Natural Selection

Definition and Overview

Basic Concept

Process by which individuals with favorable heritable traits produce more offspring, increasing allele frequencies over generations.

Population Impact

Changes genetic composition of populations; drives adaptation and speciation.

Genetic Basis

Acts on phenotypic variation linked to genotype; influences allele distribution through differential reproductive success.

Historical Background

Darwin and Wallace

Independently proposed natural selection as the mechanism of evolution in mid-19th century.

Pre-Darwinian Ideas

Earlier theories included Lamarckism and artificial selection; lacked genetic understanding.

Modern Synthesis

Integration of Mendelian genetics with Darwinian selection in 20th century; foundation of population genetics.

Mechanisms of Natural Selection

Variation

Heritable differences in traits; source: mutation, recombination, gene flow.

Differential Survival

Organisms with advantageous traits have higher survival rates.

Differential Reproduction

Higher reproductive success of individuals with favorable traits.

Inheritance

Traits passed to offspring; ensures persistence of advantageous alleles.

Types of Natural Selection

Directional Selection

Favors one extreme phenotype; shifts allele frequencies toward that trait.

Stabilizing Selection

Favors intermediate phenotypes; reduces variation around mean trait.

Disruptive Selection

Favors both extremes; may lead to polymorphism or speciation.

Balancing Selection

Maintains multiple alleles; includes heterozygote advantage and frequency-dependent selection.

Concept of Fitness

Definition

Relative reproductive success of a genotype or phenotype.

Absolute vs Relative Fitness

Absolute: number of offspring produced. Relative: fitness compared to others in population.

Components

Survival, mating success, fecundity, and offspring viability.

Role of Genetic Variation

Source of Variation

Mutation, recombination, gene flow; essential for selection to act.

Maintaining Variation

Balancing selection, heterozygote advantage, environmental heterogeneity.

Loss of Variation

Directional selection can reduce variation; genetic drift also influential.

Selection Pressure

Definition

Environmental factors influencing differential survival and reproduction.

Abiotic Factors

Temperature, humidity, toxins, climate changes.

Biotic Factors

Predation, competition, parasitism, sexual selection.

Natural Selection in Population Genetics

Allele Frequency Changes

Selection alters genotype frequencies; quantified by changes in allele proportions.

Hardy-Weinberg Principle

Baseline model assuming no selection; deviations indicate evolutionary forces.

Selection Coefficients

Measure strength of selection against or for alleles.

Mathematical Models

Selection Equation

Models allele frequency change per generation based on fitness differences.

Wright-Fisher Model

Incorporates genetic drift and selection in finite populations.

Fitness Landscapes

Visual models representing fitness peaks and valleys; adaptive evolution paths.

Δp = p * (w̄_A - w̄) / w̄where:p = allele frequency,w̄_A = average fitness of allele A,w̄ = mean population fitness,Δp = change in allele frequency per generation 

Examples in Nature

Industrial Melanism

Dark morphs of peppered moth increased during pollution, demonstrating directional selection.

Sickle Cell Anemia

Heterozygote advantage confers malaria resistance; example of balancing selection.

Antibiotic Resistance

Bacterial populations evolve resistance under drug selection pressure.

Limitations and Constraints

Genetic Correlations

Linked traits may constrain independent evolution.

Environmental Changes

Rapid shifts may outpace adaptive response.

Trade-Offs

Fitness gains in one trait may reduce others.

Implications for Evolutionary Biology

Adaptation

Natural selection shapes traits improving survival and reproduction.

Speciation

Divergent selection pressures contribute to reproductive isolation.

Conservation Genetics

Understanding selection aids in managing genetic diversity in threatened populations.

References

• Fisher, R.A., "The Genetical Theory of Natural Selection," Oxford University Press, 1930, pp. 1-272.
• Mayr, E., "Systematics and the Origin of Species," Columbia University Press, 1942, pp. 1-334.
• Hartl, D.L. & Clark, A.G., "Principles of Population Genetics," 4th Edition, Sinauer Associates, 2007, pp. 1-682.
• Endler, J.A., "Natural Selection in the Wild," Princeton University Press, 1986, pp. 1-336.
• Charlesworth, B. & Charlesworth, D., "Elements of Evolutionary Genetics," Roberts and Company Publishers, 2010, pp. 1-481.