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.

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