Genetic Drift and Population Genetics

Change Not Always Resulting from Selection

  • Example: Variation in US Sparrows

    • Context: A colony of sparrows was released in 1852 in Brooklyn, NY.

    • Observation: By 1940 (less than 100 generations later), differences in size among sparrows were noted.

    • Question raised: Is natural selection responsible for these differences?

    • Concepts Introduced: These variations can highlight important concepts of "genetic drift" and "founder effects" which will be explored later.

Genetic Drift 

Definition:

  • Genetic drift refers to random changes in gene frequencies between generations.

Characteristics:

  • Genetic drift is directionless—meaning neutral alleles can:

  • Increase in frequency

  • Decrease in frequency

  • Stay constant in frequency in each generation.

  • Population Size Effect:

    • The rate of change due to genetic drift is influenced by the size of the population.

    • Drift is more pronounced in small populations.

Illustrative Example of Genetic Drift

Population Case Study:

  • Consider a theoretical population of 10 rabbits:

  • 2 BB (brown)

  • 6 Bb (brown)

  • 2 bb (white)

  • Random Reproduction Scenario:

  • By random chance, only 5 rabbits (circled) reproduce, leading to a change in allele frequency.

  • In the next generation, if only 2 rabbits reproduce by random chance, the b allele can be completely lost.

Case study of Genetic Drift

  • Investigators: Dobzhansky and Pavlovsky (1957) conducted experiments with the fruit fly Drosophila pseudoobscura.

  • Study Design:

    • Two different initial population sizes:

    • 10 populations with 4,000 initial members.

    • 10 populations with 20 initial members.

    • Allele Composition:

    • 50% with PP chromosome

    • 50% with AP chromosome inversions.

    • Chromosome Structure Variants:

    • Normal: ABCDEFG

    • Inversion: ABCFEDG

Natural Populations and Genetic Drift

Factors Affecting Genetic Drift:

  1. Small Population Inception:

    • Genetic drift often affects populations that are small at their inception, known as the Founder effect.

  2. Periodic Population Reduction:

    • Genetic drift also impacts populations that occasionally become small, referred to as the Bottleneck effect.

  3. Application to Human Populations:

    • Many examples illustrating genetic drift have been documented among human populations.

Founder Effect

Case Study:

  • The Old Order Amish community in eastern Pennsylvania.

    • Population Statistics:

    • Contributions to the residency percentage based on adherents reported by the Old Order Amish Church vs. total population as recorded by the U.S. Census Bureau in 2000.

      • 0.1-2.1% reported in certain counties.

Case Study: Ellis-van Creveld Syndrome among the Amish

  • Population History:

    • The Amish are descended from about 200 European settlers who immigrated to the U.S. in the 18th century.

    • Restriction on Out-Breeding:

    • Out-breeding with the larger U.S. population has been limited, preserving the genetic mix of the original founders.

  • Genetic Characteristics:

    • Among founders was Samuel King and his wife; one of them carried the Ellis-van Creveld syndrome.

    • Syndrome Characteristics: - Phenotype: Dwarfism and polydactyly (six fingers).

    • Genetic Basis: Caused by a recessive mutant allele on chromosome 4, which has a frequency of about 0.001 in most populations.

      • Frequency in Amish Population: Approximately 0.07, likely due to genetic drift increasing the allele frequency, originally ~0.005 in the founders.

Case Study: Porphyria among Afrikaners

  • Origin of Genetic Disease:

    • Either Gerrit Jansz or his wife Ariaantje Jacobs carried a single mutant gene for porphyria, a metabolic disorder.

  • Disease Characteristics:

    • Failure to produce photoporphyrinogen oxidase leads to a metabolic block resulting in:

    • Red “port red” urine

    • Light sensitivity

    • Skin sores

    • Accumulation of toxins can affect the brain, leading to mental disorders.

    • Historical Note: The same disease affected historical figure George III.

  • Population Impact:

    • Approximately 30,000 Afrikaners are affected by this condition, while it remains rare in the European populations.

    • Notably, more carriers exist in Johannesburg than in all of Holland.

Predicting Heterozygosity in a Population

  • Formula:

    • Where:

      • H' is the predicted heterozygosity of the next generation.

      • N is the number of individuals in the population.

Heterozygosity Calculation Example

= 0.475$$

Migration or Gene Flow

  • Definition:

    • Migration introduces new alleles into a population, impacting gene frequencies.

  • Effects of Migration:

    • Increases variation within populations.

    • Promotes genetic mixing, thus preventing speciation events.

Migration Impact In Hardy-Weinberg

Immigrant Impact Calculation:

Bottleneck Effect

  • Description:

  • Occurs in declining populations where the surviving population is not representative of the original group.

  • Key Observation:

    • While the size of the population may return to its original level after a catastrophic event, its genetic diversity remains altered.

  • Example Types:

    • Causes can include events such as migration or famine.

Case Study: Northern Elephant Seal

  • Historical Context:

    • Intense hunting reduced the population of northern elephant seals to as few as 20 individuals in the 19th century.

  • Reproductive Challenges:

    • The harem-based mating system further reduces genetic variation in the current day population.

  • Genetic Findings:

    • Hoelzel et al. (1993) reported no heterozygosity across 43 loci in 61 individuals tested.

    • Estimated current heterozygosity (H) to be approximately 0.409, while in populations in South Georgia, H was estimated at 0.980.