Population Genetics

• Population: a group of individuals of the same species that live in the same area and interbreed (results to production of fertile offspring)
• Population genetics: the study of genetic variation within populations
• Population Genetics: involves the examination and modelling of changes in the frequencies of genes and alleles in populations over space and time
• Focus of Pop'n Genetics: species or population
• Gene Pool: the collection of all the alleles of all of the genes found within a freely interbreeding population
• Gene Frequency (aka allele frequency): The proportion of all alleles in all individuals in the group in question which are of a particular type.
• Genotype Frequency: The proportion of individuals in a group with a particular genotype.
• Hardy-Weinberg Equation: used to estimate frequency of alleles in a population.
• The sum of the frequencies of both alleles is 100%.
• Assumptions of the HW model:

  1. Organism is diploid.
  2. Reproduction is sexual.
  3. Generations are non-overlapping.
  4. Mating occurs at random.
  5. Population size is very large.
  6. Migration is zero.
  7. Mutation is zero.
  8. Natural selection does not affect the gene in question.
• Factors that affect Gene Frequency:

  1. mutation
  2. natural selection
  3. population size
  4. genetic drift
  5. environmental diversity
  6. migration
  7. non-random mating patterns
• Mutation: the primary source of new alleles in a gene pool.
• Natural Selection - the differential reproduction of genotypes.
• Relative Fitness - ability to survive in an environment long enough to reproduce.
• Disruptive Selection: selects against the average individual in a population
• Stabilizing Selection: favors the intermediate variants
• Directional Selection: an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype.
• Sexual selection occurs when individuals within one sex secure mates and produce offspring at the expense of other individuals within the same sex.
• Population Size: Increase in population causes increase in gene frequencies.
• Genetic Drift - occurs as the result of random fluctuations in the transfer of alleles from one generation to the next, especially in small populations formed, as a result of bottleneck effect and founder effect.
• Bottleneck effect: adverse environmental conditions
• Founder effect: geographical separation of a subset of the population
• Random Genetic Drift can continue until one allele is either fixed or lost.
• As genetic drift progresses,

  • heterozygosity decreases.
  • genetic variance within populations decreases.
  • genetic variance among populations increases.
• Migration: movement of individuals from one population to another; translated as gene flow.
• Migration can equalize gene frequency.
• Random Mating System: mate choice is independent of phenotype and genotype
• Subcategories of Non-random Mating System:

  • Positive Assortment
  • Negative Assortment
  • Inbreeding
• Positive Assortment: mate choice is dependent on similarity of phenotype.
• Negative Assortment: mate choice is dependent on dissimilarity of phenotype
• Inbreeding: mating with relatives at a rate greater than expected by chance.
• Positive Assortment:

  • increases homozygosity (prevents HW equilibrium)
  • affects only those genes related to the phenotype by which mates are chosen
• Negative Assortment:

  • yields an excess of heterozygotes (compared to HW)
  • increases the rate to equilibrium of alleles among loci (because linkage phases are disrupted by recombination in double homozygotes).
• Inbreeding alone does not change allele frequencies, but inbreeding does change genotype frequencies.
• Inbreeding can affect allele frequencies, by changing how selection operates.
• 2 possible reasons for inbreeding depression:

  1. deleterious recessive alleles
  2. overdominance
• Interbreeding:

  • can result to excess homozygotes

  • inbred individuals usually have lower fitness than outbred individuals (inbreeding depression)
• Race: geographically isolated breeding population that shares certain characteristics in higher frequencies than other population of that species, but has not become reproductively isolated from other populations of the same species.
• Species: members of populations that actually or potentially interbreed in nature, not according to similarity of appearance.
• Pre-zygotic Isolating Mechanisms: prevent the formation of viable zygotes