Transcript Systematics and the Phylogenetic Revolution

Systematics and the Phylogenetic
Revolution
Cladograms
Cladistics
• Derived
– Similarity that is inherited from the most recent
common ancestor of an entire group
• Ancestral
– Similarity that arose prior to the common
ancestor of the group
• Cladistics
– Compare derived and ancestral similarities
• Only shared derived characters are considered
informative in determining evolutionary relationships
Identification of Derived vs. Ancestral
Characters
• Comparison of any pheno- or genotype
• Recognizable character states
– “teeth” amniote vertebrates (birds, reptiles,
mammals)
• Presence in most mammals and reptiles
• Absent in birds and turtles
“Teeth”
• Nearest outgroups of amniotes (amniotic
eggs)
– Amphibians and Fish
• Teeth are present (in many others as well)
– Presence of teeth in mammals and reptiles (came after
amniotes) considered ancestral
– Absent in birds and turtles = derived
Example of Derived vs. Ancestral
Mammal common ancestry
• Hair
– Shared derived character in mammals
• Only seen in the most recent common ancestor
• Lungs
– Ancestral character
• Also present in amphibians and reptiles
 Evolved prior to the common ancestor or mammals
 Lungs don’t show a closer relationship between mammals than
reptiles or amphibians
 Doesn’t set mammals apart
 Hair suggests that all mammal species share a common ancestor
that existed more recently than the common ancestor of mammals,
amphibians and reptiles
 Sets mammals apart from other species
Constructing a Cladogram – Step 1
• Depicts hypothesis of evolutionary relationships
• Polarize the data
– Determine whether or not the characters are derived
or ancestral
• Comparison to an outgroup
– Species or group of species that is closely related to,
but not a member of, the group of study
• Lamprey, Shark, Salamander, Lizard, Tiger, Gorilla, Human
– Lamprey = outgroup
Constructing a Cladogram – Step 2
• Construction
– Clade
• Species that share a common ancestor (evolutionary
units)
– Shown by derived characteristics
• Group of clades that share derived
characteristics
– Amniotes  amniotic membrane
– Mammals  hair
What about convergent evolution?
• Characteristic has evolved independently
– No common ancestor
Shows a false relationship
• Homoplasy
– Shared character state that has not been inherited
from a common ancestor
• Frogs  no tail
– Should be related to Gorillas, humans  no tail ???????
– However, Frogs  no amniotic membrane and hair  gorillas,
humans have amniotic membrane and hair
• Frogs are closer to salamanders because one less change
occurred (just loss of tail)
Phylogenetics and Comparative
Biology
• Homologous  common ancestor 
divergent evolution
• Analogous  homoplastic structures  no
common ancestor  convergent evolution
• Parental care  homologous
– Dinosaurs, Crocodiles and birds
• Saber teeth  homoplastic
– Many cat like animals, carnivores