Mutation & Migration

Categorize mutations by type and severity.

• Single nucleotide polymorphisms (SNP) – single nucleotide is substituted; most common

• Indels – insertion or deletion of one or more nucleotides; frequently result in frameshifts

• Copy number variations (CNV) – a type of mutation in which the number of repeats of a genetic sequence varies; may apply to a short tandem repeat (ex. AGAT) or to whole genes (ex. Hemoglobin); includes inversions

• Chromosome translocation – occurs when the chromosome breaks and part is relocated to another chromosome

• Chromosome nondisjunction (aneuploidy) – chromosome pairs do not separate during cell division; may occur to a single chromosome or entire genome

Explain why and when mutations arise

• Acquired Mutation Hypothesis: Mutations occur in reaction to environment

• Random (Spontaneous) Mutation Hypothesis: Mutations occur independent of the environment

• If the random (spontaneous) mutation hypothesis is supported by the data we will see plates with the same location of resistant colonies each time, but their numbers will be different

• If the induced mutation hypothesis is supported by the data we will see plates with the same number of resistant colonies each time, but their locations will be different

• The Lederberg experiment results: Same colonies were antibiotic resistant. Found evidence that mutations are generally random and NOT induced by the selection pressure the organism experiences

• Extremely lethal mutations are not inherited. If a mutation is severe enough to kill the cell, the cell won’t be alive to pass the mutation on. Therefore, most traceable mutations are nearly neutral

• The mutation rate is the rate at which changes to the DNA occur. Mutations can include a number of different changes although the most common are substitutions

• The substitution rate is the rate at which single nucleotide substitutions occur. IF most most mutations are neutral or nearly neutral, then the substitution rate ≈ mutation rate

Calculate forward mutation rate, backward mutation rate, and allele frequency at equilibrium

• Forward mutation*, μ (p →q), at a particular rate

• Back mutation*, v (q → p),at potentially different rate

• p̂ = v/μ + v

• q̂ = μ/v + μ

• p̂ = (p-hat) frequency of allele p at equilibrium

• q̂ = (q-hat) frequency of allele q at equilibrium

• q̂ = √μ/s, if q is recessive

• s = selection pressure

• q̂ = (q-hat) frequency of allele q at equilibrium

• μ= forward mutation rate

• Remember that 1-p = q!

Use the island model to explain changes in allele frequency due to migration

• p = m(pC – pi)

• pc = allele frequency on continent

• pi = allele frequency on island

• m = migration rate (migrants/new pop total)

• Migration can introduce new alleles to a population

• Migration can overwhelm existing variation in a population

• In general, migration tends to decrease genetic dissimilarity (divergence) between populations which results increased diversity within a population

Describe the pros and cons of using migration as a tool to restore genetic variation

Key Pros (Benefits)

1. Alleviation of Inbreeding Depression:

   • In small, isolated populations, mating between closely related individuals leads to higher homozygosity, exposing deleterious recessive alleles.

   • Introducing migrant alleles restores heterozygosity, masking harmful recessive mutations and rapidly boosting overall population health, fertility, and survival (heterosis / hybrid vigor).

2. Restoration of Adaptive Potential:

   • Genetic variation is the raw material for natural selection. Small populations lose alleles rapidly through genetic drift.

   • Migration introduces novel genetic variants, equipping the population with the standing variation necessary to adapt to emerging environmental stressors, such as climate change, new pathogens, or altered habitat conditions.

3. Demographic Rescue:

   • Beyond gene flow, the physical addition of breeding individuals provides an immediate numerical boost (demographic rescue), helping to lift small populations out of the "extinction vortex" where demographic stochasticity and inbreeding compound one another.

4. Purging Deleterious Alleles:

   • Over time, increased effective population size ($N_e$) enhances the efficacy of natural selection relative to genetic drift, allowing the population to more effectively purge slightly deleterious mutations that would otherwise drift to fixation.

Key Cons (Risks)

1. Outbreeding Depression:

   • If the source and recipient populations are adapted to different local environments, crossing them can disrupt co-adapted gene complexes or break down local adaptations.

   • Extrinsic Outbreeding Depression: Hybrid offspring carry traits intermediate to both parents, making them poorly suited to the recipient environment.

   • Intrinsic Outbreeding Depression: Genetic incompatibilities between diverged genomes cause reduced fertility or viability in $F_1$ or $F_2$ generations.

2. Swamping of Local Adaptation (Genetic Swamping):

   • If high rates of migration are sustained, incoming gene flow can overwhelm local natural selection. Unique, locally adaptive alleles may be diluted or completely replaced by common alleles from the source population, reducing the population's overall fitness in its specific habitat.

3. Loss of Distinct Evolutionary Lineages:

   • Unregulated gene flow can homogenize genetically distinct populations or subspecies, erasing evolutionary unique units (ESUs) and reducing regional, landscape-level biodiversity (beta diversity).

4. Introduction of Pathogens or Invasive Traits:

   • Translocating individuals runs the risk of inadvertently introducing novel diseases, parasites, or maladaptive behaviors from the source environment to which the target population has no immunity or experience.