THE SCIENCE OF MODERN CLASSIFICATION CLADISTICS

CLADISTICS IS BASED ON EVOLUTIONARY RELATIONSHIPS 1. All organisms are related to a common ancestor 2. Mutations in DNA cause characteristics to over time 3. Accumulation of mutations are shown by branching (splitting in two) patterns from the common ancestor

What’s the evidence? 1.Scientists can base their hypotheses on shared characteristics, called a homologous feature Ex. Forelimbs of four-legged animals

What’s the evidence? 2. Scientists can base their hypotheses DNA evidence

What’s a cladogram? Cladogram: a graphic organizer showing hypothetical relationships between organisms

METHODOLOGY “How-to” Of cladistics

1. SELECT FEATURES Decide if each group has the characteristics or not Selected features should not overlap vertebra Boney skeleton limbs Amniotic egg hair Egg w/ shell

2. GROUP BY FEATURES Species go on the ends of branches Nodes are common ancestors Features are labeled

3. IDENTIFY CLADES A clade is a group or taxon that represents ALL of the branching groups that share a common ancestor Which color represents a clade?

VOCABULARY Shared (primitive) trait : a characteristic that was present in the common ancestor Derived trait : a characteristic that is different from the common ancestor Outgroup : shares a common ancestor, but is the least related in the clade

What is an example of a shared trait ? What is an example of a derived trait ? Which is the outgroup ?

4. RULE OF PARSIMONY If there are multiple possible cladograms, the simplest one (with the least number of changes) is best.

READING TREES HOW TO READ A CLADOGRAM

STRATEGY Look for the most recent common ancestor (nodes) More recent ancestors will be “higher” Older ancestors will be “lower” Don’t read “along the tips”

PRACTICE WITH CLADOGRAMS