Lecture 18

• Speciation


  The process by which new biological species arise
• General pattern of speciation

  1. Isolation of populations

  2. Divergence of populations

  3. Reproductive isolation of populations
• Speciation is not necessarily true in all cases
• Speciation begins when gene flow is somehow disrupted, and populations become genetically isolated
• Speciation is sometimes rapid (i.e., partial reproductive isolation has evolved in the laboratory) or even instant (i.e., whole genome duplication)
• In other cases, it may take millions of years for reproductive isolation to evolve
• Modes of speciation


  • Allopatric
  • Peripatric
  • Sympatric
  • Parapatric
• Allopatric speciation


  • The initial cause of isolation is geographical (i.e. a physical barrier)
  • This is believed to be the most common mode of speciation
  • Isolation may occur through dispersal or vicariance
• Allopatric speciation through vicariance

  • Kaibab squirrel and Abert's squirrel
  • Isolated by the Grand Canyon ~10,000 years ago
• Allopatric speciation through dispersal


  • Older-to-younger sequence of branches on the phylogeny corresponds to the older–younger sequence of island formation
• Sympatric speciation


  • A new species forms in the same place as the ancestral population
  • Often due to ecological specialization or speciation by polyploidy

  
• Sympatric speciation in Howea palms

  • Ecological niche differentiation
  • Reproductive isolation due to differences in flowering time
  • Morphological differences
• Autopolyploid


  Has multiple sets of chromosomes from one species due to chromosome duplication
• Allopolyploid


  Has multiple sets of chromosomes from two or more species, created by hybridization and chromosome duplication
• Secondary contact


  • When two populations (or newly formed species) that were previously separated come back into contact with one another
• Overlap of white oaks Quercus grisea and Quercus gambelii


  • Hybrid zone

  
• Possible outcomes of secondary contact

  • Species remain separate
  • Hybrid speciation
  • Species persist with gene flow
  • Species merge
• Mechanisms of reproductive isolation

  • Prezygotic barriers
  • Postzygotic barriers

  
• Prezygotic barriers may evolve due to divergent selection on the niches of the species or due to divergent sexual selection

  
• When hybrids between two divergent populations have low fitness, there is selection for stronger prezygotic isolation, which may result in reinforcement of a prezygotic barrier

  
• The causes of evolution of postzygotic isolation are less well understood
• Hybrid inviability and hybrid sterility are generally caused by genetic incompatibility of the two species, possibly because of genetic drift