Evolution of Populations

Introduction

This study guide covers the principles of population genetics and the mechanisms that drive evolution within populations. It explains how genetic variation arises and is maintained, the use of Hardy-Weinberg equilibrium in analyzing population genetics, and the major evolutionary processes including natural selection, genetic drift, migration, and non-random mating.

Population Genetics

Definition and Importance

• Population genetics is the study of genes and genotypes within populations, focusing on genetic variation and its relationship to phenotypic variation.

• It seeks to understand the extent, origin, maintenance, and changes in genetic variation over generations.

• Population: All members of the same species living in the same environment.

• Many species are divided into discrete populations across wide geographic ranges.

Gene Pool and Genetic Variation

• Gene pool: The complete set of alleles for every gene in a population.

• Polymorphic genes: Genes with two or more alleles present in the population, leading to trait variation.

• Monomorphic gene: A gene with only one allele in the population.

• Single nucleotide polymorphism (SNP): The most common type of genetic variation, involving a single nucleotide difference in a gene.

Allele and Genotype Frequencies

Definitions

• Allele frequency: The proportion of a specific allele among all alleles for a gene in a population.

• Genotype frequency: The proportion of individuals with a particular genotype in the population.

Calculating Frequencies

• For a gene with two alleles (e.g., CR and CW):

• Genotype frequencies are calculated by dividing the number of individuals with each genotype by the total population size.

• Allele frequencies are calculated by counting the number of each allele and dividing by the total number of alleles.

Example Calculation

• Suppose a population has 9 red-flowered plants (CRCR), 42 pink-flowered plants (CRCW), and 49 white-flowered plants (CWCW).

• Genotype frequency of CRCW = 42 / 100 = 0.42

• Allele frequency of CR = (2*9 + 42) / (2*100) = (18 + 42) / 200 = 60 / 200 = 0.3

• Allele frequency of CW = (2*49 + 42) / 200 = (98 + 42) / 200 = 140 / 200 = 0.7

Hardy-Weinberg Equilibrium

Concept and Equation

• The Hardy-Weinberg equilibrium describes the genetic makeup of a non-evolving population.

• Allele and genotype frequencies remain constant from generation to generation under certain conditions.

Hardy-Weinberg Equation:

• For two alleles, p (frequency of dominant allele) and q (frequency of recessive allele):

• = frequency of homozygous dominant genotype

• = frequency of heterozygous genotype

• = frequency of homozygous recessive genotype

Assumptions of Hardy-Weinberg Equilibrium

• No new mutations

• No natural selection

• Large population size (no genetic drift)

• No migration (gene flow)

• Random mating

If any of these conditions are not met, allele and genotype frequencies may change, indicating evolution.

Mechanisms of Evolution

Natural Selection

• Natural selection: Beneficial, heritable traits become more common in successive generations, leading to adaptation.

• Fitness: Measure of reproductive success; contribution to the gene pool of the next generation.

Patterns of Natural Selection

• Directional selection: Favors individuals at one extreme of a trait range (e.g., antibiotic resistance in bacteria).

• Stabilizing selection: Favors intermediate phenotypes, selecting against extremes (e.g., clutch size in birds).

• Disruptive selection: Favors two or more different genotypes, often in heterogeneous environments.

• Balancing selection: Maintains genetic diversity; two or more alleles are kept in balance over generations.

Balancing Selection Mechanisms

• Heterozygote advantage: Heterozygotes have higher fitness (e.g., sickle cell trait confers malaria resistance).

• Frequency-dependent selection: Fitness of a phenotype depends on its frequency; rare phenotypes may have higher fitness.

Sexual Selection

• Sexual selection: Selection directed at traits that increase mating success.

• Intrasexual selection: Competition among members of the same sex (often males) for mates.

• Intersexual selection: Mate choice by members of the opposite sex (often females), leading to showy male traits.

Genetic Drift

• Genetic drift: Random changes in allele frequencies, not due to fitness.

• Can lead to loss or fixation of alleles, especially in small populations.

Bottleneck Effect

• Population size is dramatically reduced by an environmental event.

• Surviving population may have different allele frequencies and reduced genetic variation.

Founder Effect

• Small group separates from a larger population to form a new population.

• Founding population may have less genetic variation and different allele frequencies.

Migration (Gene Flow)

• Migration: Movement of individuals between populations increases gene flow.

• Enhances genetic diversity within populations and reduces differences between populations.

Summary Table: Mechanisms Affecting Population Evolution

Key Equations

• Allele frequency:

• Genotype frequency:

Conclusion

Populations change over time due to mutation, natural selection, genetic drift, migration, and non-random mating. Understanding these mechanisms is essential for studying evolution and the diversity of life.

Additional info: The image of the peppered moths (Biston betularia) illustrates natural selection, where changes in environmental conditions led to shifts in population coloration due to differential survival.