Evolution of Populations

Introduction

The study of population genetics explores how genetic variation arises, is maintained, and changes within populations over time. Understanding these processes is fundamental to explaining how evolution occurs and how populations adapt to their environments.

Population Genetics

Definition and Importance

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

• It helps explain the relationship between genetic variation and phenotypic variation.

• Key questions include the extent of genetic variation, its origins, maintenance, and changes across generations.

Population and Gene Pool

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

• The gene pool is the collection of all alleles for every gene in a given population.

• Genetic variation within the gene pool is crucial for evolution and adaptation.

Polymorphism and Genetic Variation

• Polymorphic genes have two or more alleles present in the 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 (SNP).

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 frequency = (Number of individuals with a genotype) / (Total number of individuals)

• Allele frequency = (Number of copies of a specific allele) / (Total number of alleles for that gene)

Example Calculation

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

• Total individuals = 9 + 42 + 49 = 100

• Genotype frequency of CWCW = 49/100 = 0.49

• Allele frequency of CW = [(2 × 49) + 42] / (2 × 100) = (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):

• p2: Frequency of homozygous dominant genotype

• 2pq: Frequency of heterozygous genotype

• q2: 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.

Application Example

• In a human population, the gene for earwax type has two alleles: E (dominant, wet earwax) and e (recessive, dry earwax).

• If the frequency of E is 0.8, then the frequency of e is 0.2 (since ).

Mechanisms of Evolution

Natural Selection

• Natural selection is the process by which beneficial, heritable traits become more common in successive generations.

• Results in adaptations that promote survival and reproduction in a particular environment.

• Fitness: 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., clutch size in birds).

• Disruptive selection: Favors survival of 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.

Example Table: Sickle Cell Disease and Malaria Resistance

Sexual Selection

• Sexual selection is a form of natural selection directed at traits that increase mating success.

• 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

• Genetic drift refers to changes in allele frequencies due to random chance, not 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

• A small group separates from a larger population to establish a new population.

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

Migration (Gene Flow)

• Migration increases gene flow between populations.

• Enhances genetic diversity within populations and 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 explaining evolutionary processes and the diversity of life.

Additional info: The introductory image of peppered moths illustrates natural selection in response to environmental changes (industrial melanism).