Genetic Drift: How Random Chance Shapes Evolution

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1. The Bottleneck Effect: Survival by Chance

The Bottleneck Effect occurs when a random event, such as a natural disaster or disease, drastically reduces the population size. This leads to a loss of genetic diversity because only a small, random subset of individuals survives and passes on their genes.

Example of the Bottleneck Effect

Imagine a population of animals with different coat colors:

  • White (50%)
  • Green-spotted (33%)
  • Black (17%)

A tornado wipes out most of the population by chance, leaving only white and green-spotted animals. The new population now consists of:

  • White (60%)
  • Green-spotted (33%)
  • Black (0%) (extinct in this group)

Since survival was random, the black coat gene is lost, and future generations will no longer have it—even if it was previously common.

Real-Life Example

  • Cheetahs have extremely low genetic variation due to past bottlenecks, making them more vulnerable to diseases.
  • Northern Elephant Seals were hunted to near extinction, reducing their genetic diversity.

2. The Founder’s Effect: When a Few Start Fresh

The Founder’s Effect happens when a small subset of a population migrates to a new area and establishes a new gene pool that is different from the original population.

Example of the Founder’s Effect

Consider a human population where:

  • 83% have normal fingers
  • 17% have polydactyly (extra fingers/toes)

If a small group of 6 individuals moves to a new island, and half of them carry the polydactyly gene, the new island population may have:

  • 50% normal fingers
  • 50% polydactyly

Over generations, the frequency of polydactyly is much higher in the new population compared to the original one.

Real-Life Example

  • The Amish Community: Due to isolation and limited genetic variation, polydactyly (extra fingers) is more common among Amish populations.
  • Darwin’s Finches: A small group of finches migrated to the Galápagos Islands, leading to new species with distinct beak shapes based on random genetic traits.


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