Animal Classification & Phylogeny

• Systematics or taxonomy


  Study of the kinds and diversity of organisms and of the evolutionary relationships among them
• Taxon


  Any grouping of animals that shares a particular set of characteristics
• Taxonomic categories (hierarchically arranged from broader to more specific)

  • Domain
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species
• Above species level there are no definitions for each taxonomic category
• Molecular Approaches to Animal Systematics

  Relatedness of animals reflected in proteins and DNA, including nuclear and mitochondrial DNA, and ribosomal RNA
• Domains and Kingdoms


  • Ribosomal RNA studies reveal distant evolutionary relationships
  • Evolutionary conservation results in slow rates of change
  • Three major lineages (Domains): Eubacteria, Archaea, Eukarya
• Archaea


  Extremophile microbes
• Eukarya


  Organisms with compartmentalized cells, including nuclear membranes, mitochondrial and chloroplast membranes
• Horizontal gene transfer was common in the early history of life and makes the base of the tree of life net-like
• Animal Systematics


  Goal is to arrange animals into MONOPHYLETIC GROUPS using traits having a genetic basis and that can be measured (characters)
• Monophyletic group


  Single ancestral species and all descendants
• Polyphyletic group


  Some, but not all, members of a lineage included
• Paraphyletic group


  Members of a lineage found to have separate ancestry
• Evolutionary systematics


  • Traditional approach
  • Homologies useful in classification
  • Phylogenetic trees depict relationships, time, and abundance
• Phylogenetic systematics (cladistics)


  • Homologies of recent origin are most useful
  • Ancestral characters are common to all members of a group (symplesiomorphies)
  • Outgroup is a related group not included in the study group, used to help decide if a character is ancestral or more recently derived
  • Derived characters have arisen since common ancestry with the outgroup (synapomorphies)
  • Clade is a related subset within a lineage
  • Cladogram depicts a hypothesis regarding monophyletic lineage
• Phylogenetic species concept


  Group of populations that have evolved independently of other groups of populations, sharing one or more synapomorphies in a hierarchical nesting pattern
• Symmetry


  Describes how parts of an animal are arranged around a point or axis
• Sponges

  • Display cell-aggregate organization and some are asymmetrical (Monochora barbadensis)
• Coral polyp (Tubastraea sp.)


  • Radially symmetrical
• Bilateral symmetry

  • Accompanied by the formation of a distinct head (cephalization)
• Levels of organization

  • Unicellular (cytoplasmic)
  • Sponges (cell aggregate)
  • Diploblastic (ectoderm, endoderm, mesoglea)
  • Triploblastic (mesoderm, body cavities, organ development)
• Triploblastic organization

  • Mesoderm forms third tissue layer sandwiched between ectoderm and endoderm
  • Supportive, contractile, and blood cells
  • Body cavities often present
  • Exchanges by diffusion, storage, hydrostatic skeletons, elimination of wastes and reproductive products, facilitate increased body size
• Triploblastic acoelomate

  Mesoderm forms solid mass
• Triploblastic pseudocoelomate

  Body cavity not entirely lined by mesoderm, gut not associated with muscle or connective tissue
• Triploblastic coelomate


  Body cavity completely surrounded by mesoderm, mesenteries suspend visceral structures
• Animalia is monophyletic, based on molecular and embryological evidence
• Four phyla originated independently

  • Bilaterally symmetrical phyla
  • Protostome phyla (spiral, determinate cleavage, trochophore larval stage, ecdysozoans that molt a cuticle, lophotrochozoans)
  • Deuterostome phyla (radial, indeterminate cleavage)