Chapter 7 Updated

Chapter 7: Migration, Drift, Non-random Mating

Assumptions of Hardy-Weinberg Equilibrium

• Core Assumptions:

  • No Natural Selection

  • No Mutation

  • No Migration

  • Large Population Size

  • Random Mating

• If assumptions are broken, allele frequencies may change.

Migration

• Definition: Movement of alleles among different populations, not the same as seasonal migration.

• Gene Flow:

  • Transfer of alleles between different populations.

  • Migration of adults or life stages can impact gene pools.

Implications of Migration

• Hardy-Weinberg Conclusions:

  1. Allele frequencies remain constant over generations in absence of evolutionary forces.

  2. If allele frequencies are p and q, genotype frequencies will be p² (homozygous dominant), 2pq (heterozygous), and q² (homozygous recessive).

• Types of Migration:

  • Immigration: Moving into a population.

  • Emigration: Moving out of a population.

One-Island Model

• Two populations: mainland and island; migration significantly affects allele frequencies in the island population.

• Case Example:

  • Locus A with two alleles (A1 and A2):

    • Pre-migration: A1 frequency = 1.0; A1 fixed in the island population.

    • Post-migration (e.g., 200 individuals from mainland with A2):

      • New frequency after mating: A1 = 0.8, A2 = 0.2.

Consequences of Migration

• Violates allele frequency conclusion 1, indicating evolution occurred due to migration.

• Leads to an excess of homozygotes and after random mating, can re-establish the equilibrium.

Effects of Migration on Variation

• Homogenization: Migration typically homogenizes populations unless countered by other forces (e.g., natural selection).

• Rate of Gene Flow (m): Varies among populations; influences allele frequencies significantly across populations.

Genetic Drift

• Definition: Evolutionary changes in allele frequencies due to random chance and sampling errors.

• Important in small populations where it can lead to fixation of alleles.

• Supports the notion that genetic drift can lead to significant evolutionary change—even without natural selection.

Founder Effect

• Occurs when a small group establishes a new population, causing allele frequencies to differ from the source population due to chance.

• Example: Pennsylvania Amish population showed higher frequency of Ellis-van Creveld syndrome due to the founder effect.

Genetic Bottleneck

• Sudden reduction of population size which eliminates many alleles, changing genetic make-up substantially.

• Can lead to long-term low genetic diversity, as observed in cheetah populations.

Relationship Between Population Size and Genetic Drift

• Smaller populations have a higher chance of allele fixation.

• Genetic diversity decreases as populations drift over time, with consequences for overall fitness.

Non-Random Mating

• Aspects: Mating based on phenotypes, leading to assortative and disassortative patterns.

• Inbreeding: Increases homozygosity, reduces heterozygosity across loci.

• Coefficient of Inbreeding (F): Measures the likelihood of alleles being identical by descent, ranges from -1 (only heterozygotes) to 1 (homozygous fixation).

Effects of Non-Random Mating

• Inbreeding depression may occur, exposing deleterious alleles: higher mortality rates observed in first cousins vs. unrelated parents.

• Mechanisms evolved in species to prevent inbreeding, particularly in small populations with limited mate choice.

Conservation Genetics

• Importance of understanding the dynamics of populations like the Florida Panther, which show the impacts of genetic drift, non-random mating, and low genetic diversity.

• Mutational Meltdown: Interaction of inbreeding depression and genetic drift that can lead to declining effective population sizes, leading toward extinction if not mitigated through gene flow or artificial migration.