16.2 - Evolution as Genetic Change

• • If an individual produces many offspring, alleles stay in the gene pool and may increase in frequency
  • If an individual dies without reproducing, it doesn't give alleles to the gene pool
• Natural selection changes allele frequencies (1)

  • In single-allele traits:
  • Can lead to change in allele frequencies with few phenotypes
  • Easy to see the distinct change in phenotypes ratios as the allele frequency changes
• Natural selection changes allele frequencies (2)

  • In Polygenic Traits:
  • With many phenotypes, natural selection leads to the distribution of phenotypes in many ways because there aren't just two phenotypes
• Natural selection changes allele frequencies (2.2)

  • In Polygenic Traits:
  • Directional Selection - When individuals at one end of the curve have higher fitness than the middle on the other end
  • Stabilizing Selection - When individuals in the center of the curves have higher fitness than individuals at either end of the curve
  • Disruptive Selection - When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle
• Genetic Drift Changes Allele Frequencies (1)

  • A random change in allele frequency NOT due to natural selection
  • Affects smaller populations more
  • Can lead to decreased genetic diversity in a population which can decrease stability
• Genetic Drift Changes Allele Frequencies (2)

  • Bottleneck Effect
  • a random event causes a group or organisms in a population to die reducing the population dramatically
  • The organisms left in the population are not necessarily the most fit - "selected randomly"
  • The remaining organisms contribute alleles to the gene pool which may create different allele frequencies than the original population
  • Ex: wildfires, over hunting, natural disasters

  
• Genetic Drift Changes Allele Frequencies (3)

  • Founder Effect:
  • allele frequencies change due to migration of a small, random group from a population to a new habitat
  • Individuals bring alleles in different frequencies than the larger population had in the pool
  • The new population can be genetically different from the parent population and evolve with different allele frequencies

  
• Evolution v.s. Genetic Equilibrium (1)

  • Genetic Equilibrium - no genetic change
  • if allele frequencies do not change then the population will not evolve
  • Hardy-Weinberg Principle
  • Allele frequencies will remain constant unless one or more factors cause those frequencies to change
  • For Hardy-Weinberg Equilibrium to occur (no evolution) there has to be:
  • random mating
  • population must be large
  • can't be any migration in or out
  • no mutation
  • no natural selection
• Evolution v.s. Genetic Equilibrium (2)

  • What would happen if one of the five factors occurred?
  • The allele frequencies for a trait would change and evolution would occur
  • if frequencies change, it's because one of the five factors for HW Equilibrium is affecting the population and it's evolving
• Hardy-Weinberg Equation

  $p^2+$$2pq+$$q^2=$$1$
• Hardy-Weinberg Equation

  • Parts of the equation:
  • p2 = # homozygous dominant individuals in a pop.
  • 2pq = # heterozygous dominant individuals in a pop.
  • q2 = # recessive individuals in a pop.
  • p = frequency of T allele in a pop. (decimal)
  • q = frequency of t allele in a pop. (decimal)
  • p + q = 1 - represents all the alleles in the pop for this trait
• • If p & q change between points in time (even slightly), then this is genetic change -> evolution
  • If allele frequencies don't change over time then the population is in genetic equilibrium