16.2 Genetic Drift and Gene Flow

Genetic drift and gene flow are key forces shaping populations. Drift randomly changes allele frequencies, especially in small groups. It can lead to fixation or loss of traits. Gene flow, on the other hand, transfers alleles between populations through migration.

These processes impact genetic variation differently. Drift tends to reduce diversity within populations, while gene flow can increase it. The balance between drift, flow, and selection determines how populations evolve over time. Understanding these forces helps explain patterns of variation in nature.

Genetic Drift and Its Effects on Populations

Genetic drift in small populations

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• Random change in allele frequencies within a population over time
  • Occurs due to sampling error during reproduction randomly selecting certain alleles to be passed on to the next generation
  • More pronounced in small populations because random sampling has a greater impact when there are fewer individuals (island populations)
• Two main types of genetic drift:
  • Bottleneck effect reduces population size and genetic diversity
    • Occurs when a population experiences a drastic reduction in size due to events such as natural disasters (earthquakes) or human activities (overhunting)
    • Leads to a loss of genetic diversity and can result in the fixation or loss of alleles by randomly eliminating certain alleles from the gene pool
  • Founder effect establishes new population with limited genetic diversity
    • Occurs when a small subset of a larger population establishes a new population in a different location (colonizing a new island)
    • The new population may have a different allele frequency compared to the original population due to random sampling of alleles from the source population
• Genetic drift can lead to the fixation or loss of alleles in small populations
  • Fixation occurs when an allele reaches a frequency of 100% in the population by chance, becoming the only allele present
  • Loss occurs when an allele is completely eliminated from the population by random chance, even if it was previously common

Genetic drift vs natural selection

• Genetic drift and natural selection are both evolutionary forces that shape allele frequencies in populations over time
• Differences between genetic drift and natural selection:
  • Genetic drift is a random process driven by chance, while natural selection is a non-random process driven by differences in fitness (survival and reproduction)
  • Genetic drift affects all alleles equally regardless of their effect on fitness, while natural selection favors alleles that increase fitness (antibiotic resistance) and acts against those that decrease fitness (deleterious mutations)
  • Genetic drift is more pronounced in small populations where random sampling has a greater impact, while natural selection is more effective in large populations where even small fitness differences can lead to significant changes in allele frequencies
• Similarities between genetic drift and natural selection:
  • Both can lead to changes in allele frequencies over time, altering the genetic composition of populations
  • Both can result in the fixation or loss of alleles in a population, either by chance (drift) or due to fitness differences (selection)
• The relative importance of genetic drift and natural selection depends on factors such as population size, selection pressure (strength of environmental challenges), and the genetic architecture of the trait under consideration (number and effect size of genes involved)

Gene Flow and Its Impact on Genetic Variation

Gene flow and genetic variation

• Gene flow is the transfer of alleles between populations through the migration of individuals or the exchange of gametes (pollen, sperm)
• Gene flow can introduce new alleles into a population, increasing genetic variation within the population by adding previously absent alleles to the gene pool
• Gene flow can also reduce genetic differences between populations, leading to genetic homogenization by equalizing allele frequencies across populations
• Factors influencing gene flow:
  • Migration rates and patterns determine how many individuals move between populations (birds vs sedentary organisms)
  • Geographical barriers like mountains or rivers can limit gene flow between populations on either side
  • Mating preferences can influence gene flow if individuals preferentially mate with members of their own population (assortative mating)
  • Dispersal abilities of the organism affect how far individuals can travel to exchange alleles between populations (wind-dispersed seeds vs animal-dispersed seeds)
• The impact of gene flow on genetic variation depends on the balance between gene flow and other evolutionary forces such as genetic drift and natural selection
  • High gene flow can counteract the effects of drift and selection, maintaining genetic variation
  • Low gene flow allows drift and selection to drive population differentiation, reducing variation within populations

Population effects of drift and flow

• Population differentiation occurs when two or more populations become genetically distinct due to limited gene flow and the action of evolutionary forces such as genetic drift and natural selection
  • Genetic drift can lead to population differentiation by causing random changes in allele frequencies, especially in small, isolated populations (Galápagos finches)
  • Natural selection can lead to population differentiation if different populations experience different selection pressures, favoring different alleles (high-altitude adaptations in Tibetans vs lowlanders)
• Population homogenization occurs when gene flow between populations is high, leading to a reduction in genetic differences between populations
  • High levels of gene flow can counteract the effects of genetic drift and natural selection, preventing population differentiation by mixing alleles between populations
• The balance between genetic drift, gene flow, and natural selection determines the extent of population differentiation or homogenization
  • In situations where gene flow is limited (isolated habitats) and genetic drift or natural selection are strong, populations are more likely to become differentiated over time
  • In situations where gene flow is high (connected habitats) and overcomes the effects of genetic drift and natural selection, populations are more likely to become homogenized as they exchange alleles