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:
No mutations
No natural selection
Large population
No migration
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:
New genetic variation: (low impact on HW equilibrium)
Mutations, gene duplication, horizontal gene transfer.
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:
Survival traits (adaptation to environment).
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
Directional Selection (Shifts trait distribution)
Favors one extreme phenotype.
Causes allele fixation, reducing variation.
Example: Antibiotic resistance.
Stabilizing Selection (Narrows trait range)
Favors intermediate phenotype, against extremes.
Example: Clutch size in birds (too few = low reproduction, too many = resource strain).
Disruptive (Diversifying) Selection (Promotes multiple traits)
Favors both extremes, not intermediates.
Occurs in diverse environments.
Example: Different beak sizes in finches.
Balancing Selection (Maintains variation)
Balanced polymorphism: Multiple alleles persist over generations.
Two mechanisms:
Heterozygote advantage (e.g., sickle cell carriers resistant to malaria).
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
Intrasexual Selection (same-sex competition)
Males compete for mates/resources.
Example: Antlers in deer, large claws in crabs.
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:
Assortative vs. Disassortative Mating
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.