chap 23

• Systematics

  • All organisms share many characteristics:
  • Composed of one or more cells.
  • Carry out metabolism.
  • Transfer energy with ATP.
  • Encode hereditary information in DNA
• Systematics - Reconstruction and study of evolutionary relationships
• Phylogeny - Hypothesis about patterns of relationship among species.
• Cladistics

  • Ancestral characteristic - Similarity that is inherited from the most recent common ancestor of an entire group.
  • Derived characteristic - Similarity that arose more recently and is shared only by a subset of the species
• Phylogenetic Trees

  • Each branch point or node represents the divergence of two taxa from a common ancestor
  • Sister taxa are groups that share an immediate common ancestor
• Phylogenetic Trees

  • Rooted tree includes a branch to represent the most recent common ancestor of all taxa in the tree
  • Basal taxon diverges early in the history of a group and originates near the common ancestor of the group
  • Polytomy is a branch from which more than two groups emerge
• What We Can and Cannot Learn from Phylogenetic

  • Trees Phylogenetic trees show patterns of descent, not phenotypic similarity
  • Phylogenetic trees do not generally indicate when a species evolved or how much change occurred in a lineage
  • It should not be assumed that a taxon evolved from the taxon next to it
• Clades

  • Cladogram
  • Depicts a hypothesis of evolutionary relationships.
  • Clade
  • Species that share a common ancestor as indicated by the possession of shared derived characters. • Evolutionary units and refer to a common ancestor and all descendants. • Synapomorphy – derived character shared by clade members
• Construction of a cladogram

  • Plesiomorphies – ancestral states
  • Symplesiomorphies – shared ancestral states
• When constructing a phylogeny, systematists need to distinguish whether a similarity is the result of homology or analogy

  • Homology is similarity due to shared ancestry
  • Analogy is similarity due to convergent evolution
• Convergent evolution can occur when similar environmental pressures and natural selection produce similar (analogous) adaptations in organisms from different evolutionary lineages
• Homoplasy

  Homoplasy – a shared character state that has not been inherited from a common ancestor

  Convergent evolution, Evolutionary reversal
• Maximum parsimony assumes that the tree that requires the fewest evolutionary events (appearances of shared new derived characters) is the most likely
• Molecular homologies are determined based on the degree of similarity in nucleotide sequence between taxa
• Other Phylogenetic Methods

  • Statistical approach - Start with an assumption about the rate at which characters evolve. • Fit the data to these models to derive the phylogeny that best accords (i.e., “maximally likely”) with these assumptions.
  • Molecular clock - Rate of evolution of a molecule is constant through time. • Divergence in DNA can be used to calculate the times at which branching events have occurred
• Molecular clocks

  • To extend molecular phylogenies beyond the fossil record, and calculate when in time species diverged, we must make an assumption about how molecular change occurs over time
  • A molecular clock uses constant rates of evolution in some genes to estimate the absolute time of evolutionary change
• Molecular Clocks Between two species, we assume

  • the number of DNA changes in a gene are proportional to the time since they last shared a common ancestor
  • I.e. the longer two taxa have diverged from their common ancestor, the more mutations they will have
  • Fair assumption over long evolutionary time periods
• Taxonomy is the ordered division and naming of organisms
• Binominal Nomenclature

  The two-part scientific name of a species is called a binomial

  The first part of the name is the genus

  The second part, called the specific epithet, is unique for each species within the genus
• Hierarchical Classification from smallest to biggest

  species, genus, family, order, class, phylum, kingdom, and domain
• Cladistics

  Cladistics classifies organisms by common descent

  A clade is a group of species that includes an ancestral species and all its descendant
• A valid clade is monophyletic, signifying that it consists of the ancestor species and all its descendants.
• A paraphyletic grouping consists of an ancestral species and some, but not all, of the descendants
• A polyphyletic grouping consists of various taxa with different ancestors
• Taxonomic hierarchies are based on shared traits, ideally should reflect evolutionary relationships.
• Species concepts Biological species concept (BSC) - Defines species as groups of interbreeding populations that are reproductively isolated.

  Phylogenetic species concept (PSC) - Species is a population or set of populations characterized by one or more shared derived characters
• Label the Taxon/Root

  A) Basal

  B) Sister

  C) Polytomy

  D) Ancestral

  
• Cladogram - The derived characters between the cladogram branch points are shared by all organisms above the branch points and are not present in any below them. The outgroup (the lamprey) does not possess any of the derived characters.

  
• Lable the Clades

  A) Monophyletic

  B) Paraphyletic

  C) Polyphyletic

  
• The traditional classification included two groups that we now realize are not monophyletic: the green algae and bryophytes