Genetic Drift Study Notes

Genetic Drift

  • Definition: Genetic drift is the unpredictable fluctuation in allele frequencies within a population due to chance events

    • Examples of chance events: floods, fires, etc.

    • Small population sizes significantly increase the chance of deviation from expected statistical outcomes.

    • Importance of Large Populations: A very large population size is required for a population to maintain Hardy-Weinberg equilibrium.

    • Observation:

      • Coin Example:

        • If you flip a coin 10 times, it's plausible to get 7 heads and 3 tails (a deviation from 50/50).

        • However, flipping the coin 1,000 times means getting 700 heads and 300 tails is highly unlikely.

Types of Genetic Drift

  • Founder Effect

    • Occurs when a small number of individuals from a population establish a new population.

    • By chance, the alleles represented by these founders may differ significantly from the original population.

    • Example:

      • Consider a population of birds made up of half black and half white individuals. If two white birds are blown to an island by a storm and reproduce, the new population will consist entirely of white birds.

      • This event is classified as a chance occurrence as the storm did not favor the whiteness or blackness of the birds.

  • Bottleneck Effect

    • Occurs when a sudden environmental change reduces the population size dramatically.

    • Survivors may not represent the original population's genetic makeup, leading to significant differences in allele frequencies.

    • Graphic Representation:

      • Imagine a bottle filled with marbles that represent alleles. A disaster reduces the number of marbles dramatically—some colors may be overrepresented while others may be entirely eliminated.

      • Example:

        • In the original population, blue and white alleles are equally represented. After the bottleneck, blue may be abundant while white may be underrepresented or absent altogether.

Founder's Effects and Human Populations

  • Ellis-Van Creveld Syndrome Example

    • General population in the U.S.: Approximately 1 in 60,000 to 200,000 births experience this syndrome, which results in skeletal anomalies.

    • In certain culturally isolated populations (e.g., Old Order Amish of Lancaster County, Pennsylvania), the prevalence is about 1 in 5,000 births due to a higher representation of founders carrying the allele for this condition.

Bottleneck Examples in Humans

  • Pingelap, Micronesia Example

    • In 1775, a typhoon killed 90% of the population, resulting in only around 20 survivors.

    • Among these survivors, one was a carrier for achromatopsia, leading to high prevalence rates on the island.

      • General population prevalence: 1 in 33,000 experience achromatopsia.

      • On Pingelap, 10% have the condition, and 30% are carriers, all descending from that one male survivor.

    • Significance: This illustrates how bottlenecks can greatly increase allele frequency representation in smaller populations.

Broader Implications of Bottleneck Effects

  • Conservation and Genetic Variation

    • The bottleneck effect is particularly relevant to understanding human impacts and biodiversity loss.

    • Loss of genetic variation often leads to long-term negative consequences for species, making preservation of diverse gene pools critical.

Illustrative Examples: Literary Reference

  • Laura Ingalls Wilder's "Little House on the Prairie"

    • The narrative references the abundance of chickens available in the prairie during the late 1800s to early 1900s.

    • The author reflects on the contrast between historical availability and today's scarcity of prairie chickens, influenced by human expansion and habitat alterations.

    • Range Comparison:

      • Pre-bottleneck range of prairie chickens was extensive in the 1800s.

      • By the 1990s, populations drastically diminished due to habitat loss associated with human settlement.

Additional Data on Bottleneck Effects in Prairie Chickens

  • Statistical Data:

    • Illinois Example:

      • Population size in the 1930s to 1960s was reduced from thousands to less than 50 by the 1990s

      • Pre-bottleneck: 1,000 to 25,000 individuals, a corresponding high egg-hatching percentage (93%).

      • Post-bottleneck: Less than 50 individuals produced an egg-hatching success rate of less than 50%.

    • Summary of findings:

      • Correlation between population size reduction and genetic diversity loss, affecting allele count per locus. Loss of genetic diversity typically corresponds with a drop in reproductive success and species viability.