• Reproductive Isolation Barriers (RIBs), or isolating mechanisms, create barriers to gene flow

Types of RIBs

• Prezygotic reduce likelihood of hybrids
• Postzygotic reduce gene exhange even if hybrid zygotes are produced
  • reduced hybrid survival
  • reduced hybrid fertility

Pre-mating barriers

• Ecological Isolation:
  • Temporal isolation
  • Habitat isolation
  • Immigrants between divergent populations do not survive long enough to breed
• Potential mates meet, but do not mate:
  • Sexual isolation
  • Pollinator isolation

Pre-zygotic barriers

• Mating occurs, but no zygotes
  • Mechanical- reproductive structures don't fit
  • Females is not stimulated by males of another species to produce eggs
  • Gametic isolation - fertilisation fails

Post-zygotic barriers

• Hybrids form but have reduced fitness
  • Extrinsic (hybrids have low environmental fitness)
  • Evological inviability (poorly adapted to both parental habitats)
  • Behavioural sterility (less successful at obtaining mates)
  • In some cases, F1 hybrids can show "hybrid vigor", but this typically doesn't last multiple generations
    • Copepod example. Also mules and cattails
• Hybrid inviability:
  • Reduced survival due to genetic incompatibility
• Hybrid sterility

Haldane's Rule

Hybrid sterility is limited to heterogametic sex (e.g. XY males in mammals and most insects)

Dobzhansky-Muller incompatibilites

Speed of reproductive isolation

• Prezygotic isolation usually evolves faster than postzygotic isolation
  • Example in flies: In laboratory conditions, it was examined which flies mated, then postzygotic measured by survival of hybrids