Evolutionary Forces and Genetic Variation

• Other evolutionary forces drive evolutionary changes in populations alongside natural selection.

Learning Objectives

• Genetic drift differs from natural selection; the bottleneck effect relates to genetic drift.
• The founder effect and its relationship to the loss of genetic variation.
• Nonrandom mating affects allele and genotype frequencies in a population.
• Inbreeding coefficients interpretation.
• Being able to predict changes in genotype frequency due to inbreeding.

Key Terms and Concepts

• Mutation: Any change to the genomic sequence of an organism.
  • It can be a gene mutation (substitution, insertion, deletion) or a chromosome mutation alteration (duplication, deletion, inversion, translocation).
• Genetic Drift: Evolution arising from random changes in the genetic composition of a population from one generation to the next.
  • Allele frequencies "drift" randomly away from their starting value.
• Fixed Allele: The allele that remains in a population when all alternative alleles have disappeared.
  • No genetic variation exists at a fixed locus within a population because all individuals are genetically identical at that locus.
• Gene Flow: The movement (or migration) of alleles from one population to another.
• Genetic Bottleneck: An event in which the number of individuals in a population is drastically reduced.
• Founder Effect: describes the loss of allelic variation that happens when founding a new population from a small number of individuals
• Inbreeding coefficient (F): The probability that the two alleles at any locus in an individual will be identical because of common descent.
• Inbreeding depression: Reduction in the average fitness of inbred individuals (relative to that of outbred individuals.
  • This arises because rare recessive alleles become expressed in a homozygous state.

Quantitative Toolkit

• Mutation: Δq = μρ, Δq = μρ - να
• Inbreeding:
  • Frequency of aa = q² + Fpq
  • Frequency of Aa = 2pq - 2Fpq
  • Frequency of AA = p2 + Fpq
• Gene Flow: Δq = m(q₁ - q)

Mechanisms Affecting Genetic Variation

• Within populations:
  • Mutation increases genetic variation.
  • Migration, and some types of natural selection, increase and decrease genetic variation.
  • Genetic drift, and some types of natural selection, decrease genetic variation.
• Between populations:
  • Mutation and genetic drift increase genetic variation.
  • Migration, and some types of natural selection, decreases genetic variation.

Mutation Details

• All new alleles arise by mutation and changes allele frequencies very, very slowly.
• Mutation rate = number of mutations per biological unit (per cell division, per gamete, per round of replication).
• Mutation rates are usually low: achondroplasia has 0.00004 mutations per gamete or 4 per 100,000 gametes.
• If the mutation rate (μ) is known, allele frequencies can be calculated in future generations:
  • Δq = μρ
  • Δq = μp - να

Types of Mutations

• Beneficial mutations: Mutations that increase the fitness of the individual that possesses the mutation.
• Neutral mutations: Mutations that neither increase nor decrease the fitness of the individual that possesses the mutation.
• Detrimental mutations: Mutations that reduce the fitness of the individual that possesses the mutation.

Gene Flow Information

• Gene flow's effect on a population prevents populations from becoming genetically different from one another and increases genetic variation within a population.
• Unidirectional gene flow can be modeled like mutation and the changes in allele frequency can be calculated.
  • Δq = m(q₁ - q₁₁)
• Bidirectional gene flow reaches an equilibrium where allele frequencies no longer change.

Black-Footed Ferrets

• The only native ferret species to North America, and the rarest and most endangered land mammals in North America.
• Elizabeth Ann is a clone from the skin cell of a ferret found in 1988.
• Cloning was a method of to bring back genetic diversity to this inbred population.
• Extermination of prairie dog populations limited their primary food source.
• Sylvatic plague and canine distemper impacted ferret populations with the last population lost in 1974.
• Listed as an endangered species in 1967.

Genetic Drift Information

• Reductions in population can result in genetic drift.
• Loss of heterogeneity can result in disease, further population declines, and eventually extinction.
• Population regrowth is subject to the 'founding' population.
• Genetic drift occurs in natural populations when they are small and have low connectivity.

Genetic Drift and Mexican Tetra (Cavefish)

• Cave fish live in dark caves, lack pigmentation, and lost their eyes.
• Darwin thought that natural selection couldn't explain this due to how costly it becomes when maintaining eyes.
• The loss in skin color occurred due to genetic drift.

Genetic Drift Details

• Genetic drift is a random change in the relative frequency of alleles over time.
• Whether an allele is passed to the next generation is not due to fitness and merely due to chance.
• The probability of an allele becoming fixed is equal to the allele's frequency in the population.

Genetic Bottleneck Details

• A genetic bottleneck occurs when there is a drastic reduction in population size.
• The resulting gene pool may no longer be reflective of the original population's gene pool.
  • This can happen due to a sudden change in the environment.
• Drift operates on smaller populations.
• Understanding the bottleneck effect can increase understanding of how human activity affects other species.

Cheetahs Details

• Cheetahs rapidly expanded their range into Asia, Europe and Africa 100,000 years ago.
• 10,000 years ago they had another bottleneck due to climate change and the last glacial maxima.
• Current population decline occurs due to hunting and habitat loss again have led to another genetic bottleneck.
• Cheetahs have very little genetic diversity remaining with 90% similarity.
• Similar to the ferrets, cheetahs also lost a significant proportion of their range.
• The genetic bottleneck effect lead to a loss of variability.
• Cheetah are already unique among felids; at a variety of genetic loci, they exhibit uniformity.
• Other issues:
  • Impacted sperm viability
  • Increases disease susceptibility (infectious peritonitis) with 60% as fatal.
• This ends up furthering reducing population size and genetic variance.
• It can lead to fixation (one allele more common than the other, potentially leading to one being lost entirely).

Founder Effect Details

• A type of bottleneck that occurs when a small number of individuals leave a larger population to colonize a new habitat.
• The colonizing individuals are a random sample of the original population and do not represent the diversity found in the larger population.

Pitcairn Island Information

• In 1789 a mutiny occurred on a British naval ship where 27 people from the ship landed on Pitcairn Island.
• The descendents of these settlers exist on the island but they have genotype frequencies that differ from British populations.

Nonrandom Mating Information

• Positive Assortative Mating is when individuals mates mostly with others who have similar genotypes, usually for some desired phenotype.
  • This decreases heterozygosity.
  • An example of this is tall people marrying other tall people.
  • Inbreeding is an extreme example of positive assortative mating
• Negative Assortative Mating happens when individuals mate mostly with individuals with dissimilar genotypes (non- randomly).
  • This increases heterozygosity.
  • Example: Primrose (heterostyly)
• Inbreeding is an extreme form of positive assortative mating.
• The degree of inbreeding in a population is measured with the Inbreeding coefficient (F).

Inbreeding Coefficient Details

• The inbreeding coefficient (F) is the probability that any two alleles will be identical by descent (ranges between 0 and 1).
• The number of generations of inbreeding affects the size of F.
• Continuing within the royal family is caused F to increase with each generation.
• Higher inbreeding coefficients correlated with lower survivorship, or decreased fitness (Inbreeding depression).

Predicting Inbreeding Results with Hardy-Weinberg

• f(AA) = p² + Fpq
• f(Aa) = 2pq-2Fpq
• f(AA) = q² +Fpq

Florida Panthers Details

• Florida Panthers (Puma concolor coryi) originally ranged throughout the southeast.
• Their population was dramatically reduced to a mere 30 individuals in the mid 1980s.
• The species exhibited inbreeding depression due to the small population size.

Modifications of HWE for Selection

• p2 +2pq + q2 = 1: This states that the genotypes sum to one.
  • This is usually not true.
• p2 +2pq + q2 = y
• Some genotypes do better, and some do worse.
• w₁p² +w22pq + w3q2 = y
• Need to remember that =1 is representing a null hypothesis.

Fitness Modifications

• The relative fitness (W) exists when modifying the HW calculations to account for selection
• Average fitness w = (p²W11 +2pqW12 +q²W22)
• Relative genotypic frequencies can then be calculated by:
  • p2W 11 / W
  • 2pqW12 / W
  • q2W22 / W

Hypothetical Mouse Example

• With the fitness values Wpp = 1.0, Wpq = 0.75, and Wqq = 0.5, the B₂ allele is lost in 33 generations.