment. It favours the average or normal phenotypes and introduces homozygosity in the population.
Stabilizing selection reduces variation but does not change the mean value. rates of evolution are typically very slow because natural selection is usually stabilizing.
2) Directional Selection: under directional selection, individuals at one end of the frequency distribution do extremely well, and so the frequency distribution of the trait in the subsequent generation is shifted from where it was in the parental generation. It occurs due to change in the environment in particular direction. It favours the phenotype which is extreme.
If directional selection operates for many generations, an evolutionary trend within the population results.
3) Disruptive selection: Under disruptive selection both extremes are favoured at the expense of intermediate varieties. it occurs when a population previously adapted to a non homologous environment is subjected to divergent selection pressure in different parts of its distributional areas. The previously homologous population break up into several different adaptive forms. Extreme values have highest fitness.
4) Balancing selection: Usually a gene is considered polymorphic if at least two alleles are present, with a frequency of at least one percent for the second most frequent allele. balancing selection is rare and is generally not considered for the explaination of the levels of genetic variations found in the natural populations.
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