Fisher's Theorem of Natural Selection, Polymorphism Natural Selection | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Natural Selection

• Natural selection in nature primarily operates through the survival of the fittest in specific environments.
• It effectively selects against weaker individuals, allowing only the strongest to survive and reproduce.
• An example of this can be seen in the study of wolves in Alaska by Murie, where weaker animals are preyed upon, leaving the fittest to reproduce.

Modern Observations on Natural Selection

• Recent evidence suggests a potential differential selection for survival among humans based on blood groups A, B, and O.
• For instance, individuals with blood group A may exhibit higher genetic carcinomas, while those with blood type O may have a higher incidence of peptic ulcers.

Factors Influencing Natural Selection

• Various factors determine the individuals that will reproduce, including differences in mortality rates, sexual activity duration, and fertility levels.
• In wild and domesticated populations, defective or detrimental genes tend to be eliminated through the survival of the fittest.

Fisher's Fundamental Theorem of Natural Selection

• Sir Ronald A. Fisher highlighted the importance of genetic variability in populations and its role in adaptation.
• The theorem states that the rate of increase in a population's fitness is directly linked to the amount of genetic variability present.
• Populations with greater genetic variation can adapt more quickly to new environments, as demonstrated by laboratory studies.

Genetic Variation and Evolution

• A Mendelian population represents the inherited variation on which natural selection acts, influencing the course of evolution.
• The response of genetic variability to changing environments shapes the evolutionary path of a species.

Mathematical Equation of Fisher's Theorem

• The rate of increase in an organism's fitness over time can be expressed as a mathematical equation.
• It illustrates that the fitness growth is proportional to the genetic variability within the population.

• Heterozygote advantage, known as overdominance for fitness, maintains equilibrium gene frequencies at intermediate levels.
• Heterozygote advantage is a key explanation for polymorphism.
• The effect of selection on polygenic characters can be measured using parameters like.
• The fitness of a population is influenced by changes in gene frequencies due to selection.
• The mean selective value of a population increases over generations due to additive genetic variance.

Additive Genetic Variance and Polymorphism

• Additive genetic variance (VA) influences average fitness and measures the effect of selection.
• Polymorphism arises from the presence of more than one allele at a locus, leading to multiple genotypes.
• Genetic polymorphism is maintained by various mechanisms, such as heterozygote advantage and frequency-dependent selection.

Heterozygote Advantage

• Frequency-dependent selection: Rare phenotypes may have advantages, leading to stable equilibrium gene frequencies.
• Examples include rare self-sterility alleles in plants and coloration patterns in snails.

Environment and Polymorphism

• In a heterogeneous environment, different alleles can be advantageous in different conditions, leading to stable polymorphism.
• Factors like dominance relations, mating preferences, and environmental heterogeneity influence the outcome.

Transition and Evolution

• Polymorphism may represent transitional stages in the evolutionary replacement of alleles due to environmental changes.

Neutral Mutation in IAS (M)

• Neutral mutation involves selection as the force responsible for polymorphism.
• Selection coefficients may be very small, making mutation rates significant.
• Mutations giving rise to protein polymorphisms are unique events.
• Probability of the same amino acid substitution by recurrent mutation is very small.
• Mutation and chance give rise to polymorphisms in finite population sizes.
• Unique mutations have a small chance of survival and spreading through the population.
• New mutants take a long time to spread through the population, contributing to polymorphism.
• Spread of mutants through the population is erratic due to chance-dependent gene frequency changes.

Opinions on Neutral Mutation

• Neutralists believe many allelic differences are effectively neutral, while selectionists hold the opposite view.
• Evidence obtained is consistent with either viewpoint, with some cases proving selection operates.
• Existence and persistence of genotypic polymorphism is attributed to selective processes by R.A. Fisher and E.B. Ford.
• Selection for heterozygotes and other selective forces contribute to the retention of frequent alleles in a population.

Biochemical Polymorphism

• Biochemical polymorphism refers to molecular variants of proteins and other compounds.
• Despite demonstrations of relationships to fertility, survival, or productivity, their existence remains a puzzle.
• Blood groups and protein polymorphism are useful in determining accuracy of parentage records.