Basic requirements

• Reproductive isolation (lack of gene flow)
• Differential selection pressures (if morphological differences are to develop)
• Migrants from one species are at a disadvantage relative to native species
• Time

Geography-related modes of speciation

• Allopatric or geographic speciation
  • Populations geographically isolated, preventing gene flow
  • Adapt to different conditions from other populations
  • Genetic differences accumulate
  • Perhaps genetic barriers to gene flow develop
  • Examples
    • Gilia races
    • Platanus (sycamore)
  • Weaknesses
    • If species are held together by gene flow (making them internally uniform) how to widely-separated populations remain so similar to each other?
    • In the absence of gene flow, identical selsctioon pressures could maintain genetic similarity among widely-separated populations, but how likely is it that all ecological variables co-vary throught a species' range?
• Local speciation, or the Peripheral Isolate Model
  • Populations peripheral to main range of a species are likely to exhibit features that promote genetic divergence (i.e., population-level processes are at work)
    • Small populations
      • subject to drift
    • Large fluctuations in population size
      • experience bottlenecks
    • Occupy ecologically marginal habitats
      • subjected to different selection pressures than central populations
  • Local adaptation equips marginal populations with different capabilities than central populations
  • Marginal populations now less fit for conditions found in central populations
    • If marginal plant is dispered back into central region, it will be selected against (less fit) relative to plants native to central region
  • Reduced fitness of marginal plants relative to central plants implies that hybrids bewteen the two will be less fit in either region relative to natives
  • Leads to selection for reproductive isolation
  • Clarkia example
    • Several species pairs studied
    • Lots of chromosomal structural changes among species
      • Give rise to morphological differences
      • Give rise to intersterility between species
    • Clarkia biloba
      • Bilobed petals
      • Ancestral chromosome number (n=8)
      • Moister habitats than C. lingulata
      • Widespread in Sierra Nevada in California
      • Self-compatible, but mostly outcrosses
    • Clarkia lingulata
      • Tongue-shaped petals
      • Derived chromosome number (n=9)
      • Drier habitat than C. biloba
      • Single canyon at southern periphery of C. biloba
      • Self-compatible, but mostly outcrosses

Sympatric Speciation