Population Genetics and Natural Selection: Study Notes

Population Genetics and Natural Selection

Introduction

Population genetics is the study of genetic variation within populations and how evolutionary forces such as natural selection, genetic drift, mutation, and gene flow influence allele frequencies over time. This section covers foundational concepts including Darwin's theory of natural selection, Mendelian genetics, variation within populations, and the Hardy-Weinberg equilibrium model.

Darwin & Mendel

Darwin's Theory of Natural Selection

Charles Darwin proposed that species evolve over time through a process called natural selection, where individuals with advantageous traits are more likely to survive and reproduce.

• Natural Selection: The differential survival and reproduction of individuals due to differences in phenotype.

• Adaptation: Evolutionary process resulting in improved ability of a population to live in a particular environment.

• Inheritance: Darwin recognized the importance of inheritance but the mechanisms were unknown during his time.

• Historical Context: Darwin was influenced by Thomas Malthus' essay on population growth and competition.

Example: Darwin's observations of finches in the Galápagos Islands led him to conclude that populations evolve from ancestral forms.

Mendelian Genetics

Gregor Mendel, through experiments with garden peas (Pisum sativum), discovered the basic principles of heredity.

• Genes: Units of inheritance passed from parents to offspring.

• Alleles: Alternate forms of a gene; some alleles can mask the expression of others (dominance).

• Phenotype: Observable traits of an organism.

• Genotype: Genetic makeup of an organism.

• Mendel's Laws: Developed rules to predict inheritance patterns.

Additional info: Mendel's work laid the foundation for modern genetics, complementing Darwin's theory and leading to the synthesis of evolutionary biology.

Variation Within Populations

Genetic and Phenotypic Variation

Variation within populations is essential for evolution and can be studied using morphological and molecular approaches.

• Genotypic Variation: Differences in genetic makeup among individuals.

• Phenotypic Variation: Observable differences resulting from genetic and environmental influences.

• Phenotypic Plasticity: The ability of an organism to change its phenotype in response to environmental conditions.

• Locally Adapted Populations: Populations within a species may be genetically distinct and adapted to local environments.

Example: Potentilla glandulosa (sticky cinquefoil) exhibits phenotypic plasticity in response to environmental variation.

Hardy-Weinberg Equilibrium Model

Principles and Equations

The Hardy-Weinberg equilibrium model provides a mathematical framework to study allele and genotype frequencies in populations under ideal conditions.

• Definition: In a large, randomly mating population with no evolutionary forces acting, allele and genotype frequencies remain constant from generation to generation.

• Allele Frequencies: For two alleles (e.g., S and A), the sum of their frequencies equals 1.

Key Equations:

• Allele frequency equation: $p + q = 1.0$

• Genotype frequency equation: $p^2 + 2pq + q^2 = 1.0$

Example: If the frequency of S allele is 0.81, SA is 0.18, and AA is 0.01, then:

• Frequency of S allele ($p$): $p = 0.81 + 0.5 imes 0.18 = 0.90$

• Frequency of A allele ($q$): $q = 0.01 + 0.5 imes 0.18 = 0.10$

Conditions for Hardy-Weinberg Equilibrium

For a population to remain in Hardy-Weinberg equilibrium, the following conditions must be met:

• Random mating

• No mutations

• Large population size

• No immigration or emigration (gene flow)

• No natural selection (equal fitness among genotypes)

Additional info: In reality, these conditions are rarely all met, so allele frequencies often change over time.

Calculating Gene Frequencies: Example Table

Gene frequencies can be calculated from observed genotype frequencies. For example, in a population of Harmonia beetles with multiple color pattern alleles:

Genotype	Frequency (%)
SS	81
SA	18
AA	1

Additional info: From these frequencies, allele frequencies can be calculated as shown above.

Summary Table: Hardy-Weinberg Conditions

Condition	Description
Random Mating	Individuals pair by chance, not according to genotype or phenotype.
No Mutations	No new alleles are generated by mutation.
Large Population Size	Reduces the impact of genetic drift.
No Immigration/Emigration	No gene flow into or out of the population.
No Natural Selection	All genotypes have equal fitness.

Conclusion

Population genetics integrates Darwin's theory of natural selection and Mendelian genetics to explain how genetic variation is maintained and how populations evolve. The Hardy-Weinberg equilibrium model serves as a baseline to detect evolutionary change and understand the forces that shape genetic diversity in natural populations.