Population Genetics

Population Genetics

• Study of genes and genotypes in populations.

• Combines natural selection, Mendel’s laws, and molecular genetics.

• Focuses on:

  • Genetic variation within a gene pool.

  • How variation changes across generations.

Key Terms

• Gene: DNA sequence coding for RNA/protein; found at a locus.

• Alleles: Different gene variants.

• Diploid organisms have two copies of each gene:

  • Homozygous: Identical alleles.

  • Heterozygous: Different alleles.

• Gene pool: All alleles in a population.

• Genotype: An individual’s allele combination.

• Phenotype: Observable traits.

Populations & Genetic Variation

• Population: Group of the same species in a shared environment that can interbreed.

• Species with wide geographic ranges may have distinct populations.

• Polymorphism: The presence of two or more alleles at a frequency >1%.

• Most variation comes from SNPs (Single Nucleotide Polymorphisms).

• Example: Horse coat color (MC1R gene affects eumelanin vs. pheomelanin).

Hardy-Weinberg Equation

• Mathematical model for allele & genotype frequencies in a population.

• Predicts stability across generations if equilibrium conditions are met:

  1. No mutations

  2. No natural selection

  3. Large population

  4. No migration

  5. Random mating

• Equilibrium = no evolution, but real populations rarely meet these conditions.

Disequilibrium & Evolution

• If a population is not in HW equilibrium, it suggests evolutionary forces are at play.

• Used as a baseline to detect changes in allele frequencies over time.

Microevolution

• Microevolution: Changes in a population’s gene pool over generations due to:

  1. New genetic variation: (low impact on HW equilibrium)

     • Mutations, gene duplication, horizontal gene transfer.

  2. Mechanisms altering allele prevalence: (major impact)

     • Natural selection, genetic drift, migration, non-random mating.

Natural Selection & Adaptation

• Natural Selection: Beneficial heritable traits become more common over generations.

• Adaptation: Evolutionary changes improving survival & reproduction.

Reproductive Success

• Reproductive Success: Likelihood of contributing fertile offspring.

• Influenced by:

  1. Survival traits (adaptation to environment).

  2. Reproductive traits (mate attraction, gamete viability).

Fitness

• Fitness: Relative reproductive success of a genotype.

  • Higher fitness → More offspring → Increased allele prevalence.

• Mean fitness: Average reproductive success in a population.

Types of Natural Selection

1. Directional Selection (Shifts trait distribution)

   • Favors one extreme phenotype.

   • Causes allele fixation, reducing variation.

   • Example: Antibiotic resistance.

2. Stabilizing Selection (Narrows trait range)

   • Favors intermediate phenotype, against extremes.

   • Example: Clutch size in birds (too few = low reproduction, too many = resource strain).

3. Disruptive (Diversifying) Selection (Promotes multiple traits)

   • Favors both extremes, not intermediates.

   • Occurs in diverse environments.

   • Example: Different beak sizes in finches.

4. Balancing Selection (Maintains variation)

   • Balanced polymorphism: Multiple alleles persist over generations.

   • Two mechanisms:

     1. Heterozygote advantage (e.g., sickle cell carriers resistant to malaria).

     2. Negative frequency-dependent selection (rare traits favored, e.g., predator-prey dynamics).

Sexual Selection

• Enhances reproductive success rather than survival.

• More intense in males (higher reproductive variability).

• Leads to secondary sex characteristics (traits aiding reproduction).

Types of Sexual Selection

1. Intrasexual Selection (same-sex competition)

   • Males compete for mates/resources.

   • Example: Antlers in deer, large claws in crabs.

2. Intersexual Selection (mate choice)

   • Females choose based on traits signaling genetic quality.

   • Example: Peacock feathers.

• Sexual Dimorphism: Visible differences between sexes due to sexual selection.

Cryptic Female Choice

• A type of intersexual selection occurring at or after mating.

• Female-driven mechanisms influence sperm success in fertilization.

• May function to prevent inbreeding.

The Cost of Reproduction

• Sexual selection explains traits that increase reproductive success but decrease survival.

• Traits that increase predation risk will be less common in predator-rich environments.

Genetic Drift

• Random changes in allele frequencies, independent of fitness.

• Leads to either fixation (100%) or loss (0%) of alleles.

• Most impactful in small populations, reducing genetic diversity.

• Drastic changes may occur after a population reduction (e.g., bottleneck, founder effect).

Bottleneck Effect

• Dramatic population reduction, followed by recovery.

• Random loss of alleles, not based on fitness.

• Surviving population may have different allele frequencies from the original.

• Example: Cheetahs have low genetic variation due to past bottlenecks.

Founder Effect

• Small group separates from a larger population to establish a new colony.

• Founding populations have less genetic variation than the original.

• Allele frequencies may differ randomly from the original population.

• Example: Amish populations have higher frequencies of certain genetic disorders.

Migration & Gene Flow

• Gene Flow: Movement of alleles between populations.

• Effects of Migration:

  • Reduces genetic differences between populations.

  • Increases genetic diversity within a population.

Non-Random Mating

• Individuals choose mates based on genotypes/phenotypes.

• Affects genotype proportions, deviating from Hardy-Weinberg predictions.

• Two main types:

  1. Assortative vs. Disassortative Mating

  2. Inbreeding

Assortative vs. Disassortative Mating

• Assortative Mating: Similar phenotypes mate preferentially.

  • Increases homozygosity (more identical alleles).

• Disassortative Mating: Dissimilar phenotypes mate preferentially.

  • Increases heterozygosity (more genetic diversity).

Inbreeding

• Mating between genetically related individuals.

• Increases homozygosity, decreases heterozygosity.

• Can increase harmful recessive traits.

Inbreeding Depression

• Reduced fitness due to increased homozygosity of harmful alleles.

• More common in shrinking populations.

• Example: Florida panthers suffer from poor sperm quality, low genetic diversity, and physical abnormalities due to inbreeding.