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 introduces foundational concepts in evolutionary biology, focusing on the synthesis of Darwinian natural selection and Mendelian genetics, and the Hardy-Weinberg equilibrium model.

Darwin & Mendel

Darwin's Theory of Natural Selection

Charles Darwin proposed that populations evolve over time through the process of 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 did not know the mechanisms.

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

Mendelian Genetics

Gregor Mendel's experiments with garden peas (Pisum sativum) established the principles of inheritance, showing that traits are passed from parents to offspring in discrete units called genes.

• Gene: The basic unit of heredity; a segment of DNA that codes for a trait.

• Allele: 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.

• Example: Mendel's rules allow prediction of inheritance patterns in offspring.

Additional info: The synthesis of Darwin's and Mendel's ideas led to modern evolutionary biology, integrating natural selection with genetic inheritance.

Variation Within Populations

Genetic and Phenotypic Variation

Variation within populations is essential for evolution. It arises from differences in genotypes and can be studied using morphological and molecular approaches.

• Genotypic Variation: Differences in genetic makeup among individuals.

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

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

• Example: Potentilla glandulosa (sticky cinquefoil) shows local adaptation and genetic distinctiveness among populations.

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.

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

• Allele Frequency Equation:

• Genotype Frequency Equation:

• p: Frequency of one allele (e.g., S)

• q: Frequency of the alternative allele (e.g., A)

• p^2: Frequency of homozygous genotype (SS)

• 2pq: Frequency of heterozygous genotype (SA)

• q^2: Frequency of homozygous genotype (AA)

• Example: If S = 81%, SA = 18%, AA = 1%, then allele frequencies can be calculated using the equations above.

Conditions for Hardy-Weinberg Equilibrium

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

• Random mating

• No mutations

• Large population size

• No immigration or emigration (gene flow)

• Equal fitness among all genotypes (no natural selection)

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

Calculating Allele Frequencies: Example Table

The following table summarizes genotype and allele frequencies in a sample population (e.g., Harmonia beetles):

To calculate allele frequencies:

• Frequency of S allele:

• Frequency of A allele:

Evolution by Natural Selection and Change Due to Chance

Natural Selection and Genetic Drift

Evolution occurs when allele frequencies change over time due to natural selection or random events (genetic drift).

• Natural Selection: Favors alleles that confer higher fitness, leading to adaptation.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

• Example: Small, isolated populations are more likely to deviate from Hardy-Weinberg equilibrium due to chance events.

Additional info: Other evolutionary forces include mutation and gene flow, which also affect genetic variation.