Speciation and Systematics


 A. Biological Species Concept

  1. Groups of populations that can actually or potentially exchange genes with one another and that are reproductively isolated from other groups
  2. types of reproductive barriers
  3. allopatry vs sympatry
  4. problems: asexual species, weak hybridization barriers, only living species
  5. advantage: clear tie to concept of fitness and common gene pool

B. Phylogenetic Species Concept

  1. monophyletic group composed of a cluster of individual organisms within which there is a pattern of ancestry and descent
  2. members of group share derived characters that distinguish them from other groups
  3. problems: how many shared derived characters make a species
  4. advantages: can use on fossil organisms and asexual species

C.  Process of Speciation

  1. genetic separation
  2. phenotypic differentiation
  3. geographic isolation: allopatry vs sympatry
  4. prezygotic vs postzygotic barriers to reproduction


Goal of systematics is to discover the relationships among organisms that result from having common ancestors

  1. organisms are mosaics of ancient and recent features, systematics infers historical relationships from similarities
  2. convergence (look the same but different ancestor, common function), form of homoplasy
  3. divergence (look different but common ancestor)
  4. structure homologous if inherited from a common ancestor--may be modified
  5. a. same developmental pathway
    b. share same relative position to other structures such as nerves and blood vessels
    c. more complex the structure, stronger the argument
  6. genes orthologous if share similar DNA sequence (more sequence, stronger argument)
  7. first steps to a phylogenetic tree:
  8. a. determine the outgroup = taxon not within group being studied but close
    b. outgroup choice roots the tree
    c. implications of outgroup choice are ancestral character states (plesiomorphic) vs derived character states (apomorphic)
  9. cladistics--
  10. a. look only at shared derived character states (synapomorphies)
    b. best tree is one that requires fewest changes in character states to get to current species, i.e. parsimony
    c. ignores shared ancestral characters, give no information on evolutionary path
  11. phenetics
  12. a. group species by similarity of their appearance
    b. may not give same phylogenetics
    c. does not explicitly emphasize evolutionary path
    d. molecular data often measures level of genetic similarity between two groups and is therefore a phenetic system of classification
  13. crucial point: phylogenies inferred from different types of data should, under common descent, be concordant