Population Genetics

Introduction to Population Genetics

Population genetics is the study of genetic variation within populations and how this variation changes over time. It integrates principles of Mendelian genetics with evolutionary theory to explain the distribution and behavior of genes in populations.

• Population: A group of individuals of the same species that occupy the same region and can interbreed.

• Gene Pool: The complete set of alleles for all genes in a population.

• Population geneticists analyze changes in the gene pool due to factors such as mutation, selection, drift, migration, and mating patterns.

Genetic Variation in Populations

Monomorphic and Polymorphic Genes

Genetic variation is observed as differences in alleles and phenotypes within a population.

• Monomorphic Gene: Exists predominantly as a single allele (≥99% of cases).

• Polymorphic Gene: Exists as two or more alleles in a population.

• Single-Nucleotide Polymorphism (SNP): A variation at a single base pair in DNA; accounts for ~90% of human genetic variation.

• Example: The human beta-globin gene has multiple alleles, including HbA (normal), HbS (sickle cell), and loss-of-function alleles.

Allele, Genotype, and Phenotype Frequencies

Population genetics relies on quantifying the frequencies of alleles, genotypes, and phenotypes.

• Allele Frequency: Number of copies of an allele divided by the total number of alleles for that gene.

• Genotype Frequency: Number of individuals with a specific genotype divided by the total number of individuals.

• Phenotype Frequency: Proportion of individuals with a particular phenotype; natural selection acts on phenotypes.

• Example Calculation: In a population of 100 frogs (64 GD GD, 32 GD GL, 4 GL GL): - Frequency of GL allele: - Frequency of GL GL genotype:

The Hardy-Weinberg Equilibrium

Hardy-Weinberg Principle and Equation

The Hardy-Weinberg equilibrium (HWE) provides a mathematical model for predicting genotype frequencies from allele frequencies in an ideal population.

• Equation: and Expands to: Where p = frequency of one allele, q = frequency of the other allele.

• Genotype Frequencies: - Homozygote 1: - Heterozygote: - Homozygote 2:

• Example: If , : - (64%) - (32%) - (4%)

Conditions for Hardy-Weinberg Equilibrium

• No new mutations

• No genetic drift (infinitely large population)

• No migration

• No natural selection

• Random mating

In reality, these conditions are rarely met, so populations often deviate from HWE.

Testing for Hardy-Weinberg Equilibrium

• Use a chi-square test to compare observed and expected genotype frequencies.

• If the null hypothesis (population is in HWE) is not rejected, the population is considered in equilibrium for that gene.

Microevolution: Forces That Change Allele Frequencies

Overview of Microevolution

Microevolution refers to changes in a population's gene pool from generation to generation, driven by several factors:

• Mutation

• Genetic drift

• Migration (gene flow)

• Natural selection

• Nonrandom mating

Table: Factors That Govern Microevolution

Natural Selection

Principles of Natural Selection

• Allelic variation arises from mutations and other sources.

• Some alleles confer higher survival or reproductive success.

• Individuals with beneficial alleles are more likely to reproduce.

• Over generations, allele frequencies shift, leading to adaptation (adaptive evolution).

Darwinian Fitness and Selection Models

• Darwinian Fitness (w): Relative likelihood that a genotype will survive and contribute to the next generation.

• Assign the highest fitness value as 1.0; others are relative to this.

• Example: If AA produces 5 offspring, Aa 4, aa 1: - - -

General Selection Model

• Mean Fitness ():

Patterns of Natural Selection

• Directional Selection: Favors one extreme phenotype (e.g., dark-colored butterflies become more common due to predation pressure).

• Balancing Selection: Maintains two or more alleles (e.g., heterozygote advantage in sickle cell anemia).

• Disruptive Selection: Favors two or more different phenotypes (e.g., shell color in snails varies by habitat).

• Stabilizing Selection: Favors intermediate phenotypes (e.g., optimal clutch size in birds).

Examples and Applications

• Directional Selection: DDT resistance in mosquitoes increases over generations.

• Balancing Selection: Sickle cell allele (HbS) is maintained at high frequency in malaria-endemic regions due to heterozygote advantage.

• Disruptive Selection: Cepaea nemoralis snails show different shell colors favored in different habitats.

• Stabilizing Selection: Birds laying an intermediate number of eggs have the highest fitness.

Genetic Drift

Definition and Consequences

Genetic drift is the random fluctuation of allele frequencies from one generation to the next, especially significant in small populations.

• Can lead to fixation (frequency = 1) or loss (frequency = 0) of alleles.

• Reduces genetic variation over time.

Calculating Mutation and Fixation Probabilities

• Average Number of New Mutations: (N = population size, = mutation rate)

• Probability of Fixation: for a new mutation

• Probability of Elimination:

Bottleneck and Founder Effects

• Bottleneck Effect: Sudden reduction in population size (e.g., natural disaster) leads to loss of genetic diversity.

• Founder Effect: Small group establishes a new population with reduced genetic variation and potentially different allele frequencies (e.g., Old Order Amish and Ellis-van Creveld syndrome).

Migration (Gene Flow)

Effects of Migration

• Migration introduces new alleles and changes allele frequencies in recipient populations.

• Gene flow tends to reduce differences between populations and increase genetic diversity within populations.

Migration Formula

• Change in allele frequency: Where = change in allele frequency in the conglomerate, = donor allele frequency, = recipient allele frequency, = proportion of migrants.

• Example: , , New allele frequency:

Nonrandom Mating

Types and Effects

• Random Mating: Individuals pair by chance, regardless of genotype or phenotype.

• Assortative Mating: Individuals select mates based on similarity (positive) or dissimilarity (negative) of traits.

• Inbreeding: Mating between genetically related individuals; increases homozygosity and can lead to inbreeding depression.

• Outbreeding: Mating between unrelated individuals; increases heterozygosity.

Inbreeding Coefficient (F)

• Quantifies the probability that two alleles are identical by descent.

• Calculated by tracing inbreeding paths in a pedigree.

Effects of Inbreeding on Genotype Frequencies

• Modified genotype frequencies: - - - Where f = inbreeding coefficient.

• Example: , , - - -

• Inbreeding increases homozygosity and decreases heterozygosity.

Sources of New Genetic Variation

Mechanisms Generating Variation

Mutation and Its Effects

• Mutations are random changes in DNA sequence or chromosome structure/number.

• Types: beneficial, neutral, or deleterious (most are neutral or deleterious).

• Mutation Rate: Probability a gene will be altered per generation (typically to ).

Exon Shuffling

• Exons (coding regions) from one gene are inserted into another, creating new proteins with novel domain combinations.

• Can be promoted by transposable elements or nonhomologous recombination.

• Example: A gene acquires a new exon, resulting in a protein with a new functional domain.

Horizontal Gene Transfer

• Genetic material is transferred between organisms without reproduction.

• Common in prokaryotes; can occur between prokaryotes and eukaryotes.

• Mechanisms: engulfment, conjugation, transduction, transformation.

Changes in Repetitive Sequences

• Short sequences (microsatellites/STRs, minisatellites) repeated in the genome.

• Repeat number can change via replication errors or recombination.

• Contribute to genetic diversity and are used in DNA fingerprinting.

Summary Table: Key Equations in Population Genetics

Conclusion

Population genetics provides a framework for understanding how genetic variation is generated, maintained, and altered in populations. It combines mathematical models with empirical data to explain evolutionary processes and the genetic structure of populations.