Transcript Modern Evolutionary Classification

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