Taxonomy, Speciation and Extinction


  • Taxonomy: names organisms, assigns hierarchical relationships
  • Populations: the individuals of a species in a (contiguous) area
  • Cladogenesis: evolution and differentiation of species lineages

Species Concepts

    Morphological Species: distinguishable from other species by its morphology
    Biological Species: potentially isolated from interbreeding with other species (Mayr, 1963)
      Biological Species Concept:
    • Does not apply to fossil species
    • Does not apply to asexual species
    • Does not apply to many plants (hybridization)

Other Species Concepts

  • Evolutionary Species: actually isolated from interbreeding
      A separate evolutionary lineage
  • Phylogenetic Species: shared apomorphic (derived) characters that neither their ancestors nor other species have
  • Polytypic "Species": distinct forms (color, behavior) that might interbreed snakes [r]
  • Subspecies: populations within a species that are morphologically or genetically distinct Cassowary
  • Ecotypes: morphologically distinct (plants) characteristic of a habitat Aradopsis

Higher Classifications: Based on decreasing degree of relationship
Kingdom Animalia
Phylum Chrodata
Class Mammalia
Order Primates
Family Hominidae
Genus Homo
Species Homo sapiens
Subspecies sapiens


The Fossil Record (Eldredge and Gould, 1972; Stanley, 1979)
  • Species Selection: differential survival of species
  • Punctuated Equilibrium: lineages undergo long periods of little morphological change (stasis) interspersed with brief intervals of rapid change (punctuation, speciation) (FIG) (text)
  • Micro- vs. Macroevolution
      Microevolution: Natural Selection - differential survival due to inherited traits (individual selection)
      Macroevolution: differential survival of branches within lineages

    Microevolution individuals and populations generations
    Macroevolution species evolutionary time scale


    Genetic Drift: random changes in the genetic composition of a population, important in small populations (< 100)
    Founder effect: individuals colonizing an area are a subsample of the genetic diversity of the population (FIG) [+ r]
  • Selection Pressure: over generations, gene frequency for traits favoring survival increase (FIG)
  • Gene Flow: movement of genes in populations due to mating
      May act to homogenize genetic composition of species, preventing speciation
    Geographic variation barriers to gene flow determine pattern
      Cline: geographical variation of a trait along an environmental gradient [+ r]

Allopatric Speciation: genetic differentiation of isolated populations
  • Vicariant Isolation: environmental change creates barrier to genetic exchange (FIG)
  • Founder Events: [FIG] dispersal creates new population
  • Results of Re-contact
    1. No Interbreeding (speciation complete)
    2. Interbreeding successful (populations merge) reticulate evolution (FIG)
    3. Interbreeding partially successful Reinforcement mechanisms continue speciation

Sympatric Speciation: divergence of populations in physical contact
Parapatric Speciation: disruptive selection along gradient - species on opposite ends of gradient

Sympatric Speciation:

  • Single mutation effecting reproduction
      Flower morphology, color influences pollinator
  • Chromosomal Changes: greatly reduces production of fertile offspring when mated with parent population
    • inversion, translocation
        Disrupts meiosis and leads to Aneuploidy
    • Aneuploidy: change in chromosome number +/- one
    • Polyploidy: tetraploid hexaploid (Larrea)
      • Autopolyploidy: within a population
      • Allopolyploidy: hybridization of different species

Phyletic Speciation (chronospecies) descendents diverge from ancestors with time (FIG)

Diversification: increasing differences among recently-separated species
  • Ecological Diversification
    • Competitive Exclusion: similar species cannot coexist
    • Sibling (cryptic) species: reproductively isolated but ecologically equivalent
    • Character displacement: sibling species in contact have different morphologies (FIG)

  • Adaptive Radiation: differentiation into multiple species in sympatric habitats or niches (FIG) [.] (Hawaiian Silversword

Extinction threshold

    "The Evil Twin of Speciation"
    Red Queen Hypothesis: "running as fast as we can to stay in place"

  • Ecological Processes: habitat and population factors effecting likelihood of extinction
    • Competition: arrival of superior competitors (Pliocene Interchange)
    • Predators (parasites): may cause extinction if an alternate host supports population

  • High Risk Groups: large carnivorous (more than small herbivorous mammals)


Small Populations: lower birth/death ratios
Pica, Smith 1974,1980

Population Size
Birth Rate
Death Rate
Time to Extinction yr
1 0.35 0.00 2.9
2 0.35 1.63 6.9
3 0.35 2.44 46.2
4 0.35 2.84 405.1
5 0.35 3.15 3751.5

Recent Extinctions next

Extinctions in the Fossil Record


Species Selection: differential survival of species
  • Process of Species Selection:
      strong selection pressure
      rapid environmental change

  • Examples of Species Selection
    • Mesozoic diversification of Angiosperms
    • Cenozoic diversification of Mammals (FIG)
    • Mollusk larval distribution planktotrophic vs. nonplanktotrophic (FIG)
    • Bivalves vs. brachiopods (FIG)
      Other Families