Speciation Notes

Learning Objectives

  • Compare allopatric and sympatric speciation processes.
  • Explore how autopolyploidy and allopolyploidy lead to rapid sympatric speciation.
  • Evaluate sticklebacks as an example of speciation in action.
  • Compare local adaptation to acclimation and consider which process leads to speciation.

Allopatric Speciation

  • Involves geographic separation.
  • Selection and/or drift occurs.
  • Example: Kaibab squirrel (North Rim) and Abert’s squirrel (South Rim) separated by the Grand Canyon.

Sympatric Speciation

  • Occurs without geographic separation.
  • A polymorphism appears.
  • Assortative mating takes place, driven by sexual selection.

Sympatric Speciation: Habitat Differentiation

  • Polymorphism appears.
  • Assortative mating occurs.
  • Examples of habitat differentiation driving sympatric speciation.

Sympatric Speciation: Autopolyploidy

  • Cell division error within a species leads to an increase in chromosome number.
  • A diploid organism (2n = 12) experiences an error leading to diploid gametes and a tetraploid species (4n = 48).
  • New species is likely established through self-fertilization.

Sympatric Speciation: Allopolyploidy

  • Hybridization occurs between two species; chromosomes can't pair up properly.
  • Error doubles the chromosomes, and hybrids can now interbreed.

Sympatric Speciation: Allopolyploidy Details

  • Two sets of chromosomes exist that can't pair properly.
  • Functionally diploid: two unpaired sets of chromosomes + one unpaired chromosome.
  • Functionally tetraploid with two sets of paired chromosomes.
  • New species is likely established through self-fertilization.

Polyploidy as a Speciation Mechanism

  • Polyploidy is an incredibly successful speciation mechanism in plants because:
    • It happens nearly instantaneously.
    • It provides higher genetic diversity.
    • It leads to reduced inbreeding depression.
  • Polyploid African clawed frog (Xenopus) as an example.
  • Question: Why are there so few animal polyploids?

Sticklebacks and Speciation

  • Limnetic sticklebacks feed on plankton in open water of lakes.
  • Benthic sticklebacks feed on invertebrates near the lake bottom.
  • Speciation example demonstrated by Rundle et al. 2000.
  • Probability of spawning:
    • Same lake vs. Different lakes
    • Limnetic x Limnetic, Limnetic x Benthic, Benthic x Benthic.

Adaptation, Acclimation, Speciation

  • Adaptation: Natural selection in response to environment leads to increased frequency of phenotype in the population over time.
  • Local adaptation: Species exhibit phenotypes that differ due to local conditions, potentially leading to speciation over time.
  • Acclimation: Physiological response to environment leads to changes in individual phenotype during a single generation.

Adaptation vs. Acclimation

  • Thale cress, Arabidopsis thaliana, as an example.
  • Distinction between native and naturalized populations to determine adaptation vs. acclimation.