Changes in Allele Frequencies

1
Changes in Allele Frequencies

• Determined by Hardy-Weinberg Principle

2
Hardy-Weinberg Principle

• Two alleles for a gene (Aa), A p and a q
• p q 1 ie., p 0.6 and q 0.4
• frequencies of genotypes are p 2 2pq q2
• ie, 0.36 for AA, 0.16 for aa, and 0.48 for Aa
• same frequency from generation to generation
• no change no microevolution

3
Assumptions of HW

• Random mating
• Large Population Size
• No Mutations
• No Interbreeding with other populations
• No selection

4
Violation of assumptions

• Natural selection produces adaptive change
• Violations of assumptions may produce
  nonadaptive change

5
Mutations

• Mutations provide new alleles
• But mutations do not change allele frequencies
  much by themselves due to low rate of mutations

6
Random Drift

• Chance errors may lead to loss of an allele from
  a population
• Most pronounced in small populations
• Founder effect-A new population founded from a
  few individuals
• Bottleneck effect-A large population diminishes
  to a few individuals
• Can spread new mutations but not from natural
  selection

7
Random Drift due to Small Population Size
8
Random Drift due to Bottleneck
9
Nonrandom mating

• Individuals may choose like (positive
  assortative) to mate
• This increases inbreeding
• Leads to higher than expected homozgyotes
  (greater chance of recessive traits being
  expressed)
• Most species are outbreeders (negative
  assortative) to avoid homozygosity
• Higher than expected heterozygosity

10
Interbreeding between Populations

• Gene flow between populations can introduce new
  alleles
• Particularly when populations live in distinct
  microhabitats

11
Natural selection -reducing allele frequency

• Operates on the phenotype
• Harmful alleles can be reduced but only to a
  point
• Harmful alleles can be hidden in heterozygotes

12
Natural selection-increasing allele frequency

• Examples include roaches and chemicals
• Roaches resistant to spray after many generations
• Peppered moth example (below)

13
Directionof Selection
14
Stabilizing selection on human birth weight
15
Sexual selection

• The differential ability of individuals with
  different genotypes to acquire mates
• May result in sex differences (sexual dimorphism)
• Female choice-females judge males to choose best
  mate
• Male competition-males compete for resources
  needed to mate (territory, food)

16
Natural selection and Genetic Variation

• Balanced polymorphism-balance of alleles
• Due to different environments affecting natural
  selection
• Examples from sickle-cell anemia and malaria
• In Africa, carriers (heterozygotes) for
  sickle-cell anemia are benefited where malaria is
  common explains higher rate of sickle-cell
  anemia in African-Americans in US