Bio 9/30

Introduction to Population Genetics

• Evolution is measured at the population level, not at the individual level.

• Upcoming focus: changes in genes/alleles over time, species creation, and population growth.

• Key question: How can we determine if the genes of a population have changed?

Monohybrid Cross

• Mating two purebred individuals results in F1 generation of heterozygous individuals.

• F2 generation yielded different phenotypes and genotypes.

• Alleles R and r represent frequencies in the population:
  $R^2 + 2Rr + r^2 = 1.0$

Allele Frequencies

• Allele frequencies multiply due to independent assortment.

• Total frequencies equal 1.0, indicating 100% of the population's alleles.

• p and q refer to frequencies of two alleles; $p + q = 1.0$.

• Hardy-Weinberg Equilibrium equation: $p^2 + 2pq + q^2 = 1.0$.

• More than two alleles: $p + q + w + z = 1.0$; genotypic frequency becomes:
  $p^2 + 2pq + 2pw + 2pz + q^2 + 2qw + 2qz + w^2 + 2wz + z^2 = 1.0$.

Hardy-Weinberg Principle

• Challenges earlier beliefs about allele fixation and distribution.

• Key aspects include:

  • Allele frequencies (p and q) remain consistent across generations.

  • Example: $p = 0.7$, $q = 0.3$; genotypic frequencies calculated as:

  • $AA: 0.49; aa: 0.09; Aa: 0.42$.

• H-W model assumptions:

  • Random mating, no mutation, no migration, infinite population size, no natural selection.

Assumptions of H-W

• Violating assumptions means H-W does not apply.

• Changes in allele frequency indicate something is affecting the population.

• Identifying violations is critical for understanding population changes.

H-W as Null Hypothesis

• H-W acts as a null hypothesis to determine genetic changes at a locus.

• If observed data deviates significantly from H-W predictions, reject H-W and assume evolution has occurred.

• Research typically involves hypothesizing causes of detected changes in populations.

• Genetic changes indicate violations of the H-W assumptions, suggesting an evolving population.