Are species "real?"

Attempts at defining the term "species"

• Morphological (or Phenetic) Species Concept
  • Members of a species are morphologically similar to each other
  • Members of different species differ from each other
    • In many characterstics
    • Characters that distinguish species are correlated with each other
  • A gap in charactersitics separate one species from another
• Biological Species Concept
  • Individual that are interfertile belong to the same species
  • Benefits of the BSC
    • It is testable
    • It emphasizes isolation as a species criterion, which is consistent with modes of speciation
  • Problems with the BSC
    • Plant often exhibit intermediate levels of interfertility within and among species
    • Cannot be applied to extinct, fossil, or asexual species
    • Hybridization is comon in plants; many morphologically distinct species would be united under the BSC
• Phylogeny-based definitions (Evolutionary species concept, Autapomorphy species concept, genealogical species concept, etc.)
  • Systematists accept the need to reflect phylogenetic relationships in classifications, but
  • No single definition is applicable across all plant groups because of the diversity of evolutionary processes occuring in plant species
  • Morphology remains a primary means of species definition
    • Interpretation of morphological patterns is moderated by knowledge of taxon-specific evolutionary processes
    • Although indirect, morphology does a pretty good job of indicating ancestor-descendent relationships, particularly when many morphological characters co-vary across species.

Case studies of species relationships

• Easily recognized species
  • Morphogical similarity parallels evolutionary history: plants that look alike
    • are most closely related
    • belong to a distinct evolutionary line
    • do not hybridize with dissimilar-looking plants
  • All species definitions are likely to yield the same classfication in these groups of plants
• Microspecies (aka sibling or cryptic species)
  • Morphologically very similar to each other
  • Little or no hybridization
  • May be autogamous or xenogamous
• Agamospecies
  • Reproduce asexually (most of the time)
  • Produce embryos from diploid cells, not from zygote
  • Often are the sterile products of hybridization between sexual species
  • Usually produce some viable pollen that can fertilize eggs of sexual relatives
  • Usually produce very small percentage of sexual seeds
• Species that hybridize like crazy
  • Usually lack internal barriers to hybridization
  • Rely on external barriers than can break down (temporal, ecological, ethological)
  • Frequently exhibit introgression
  • May form hybrid swarms
• Species that are isolated only by distance
  • Species may look different, but are fully interfertile
  • Adaptive radiation without reproductive divergence
  • Adaptive radiation without correspondingly great genetic divergence