Population Genetics and Natural Selection: Foundations and the Hardy-Weinberg Principle

Population Genetics and Natural Selection

Introduction

Population genetics explores how genetic composition changes within populations over time, providing the foundation for understanding evolution by natural selection. This section covers the historical context, mechanisms of inheritance, genetic variation, and the Hardy-Weinberg equilibrium model.

Darwin & Mendel: Historical Foundations of Evolutionary Theory

Darwin's Theory of Natural Selection

• Charles Darwin visited the Galápagos Islands in 1835 and observed that populations evolved from ancestral forms.

• Inspired by Thomas Malthus (1838), Darwin proposed that individuals with favorable traits have a competitive advantage, leading to the theory of natural selection.

• Natural selection is the process where differential survival and reproduction lead to adaptation—populations become better suited to their environments over generations.

Key Point: The mechanism of inheritance was unknown during Darwin's time, limiting the explanatory power of his theory.

Gregor Mendel and Mendelian Genetics

• Gregor Mendel, an Augustinian monk, used scientific experimentation and mathematics to study inheritance in garden peas (Pisum sativum).

• He discovered that traits are passed from parent to offspring in discrete units called genes.

• Genes exist in alternative forms called alleles. Some alleles are dominant and mask the expression of others (recessive).

• Mendel developed rules to predict inheritance patterns, forming the basis of classical genetics.

The Synthesis: Modern Evolutionary Ecology

• The integration of Darwin's natural selection and Mendelian genetics led to the modern synthesis in evolutionary biology.

• Key terms: allele (variant of a gene), genotype (genetic makeup), phenotype (observable traits).

Variation Within Populations

Genetic and Phenotypic Variation

• Variation among individuals in a population arises from the combined effects of genes and environment.

• Traditional studies focused on morphological (physical) traits, while modern approaches use molecular techniques to analyze genetic differences.

Variation in Plant Populations

• Ecotypes: Locally adapted and genetically distinctive populations within a species.

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

• Example: Potentilla glandulosa (sticky cinquefoil) shows variation in growth and flowering time depending on elevation and climate.

Common Garden Experiment

• A method to distinguish genetic from environmental influences on phenotypic variation.

• Individuals from different populations are grown in the same environment to observe whether differences persist (indicating genetic basis).

Hardy-Weinberg Equilibrium Model

Principle and Purpose

• The Hardy-Weinberg equilibrium provides a mathematical model to study genetic variation in populations.

• It predicts genotype and allele frequencies under ideal conditions, serving as a null hypothesis for detecting evolutionary change.

• Evolution is defined as a change in gene (allele) frequencies in a population over time.

Hardy-Weinberg Equations

• For a gene with two alleles, S and A:

$p + q = 1.0$

$p^2 + 2pq + q^2 = 1.0$

• p = frequency of allele S

• q = frequency of allele A

• p2 = frequency of SS genotype

• 2pq = frequency of SA genotype

• q2 = frequency of AA genotype

Calculating Gene Frequencies: Example

• Given genotype frequencies: SS (81%), SA (18%), AA (1%)

• Calculate allele frequencies:

$p = ext{frequency of S} = rac{2 imes ext{SS} + ext{SA}}{2 imes ext{total individuals}}$

$q = ext{frequency of A} = rac{2 imes ext{AA} + ext{SA}}{2 imes ext{total individuals}}$

• Plug in the values to solve for p and q.

Conditions for Hardy-Weinberg Equilibrium

• Random mating

• No mutations

• Large population size

• No immigration or emigration (no gene flow)

• No natural selection (equal fitness among genotypes)

Note: In reality, at least one of these conditions is often violated, leading to changes in allele frequencies (evolution).

Application and Limitations

• Large, stable populations are more likely to approximate Hardy-Weinberg equilibrium.

• Small, isolated populations are more susceptible to evolutionary forces such as genetic drift.

Table: Hardy-Weinberg Genotype Frequencies

Genotype	Frequency (Equation)	Description
SS	$p^2$	Homozygous for S allele
SA	$2pq$	Heterozygous (one S, one A allele)
AA	$q^2$	Homozygous for A allele

Key Terms and Concepts

• Allele: Alternative form of a gene.

• Genotype: Genetic makeup of an organism (e.g., SS, SA, AA).

• Phenotype: Observable traits of an organism.

• Gene pool: The total collection of genes and alleles in a population.

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

• Gene flow: Movement of alleles between populations due to migration.

• Mutation: Random change in DNA sequence, introducing new alleles.

• Selection: Differential survival and reproduction of individuals with certain genotypes.

Summary

• Population genetics provides the framework for understanding how evolutionary processes such as natural selection, genetic drift, and gene flow affect genetic variation.

• The Hardy-Weinberg equilibrium serves as a baseline to detect when and how populations are evolving.