Evolutionary Biology Unit 6 Review: Genetic Drift, Gene Flow & Non-random Mating

Key Concepts

• Genetic drift random changes in allele frequencies over generations due to sampling effects in finite populations
• Gene flow transfer of alleles between populations through migration or interbreeding
• Non-random mating occurs when individuals mate with others based on specific traits or preferences rather than by chance
  • Includes assortative mating where similar phenotypes mate more frequently
  • Also includes disassortative mating where dissimilar phenotypes mate more often
• Hardy-Weinberg equilibrium assumes no genetic drift, gene flow, or non-random mating in a population
• Founder effect occurs when a small group establishes a new population leading to reduced genetic diversity (Amish populations)
• Bottleneck effect drastic reduction in population size that can lead to loss of rare alleles and decreased genetic variation (cheetahs)
• Effective population size ($N_e$) number of individuals in an idealized population that would experience the same amount of genetic drift as the actual population

Genetic Drift Explained

• Genetic drift changes in allele frequencies due to random sampling of gametes in finite populations
• Occurs in all populations but has a more pronounced effect in smaller populations
• Can lead to fixation where one allele reaches a frequency of 100% or loss where an allele disappears from the population
• Two main types: sampling error during gamete formation and random changes in allele frequencies over generations
  • Sampling error occurs because gametes contain only a sample of the parent's alleles
  • Generational changes result from the cumulative effects of sampling error over time
• Genetic drift is a random process and does not have a predictable direction or outcome
• Can cause populations to diverge genetically over time even in the absence of selection or mutation (Galapagos finches)
• Interacts with other evolutionary forces like selection, mutation, and gene flow to shape genetic variation within and among populations

Gene Flow: Definition and Mechanisms

• Gene flow transfer of alleles between populations through migration or interbreeding
• Can introduce new alleles into a population or change the frequencies of existing alleles
• Tends to reduce genetic differences between populations and counteracts the effects of genetic drift and local adaptation
• Gene flow is mediated by the movement of individuals, gametes, or even whole populations
  • Physical movement of individuals between populations is the most common mechanism (migratory birds)
  • Pollen and seeds can also disperse between plant populations
• Rate and patterns of gene flow depend on factors like dispersal ability, geographic barriers, and mating systems
• Admixture occurs when two previously isolated populations interbreed resulting in the exchange of alleles (African-American populations)
• Isolation by distance pattern where gene flow decreases with increasing geographic distance between populations

Non-random Mating Patterns

• Non-random mating occurs when individuals mate with others based on specific traits or preferences rather than by chance
• Can lead to changes in allele frequencies and genotype proportions that deviate from Hardy-Weinberg expectations
• Assortative mating occurs when individuals with similar phenotypes mate more frequently than expected by chance
  • Positive assortative mating individuals with similar traits mate more often (tall people mating with other tall people)
  • Negative assortative mating individuals with dissimilar traits mate more often (MHC dissimilar mates in humans)
• Inbreeding mating between closely related individuals
  • Increases homozygosity and can lead to inbreeding depression (reduced fitness)
  • Common in small, isolated populations or those with limited dispersal (island species)
• Outbreeding mating between distantly related or unrelated individuals
  • Can introduce new alleles and increase heterozygosity (hybrid vigor)
• Sexual selection occurs when traits that confer a mating advantage are favored leading to non-random mating (peacock tail)

Evolutionary Impacts and Examples

• Genetic drift, gene flow, and non-random mating can have significant impacts on the evolution of populations and species
• Genetic drift can lead to the fixation of alleles, loss of genetic diversity, and population differentiation
  • Founder effects during species introductions or colonization events (Drosophila in Hawaii)
  • Bottlenecks due to environmental catastrophes, overhunting, or habitat fragmentation (northern elephant seals)
• Gene flow can homogenize populations, counteract local adaptation, and spread advantageous alleles
  • Introgression of adaptive alleles between closely related species (Heliconius butterflies)
  • Spread of pesticide resistance alleles in insects or antibiotic resistance genes in bacteria
• Non-random mating can alter genotype frequencies, maintain variation, or lead to speciation
  • Assortative mating can maintain color polymorphisms in populations (peppered moths)
  • Disassortative mating at immune genes (MHC) in vertebrates promotes offspring disease resistance
  • Inbreeding can lead to local adaptation but also increase extinction risk in small populations (Isle Royale wolves)
  • Sexual selection can drive rapid evolution of mating-related traits and behaviors (bowerbirds)

Mathematical Models and Calculations

• Mathematical models help predict and quantify the effects of genetic drift, gene flow, and non-random mating on populations
• Wright-Fisher model assumes discrete generations, random mating, no selection, and constant population size
  • Probability of fixation of a neutral allele is equal to its initial frequency ($p$)
  • Time to fixation depends on population size and is approximately $4N_e$ generations
• Moran model assumes overlapping generations and is useful for modeling bacterial or viral populations
• $F_{ST}$ measures the degree of genetic differentiation among populations
  • Ranges from 0 (no differentiation) to 1 (complete differentiation)
  • Calculated as $F_{ST} = \frac{\sigma^2_S}{\sigma^2_T}$ where $\sigma^2_S$ is the variance in allele frequencies among subpopulations and $\sigma^2_T$ is the total variance
• Migration rate ($m$) is the proportion of individuals in a population that are immigrants
  • Relationship between migration rate and $F_{ST}$ is given by $F_{ST} = \frac{1}{1+4Nm}$ where $N$ is the population size
• Inbreeding coefficient ($F$) measures the probability of identity by descent of two alleles at a locus
  • Calculated using path analysis in pedigrees or estimated from genotype frequencies in populations

Research Methods and Tools

• Various experimental and observational approaches are used to study genetic drift, gene flow, and non-random mating in natural populations
• Molecular markers like microsatellites, SNPs, and DNA sequences are used to quantify genetic variation and infer evolutionary processes
  • Microsatellites are highly variable repetitive DNA sequences used for population genetic studies
  • SNPs (single nucleotide polymorphisms) are single base pair changes used for genome-wide association studies and population structure analysis
• Experimental evolution studies manipulate populations in the lab to test evolutionary hypotheses
  • Drosophila experiments have demonstrated the effects of population size on the rate of genetic drift
• Computer simulations model the effects of different evolutionary scenarios on populations
  • Coalescent simulations trace alleles back in time to their most recent common ancestor
  • Forward simulations model the effects of drift, selection, and migration on future allele frequencies
• Genome-wide sequencing technologies allow for high-resolution studies of genetic variation and evolutionary history
  • Whole-genome sequencing provides complete genetic information for individuals or populations
  • RADseq (restriction site-associated DNA sequencing) targets a subset of the genome for cost-effective population genomic studies

Real-world Applications and Case Studies

• Understanding genetic drift, gene flow, and non-random mating has important applications in conservation biology, agriculture, and human health
• Conservation biologists use population genetic principles to manage small or fragmented populations
  • Genetic rescue involves introducing individuals from other populations to increase diversity and fitness (Florida panther)
  • Captive breeding programs aim to minimize inbreeding and maintain genetic diversity (California condor)
• Agricultural scientists consider the effects of drift and gene flow when developing crop varieties and livestock breeds
  • Maintaining diverse seed banks and germplasm collections helps preserve genetic resources for future breeding efforts
  • Marker-assisted selection uses molecular markers to select for desirable traits and minimize inbreeding in animal and plant breeding
• Human geneticists study the effects of drift, admixture, and non-random mating on human populations
  • Founder effects have led to high frequencies of certain genetic disorders in some populations (Tay-Sachs in Ashkenazi Jews)
  • Admixture mapping uses differences in allele frequencies between ancestral populations to identify disease-associated genes (African-American populations)
  • Consanguineous marriages (between close relatives) increase the risk of recessive genetic disorders by increasing homozygosity (Pakistani populations)
  • Non-random mating patterns can influence the distribution of traits like height, intelligence, and psychiatric disorders in human populations