Populations and Gene Pools

• Concepts

  • A population is a localized group of interbreeding individuals

  • Gene pool is a collection of alleles in the population

  • Allele frequency is how common that allele in the population

Evolution of Populations

• Evolution

  • Change in allele frequencies in a population

• Non-evolving population

  • Remove all agents of evolutionary change

    • Very large population size

      • No genetic drift

    • No migration

      • No gene flow

    • No mutation

      • No genetic change

    • Random mating

      • No sexual selection

    • No natural selection

      • Everybody is equally fit

Hardy-Weinberg Equilibrium

• Hypothetical, non-evolving population

  • Preserves allele frequencies

• Serves as a model (null hypothesis)

  • Natural populations rarely in H-W equilibrium

  • Useful model to measure if forces are acting on a population

    • Measuring evolutionary change

Hardy-Weinberg Theorem

• Counting alleles

  • Assume 2 alleles

    • B,b

  • Frequency of dominant allele

    • Variable p

  • Frequency of recessive allele

    • Variable q

    • Frequencies must ad to 1

      • p+q=1

• Counting individuals

  • Frequency of homozygous dominant

    • p*p=p^2

  • Frequency of homozygous recessive

    • q*q=q^2

  • Frequency of heterozygotes

    • (p*q)+(q*p)=pq

    • Frequency of all individuals must add up to 1

      • p^2+qp+q^2=1

Application of H-W Principle

• Sickle cell anemia

  • Inherit mutation in gene coding for hemoglobin

    • Oxygen-carrying blood protein

  • Low oxygen levels cause RBC to sickle

    • Breakdown of RBC

    • Clogging small blood vessels

    • Damage to organs

  • Often lethal

Sickle Cell Frequency

• High frequency of heterozygotes

  • 1 in 5 in Central Africans

  • Unusual for allele with severe detrimental effects in homo zygotes

    • 1 in 100

    • Usually die before reproductive age

Heterozygote Advantage

• In tropical Africa, where malaria is common

  • Homozygous dominant (normal)

    • Die or reduced reproduction from malaria

  • Homozygous recessive

    • Die or reduced reproduction from sickle cell anemia

  • Heterozygote carriers

    • Relatively free of both

    • Survive and reproduce more