Genetic Drift Notes

Genetic Drift

• Genetic drift is a random change in allele frequencies within a population, particularly impactful in smaller populations.
• It is distinct from natural selection, as it's driven by chance rather than adaptive advantages.
• Genetic drift leads to a reduction in genetic variation over time.

Types of Genetic Drift

1. Founder Effect

• Definition: When a small subset of a larger population establishes a new, isolated population, the allele frequencies in the new group may differ from the original population due to chance.
• This new population carries only a fraction of the original population's genetic diversity.
• Example: Amish Population
  • A small group of Amish settlers migrated to Pennsylvania in the 1700s.
  • They carried a recessive allele for Ellis-van Creveld syndrome, a rare genetic disorder causing short limbs and extra fingers.
  • Due to the founder effect and subsequent inbreeding within the isolated Amish community, the frequency of this allele is significantly higher than in the general population.
• Example: Darwin’s Finches
  • A small number of finches colonized the Galápagos Islands from South America.
  • These finches represented only a fraction of the genetic diversity of the mainland finch population.
  • The descendants of these finches underwent adaptive radiation, evolving into multiple species, but their genetic variation was shaped by the limited gene pool of the founders.
• Example: Pitcairn Islanders
  • The Pitcairn Islands were settled by a small group of British sailors and Tahitian women in 1790.
  • The descendants of these settlers exhibit unique genetic traits due to the limited gene pool of the founding individuals.
• Example: Afrikaner Population (South Africa)
  • Dutch settlers in the 1600s established a small founding population in South Africa.
  • The Afrikaner population exhibits a high frequency of Huntington’s disease, a rare genetic disorder, likely due to the founder effect from those early settlers.

2. Bottleneck Effect

• Definition: A sharp reduction in the size of a population due to environmental events (such as famines, earthquakes, floods, fires, disease, or droughts) or human activities (such as genocide).
• The surviving population no longer represents the genetic diversity of the original population.
• This can lead to a significant loss of genetic variation.
• Example: Cheetahs
  • Cheetahs experienced a severe population bottleneck around 10,000 years ago.
  • Only a small number of individuals survived, leading to drastically reduced genetic diversity.
  • Modern cheetahs exhibit remarkably low genetic variation; even unrelated individuals can have nearly identical genes.
  • This lack of diversity makes them highly vulnerable to diseases and reduces their ability to adapt to environmental changes.
• Example: Northern Elephant Seals
  • Hunted nearly to extinction in the 1800s, with only approximately 20 individuals surviving.
  • The population has rebounded to over 200,000, but they possess very limited genetic variation, making them vulnerable to diseases.
• Example: American Bison
  • Nearly wiped out in the 1800s due to overhunting.
  • The population was reduced from millions to just hundreds of animals, representing a severe bottleneck.
  • Conservation efforts have increased the population, but genetic diversity remains low.
• Example: Greater Prairie Chicken (USA)
  • Habitat destruction in Illinois caused a major population crash.
  • As numbers declined, fertility and hatch rates also decreased, likely due to inbreeding and loss of genetic diversity.
  • Genetic rescue efforts involving the introduction of chickens from other states helped improve survival rates.
• Example: Human History: Toba Catastrophe Theory
  • Approximately 70,000 years ago, the Toba supervolcano eruption may have caused a global bottleneck in the human population.
  • The human population may have dropped to just a few thousand individuals.
  • This could explain the relatively low genetic variation observed in humans compared to other species.

Why Genetic Drift Matters

• Genetic drift can cause alleles to either disappear from a population or become more common purely by chance, irrespective of their adaptive value.
• Reduces the genetic variation within a population, potentially limiting its ability to adapt to future environmental changes or selective pressures.

Summary Table