For full credit, write four small paragraphs explaining those four forms of sele
ID: 57470 • Letter: F
Question
For full credit, write four small paragraphs explaining those four forms of selection. Each paragraph should:
Explain what fitness values of the three genotypes define that particular form of selection (complete dominance, incomplete dominance, overdominance or underdominance).
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.
Based on the equilibria, state whether the selection model is likely to explain polymorphism in natural populations.
Explanation / Answer
An individual's fitness is found out by it's phenotype.
In complete dominance, fitness of AA and Aa genotypes are equal to each other. And the fitness of aa genotype is less than AA and Aa.These fitness values indicate that the dominant allele has an advantage over the recessive allele. This type of selection is directional selection and the individuals having the dominant allele A are favoured. Polymorphism occurs in natural populations, with one individual showing the dominant phenotype and the other showing the recessive phenotype.
In incomplete dominance, fitness is in the order:- fitness of AA>Aa>aa. This shows that the fitness value of homozygous dominant is twice as much the heterozygous individual. This type of selection is directional, in which the phenotype AA is favoured. This selection model gives three phenotypes, one each for homozygous dominant and homozygous recessive and one for heterozygous individual. Eg:- cross between red and white snapdragon gives pink.
In overdominance , fitness of heterozygote is highest. Fitness is in the order Aa>AA>=aa. This type of selection is stabilising. In such a selection, the intermediate phenotype Aa is favoured. This type of selection also gives three phenotypes, one and the best one for the heterozygous and slowly and steadily the homozygous individuals will be removed from the population. In this type of selection, genetic polymorphism is conserved.
In underdominance, homozygous individuals have higher advantage over the heterozygote. Fitness is in the order AA>=aa>Aa. This type of selection is disruptive and the individual having homozygous phenotype is favoured over the heterozygous phenotype. Genetic polymorphism is not conserved and heterozygous individuals undergo elimination.
Three types of equilibria are there, based on the type of selection. Stabilising selection, directional selection, and disruptive selection. For stabilising selection or stable equilibria, the intermediate phenotype (in this case, Aa) is favoured. Starting conditions which lead to stable equilibria is presence of alleles of both phenotypes in equal proportion.
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