Relatedness = recency of
common ancestry
• You are more closely related to your first
cousins than your second cousins
because….
– grandparents vs. great-grandparents
• You are more closely related to a
chimpanzee than to a worm because….
– ≈ 6 Million years ago (Ma) vs. ≈ 600 Ma
Is a frog more closely related to a
trout or a human?
Why might you go wrong?
If you look “along” the top
Trout
Frog
Lizard
Mouse
Human
• But this is not how evolution happened
• All these species are alive today: A living
trout is not an ancestor of a frog
• The order “along the top” can change
without changing the content of the tree
You can change the order
without changing the tree
Fish
Frog
Lizard Mouse Human Fish
Human Mouse
Lizard
Frog
On this tree, is a frog more closely
related to a trout or a human?
Trout
Human
Mouse
Lizard
Frog
-> The same tree depicts the same
relationships
Is a gibbon more closely related
to a human or a macaque?
Don’t be distracted
by similarity
Remember not to look “along
the top”
Monophyletic group
(clade)
Non-monophyletic
group
Members are more
closely related to
each other than to
any organisms
outside the group
Some members are
more closely related
to organisms outside
the group
Biological classifications should
reflect evolutionary relationships
•
•
•
•
They should mirror phylogenetic trees
Only clades should be named
Most named groups you know of are clades
But there are a few exceptions:
Fish, invertebrata, reptiles, protista, prokaryota