Relatedness = recency of common ancestry

– grandparents vs. great-grandparents

– ≈ 6 Million years ago (Ma) vs. ≈ 600 Ma

• You are more closely related to your first cousins than your second cousins because….

• You are more closely related to a chimpanzee than to a worm because….

Is a frog more closely related to a Is a frog more closely related to a trout or a human?trout or a 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

TroutTrout FrogFrog LizardLizard MouseMouse HumanHuman

Why might you go wrong?If you look “along” the top

You can change the order without changing the tree

Fish FrogLizardMouseHumanFishFish FrogFrog LizardLizard MouseMouse HumanHuman

Trout FrogLizardMouseHuman

-> The same tree depicts the same relationships

On this tree, is a frog more closely related to a trout or a human?

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”

Non-monophyletic group

Some members are more closely related to organisms outside

the group

Monophyletic group(clade)

Members are more closely related to

each other than to any 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