Evolution of Populations

Introduction

The study of population genetics is central to understanding how populations evolve over time. It examines the genetic composition of populations, the sources and maintenance of genetic variation, and the mechanisms that drive evolutionary change. This guide covers key concepts such as genetic variation, Hardy-Weinberg equilibrium, patterns of natural selection, sexual selection, genetic drift, and migration.

Population Genetics

Definition and Scope

• Population genetics is the study of genes and genotypes within populations.

• It seeks to understand the extent of genetic variation, its origins, maintenance, and changes across generations.

• Genetic variation is closely linked to phenotypic variation—observable traits in organisms.

Population and Gene Pool

• A population consists of all members of the same species living in a specific environment.

• The gene pool is the complete set of alleles for every gene in a population.

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

Genetic Variation: Polymorphism

• Polymorphic genes have two or more alleles present in a population, leading to trait variation.

• Monomorphic genes have only one allele in the population.

• The most common type of genetic polymorphism is 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 a 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 (CRCR), 42 pink-flowered (CRCW), and 49 white-flowered (CWCW) plants.

• Genotype frequency of CWCW:

• Allele frequency of CW:

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 (dominant) and q (recessive):

• = frequency of homozygous dominant genotype

• = frequency of heterozygous genotype

• = frequency of homozygous recessive genotype

Conditions for Hardy-Weinberg Equilibrium

• No new mutations

• No natural selection

• Large population size (no genetic drift)

• No migration (gene flow)

• Random mating

In reality, these conditions are rarely met, so allele and genotype frequencies change over time, indicating evolution.

Application Example

• If the frequency of the dominant allele E for earwax type is 0.8, the frequency of the recessive allele e is:

Mechanisms of Evolution

Natural Selection

• Natural selection increases the frequency of beneficial, heritable traits in successive generations.

• Results in adaptations that promote survival and reproduction in specific environments.

• Fitness is a measure of reproductive success, including survival to reproductive age and traits associated with reproduction.

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., optimal 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: Heterozygote Advantage

• Example: Sickle cell trait in humans. Heterozygotes (HbA/HbS) are resistant to malaria, while homozygotes may suffer from malaria or sickle cell disease.

Balancing Selection: Frequency-Dependent Selection

• The fitness of a phenotype depends on its frequency in the population; rare phenotypes may have higher fitness.

Sexual Selection

Definition and Types

• Sexual selection targets traits that improve mating success in sexually reproducing species.

• Often affects male characteristics more intensely.

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

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

Genetic Drift

Definition and Effects

• Genetic drift is the change in allele frequencies due to random chance, not fitness.

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

Bottleneck Effect

• A population is dramatically reduced by an environmental event, then rebuilds.

• Survivors may have different allele frequencies, reducing genetic variation.

Founder Effect

• A 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)

Role in Evolution

• Migration increases gene flow between populations.

• Enhances genetic diversity within populations.

• Reduces differences in allele frequencies between populations.

Summary Table: Mechanisms Affecting Population Evolution

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 at the beginning illustrates directional selection, where environmental changes (e.g., pollution) favored the survival of darker moths over lighter ones.