Speciation

• Speciation


  An evolutionary process that results in the formation of a new species from a pre-existing species
• Speciation


  • Occurs when reproductive isolating mechanisms prevent two breeding organisms from producing fertile, viable offspring
  • There are two basic mechanisms: Allopatric speciation (geographical isolation) and Sympatric speciation (reproductive isolation)
• Allopatric speciation


  1. Geographical barrier physically isolates populations of an ancestral species

  2. Two populations begin to evolve separately due to mutation, genetic drift and natural selection

  3. Eventually the two populations reach a degree of genetic divergence whereby they can no longer interbreed
• Sympatric speciation


  1. Divergence of species within the same geographical location (without a physical barrier)

  2. May result from reproductive isolation due to genetic abnormalities

  3. Typically a chromosomal error arises which prevents successful reproduction with any organism lacking the same error
• Sympatric speciation is most commonly caused by a meiotic failure during gamete formation
• Polyploidy


  Having more than two complete sets of chromosomes
• Fertile polyploid offspring typically require two polyploid parents
• Polyploidy is more common in plant species as they may lack separate sexes or can reproduce asexually
• Polyploid crops may be desirable to farmers as they allow for the production of seedless fruits, demonstrate improved longevity and disease resistance
• Allium species


  • Diploid (2n) Allium cepa (garden onion)
  • Triploid (3n) Allium carinatum (keeled garlic)
  • Tetraploid (4n) Allium tuberosum (chinese chives)
  • Pentaploid (5n) Allium oleraceum (field garlic)
  • Octoploid (8n) Allium nutans (blue chives)