Modern Evolutionary
Classification
Section 18-2

Which Similarities are Most
Important?
 Taxonomic groups above species were
“invented” to distinguish between one family, phylum, etc., and another
 Taxonomists have always tried to group organisms according to biologically important characteristics

 Grouping organisms based on visible similarities and differences has its drawbacks
 Ex. Dolphins
 Fish? - live in water, have finlike limbs
 Mammal? - breathe air and feed young

Evolutionary Classification
 Phylogeny - study of evolutionary relationships among organisms
 Prompted by Darwin and his descent with modification
 Organisms grouped into categories that represent evolutionary descent
 Evolutionary classification

What this means…
 Species in a genus are more closely related to each other than they are with species of another genus
 Reasoning: all members of a genus share a common ancestor

What this means…
 All genera in a family share a common ancestor
 Ancestor is further in past than ancestor of any genus in family

Crustacean or Mollusks?
 Limpets and barnacles first classified on visible similarities
 Similar shells, hole in shell, attached to rocks

Crustacean or Mollusks?
 Barnacles - molt (shed exoskeleton), have joined limbs, body segments
 Limpets - internal anatomy like snails, don’t molt
 Barnacles are crustaceans, limpets are mollusks

Classification Using Cladograms
 Shift made from evolutionary classification to cladistic analysis

Uses characteristics that are “evolutionary innovations” or new characteristics that arose as lineages evolve over time
 called derived characteristics - not found in older lineages

Classification Using Cladograms
 Can use derived characteristics to construct a cladogram
 Diagram that shows evolutionary relationships
 Shows how one lineage branched into others

Similarities in DNA and RNA
 Because DNA and RNA are so similar across all forms of life, these molecules provide an excellent way of comparing organism

Similarities in DNA and RNA
 Genes show important similarities at the molecular level
 DNA similarities can be used the help determine classification and evolutionary relationships

Similarities in DNA and RNA
 Scientists can compare DNA sequences to trace history of genes over millions of years
 DNA evidence can show evolutionary relationships of species and how species have changed
 More similar DNA sequences - more recently shared common ancestor

Molecular Clocks
 Comparisons of DNA can be used to mark the passage of evolutionary time
 Done with molecular clocks
 Uses DNA comparisons to estimate length of time two species have been evolving independently

Molecular Clocks
 By analyzing neutral gene mutations, and looking for dissimilarities, one can tell how long ago they shared a common ancestor