Causes (and effects)

• Drift
  • Reduction in allele diversity
  • Populations differ (randomly) in which alleles are fixed and which are lost
• Genetic bottleneck
  • Reduction in allele diversity
  • Populations differ (not necessarily randomly) in which alleles are fixed
• Founder effect
  • Reduction in allele diversity
  • Populations differ (not necessarily randomly) in which alleles are fixed
• Selection
  • Ecotypes
    • Distinct ecological conditions
    • No intergrading of ecological conditions between populations
    • Specific selection pressure on population
    • Selection for adaptive traits
    • Genetic basis for adaptive traits
    • Streptanthus example
      • Photo of Streptanthus
      • Serpentine habitats
  • Clines
    • Continuous gradient in ecological conditions
    • Selection pressure on each population, but in proportion to gradient in ecological conditions
    • Selection for adaptive traits
    • Genetic basis for adaptive traits
    • Achillea example: Clausen, Keck & Heisey
      • Two Achilllea photos
      • Photos from a transect across California