Population Genetics: Mechanisms of Evolution in Populations

Population Genetics

Introduction

Population genetics is the study of genetic variation within populations and examines how the frequencies of alleles and genotypes change over time. This field provides the foundation for understanding evolutionary processes and how populations, rather than individuals, evolve.

How Populations Evolve

Populations, Not Individuals, Evolve

• Natural selection acts on individuals, but only populations evolve over generations.

• Evolution acts on heritable traits that vary between individuals in a population.

• Individuals in a population may have different phenotypes (observable characteristics) due to different genotypes (genetic makeup).

• A population is a group of individuals of the same species that live in the same area and interbreed.

• Example: Variation in coat color among horses is due to genetic differences; over time, the frequency of certain coat colors may change in the population.

Genetic Variation in Populations

Sources of Genetic Variation

• Sexual reproduction leads to recombination, producing novel genotypes.

• Independent assortment during meiosis shuffles alleles, creating new combinations without changing allele frequencies.

• Mutation introduces new alleles by altering DNA sequences. Mutations can occur in somatic cells (not inherited) or germ cells (heritable).

• Mutations may involve changes in the number or position of genes and are occasionally beneficial.

• Genetic variation is essential for evolution to occur, but its presence alone does not guarantee evolution.

Determining if a Population is Evolving

Using Allele Frequencies and the Hardy-Weinberg Equation

• Variation within a population is necessary for evolution, but not all variation leads to evolutionary change.

• To determine if a population is evolving, scientists measure the frequency of alleles and genotypes and compare them to expected values under Hardy-Weinberg equilibrium.

Allele Frequencies

Calculating Allele Frequencies

• The total number of alleles in a population equals the number of individuals multiplied by 2 (for diploid organisms).

• The sum of all allele frequencies in a population equals 1.

• Formula: $p + q = 1$ where p is the frequency of one allele (e.g., dominant), and q is the frequency of the other allele (e.g., recessive).

• Example: In a population of 500 flowers with incomplete dominance:

  • Homozygous dominant (CRCR): 320 individuals

  • Heterozygous (CRCW): 160 individuals

  • Homozygous recessive (CWCW): 20 individuals

  • Total alleles: 500 x 2 = 1000

  • Number of CR alleles: (320 x 2) + 160 = 800

  • Number of CW alleles: (20 x 2) + 160 = 200

  • Frequency of CR (p): 800/1000 = 0.8

  • Frequency of CW (q): 200/1000 = 0.2

  • Check: 0.8 + 0.2 = 1

Hardy-Weinberg Principle

Definition and Equation

• The Hardy-Weinberg Principle (or equilibrium) states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary influences.

• The Hardy-Weinberg equation predicts expected genotype frequencies based on allele frequencies:

$p^2 + 2pq + q^2 = 1$

• p2: Expected frequency of homozygous dominant genotype

• 2pq: Expected frequency of heterozygous genotype

• q2: Expected frequency of homozygous recessive genotype

• Example: If p = 0.8 and q = 0.2:

  • $(0.8)^2 + 2(0.8)(0.2) + (0.2)^2 = 0.64 + 0.32 + 0.04 = 1$

Conditions for Hardy-Weinberg Equilibrium

• No mutations

• Random mating

• No natural selection

• Extremely large population size

• No gene flow (no migration of individuals into or out of the population)

Mechanisms That Alter Allele Frequencies

Overview

• Evolutionary mechanisms that can change allele frequencies include:

• Natural selection: The only mechanism that consistently causes adaptive evolution.

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

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

• Mutation: Introduction of new alleles.

Genetic Drift

• Genetic drift is the random fluctuation of allele frequencies from one generation to the next.

• It has a greater effect in small populations.

• Founder effect: When a new population is started by a small number of individuals, leading to different allele frequencies than the original population.

• Bottleneck effect: A sudden reduction in population size due to environmental events, resulting in a loss of genetic diversity.

• Example: Tristan da Cunha island was colonized by 15 individuals; among 240 descendants, 4 had retinitis pigmentosa, a much higher frequency than in the original population.

Gene Flow

• Gene flow is the transfer of alleles between populations due to the movement of individuals or gametes.

• It can introduce new alleles into a population or change existing allele frequencies.

• Gene flow tends to reduce genetic differences between populations.

• Example: Banding patterns in snake populations can change due to gene flow between populations with different patterns.

Natural Selection

• Natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution, increasing the frequency of beneficial alleles.

• It acts on phenotypic variation that is heritable.

• Modes of selection include:

  • Directional selection: Favors one extreme phenotype.

  • Disruptive selection: Favors both extreme phenotypes over intermediate forms.

  • Stabilizing selection: Favors intermediate phenotypes.

Summary Table: Mechanisms of Evolution

Mechanism	Description	Effect on Genetic Variation	Adaptive?
Natural Selection	Non-random increase in beneficial alleles	Can increase or decrease	Yes
Genetic Drift	Random changes in allele frequencies	Decreases (especially in small populations)	No
Gene Flow	Movement of alleles between populations	Can increase or decrease	No
Mutation	Introduction of new alleles	Increases	No (by itself)

Key Equations

• Allele frequency: $p + q = 1$

• Genotype frequency: $p^2 + 2pq + q^2 = 1$

Conclusion

Population genetics provides the mathematical and conceptual framework for understanding how evolutionary processes shape genetic variation in populations. By applying the Hardy-Weinberg principle and analyzing allele frequencies, biologists can determine whether a population is evolving and identify the mechanisms responsible for evolutionary change.