interpreting cladograms big idea: phylogenies depict ancestor and descendent relationships among...

INTERPRETING CLADOGRAMS

BIG IDEA: PHYLOGENIES DEPICT ANCESTOR AND DESCENDENT RELATIONSHIPS AMONG ORGANISMS BASED ON HOMOLOGY

THESE EVOLUTIONARY RELATIONSHIPS ARE REPRESENTED BY DIAGRAMS CALLED CLADOGRAMS (BRANCHING DIAGRAMS THAT ORGANIZE RELATIONSHIPS)

Interpreting Cladograms Notes

Reading Cladograms

• When an ancestral lineage splits: speciation is indicated due to the “arrival” of some new trait.

Each lineage has unique traits to itself alone and traits that are

shared with other lineages.

each lineage has ancestors that are unique to that lineage and ancestors that are shared with other lineages — common ancestors.

Read like a family tree: show patterns of shared ancestry between lineages.

Quick Question #1

What is our definition of a clade?

(look back to zoology notes #1 if you cannot remember)

A group that includes a common ancestor and all the descendants (living and extinct) of that ancestor.

Reading Cladogram: Identifying Clades

Using a cladogram, it is easy to tell if a group of lineages forms a clade.

Imagine clipping a single branch off the phylogeny all of the organisms

on that pruned branch make up a clade

So everything in the pink circle is a clade (common ancestor and all descendants)

Quick Question #2

Looking at the image to the right:

Is the green box a clade?

The blue?The pink?The orange?

Reading Cladograms: Clades

Clades are nested within one another they form a nested hierarchy.

A clade may include many thousands of species or just a few.

Interpreting Cladograms

it's easy to misinterpret cladograms as implying that some organisms are more "advanced" than others

however, cladograms don't imply this at all.when reading a cladogram, it is important to

keep three things in mind

(mis)Interpreting Cladograms: One

Evolution produces a pattern of relationships among lineages that is tree-like, not ladder-like.

(mis)Interpreting Cladograms: Two

Just because we tend to read phylogenies from left to right, there is no correlation with level of "advancement."

(mis)Interpreting Cladograms: Three

For any speciation event on a phylogeny, the choice of which lineage goes to the right and which goes to the left is arbitrary. The following phylogenies are equivalent:

Interpreting Phylogenies: Human Example

The points described above cause the most problems when it comes to human evolution.

It is important to remember that: Humans did not evolve from

chimpanzees. Humans and chimpanzees are evolutionary cousins and share a recent common ancestor that was neither chimpanzee nor human.

Humans are not "higher" or "more evolved" than other living lineages. Since our lineages split, humans and chimpanzees have each evolved traits unique to their own lineages.

Quick Question #3

What is this called?

What do you think the red lines represent?

Creation of Cladograms

Given a set of observations, phylogenetic analysis seeks to find the simplest branching relationships between organisms to depict their evolution.

Heritable traits possessed by organisms, characters, are used to compare the organisms being studied.

• Characters can

be compared across organisms • physical traits• genetic

sequences• behavioral

traits.

?BUT HOW DO WE CONSTRUCT A CLADOGRAM?

3 Alternative, mutually exclusive Cladograms

How Do We Choose Between Them?

Outgroup

(Not an Ancestor, but a Stand-in to represent the Ancestral Condition)

PP RD PCFur/Mane No Yes Yes YesToes/Foot Many Toes One Hoof One Hoof One HoofWings No No Yes YesHorn No No No YesEyes Yes Yes Yes YesTail Yes Yes Yes YesMouth Yes Yes Yes Yes

Primitive (ancestral) State Derived States

CharactersINGROUP ORGANISMS

Outgroup PP RD PCFur/Mane No Yes Yes YesToes/Foot Many Toes One Hoof One Hoof One HoofWings No No Yes YesHorn No No No YesEyes Yes Yes Yes YesTail Yes Yes Yes YesMouth Yes Yes Yes Yes

CharactersINGROUP ORGANISMS

Fur/ManeOne Hoof

Wings

Horn

EyesTailMouth

Ancestral characters shared by all taxa link organisms together

Derived character states found in only one organism separate them from other organisms

3 Steps (evolutionary transitions from ancestral derived) to explain this tree

Outgroup PPRD PCFur/Mane No YesYes YesToes/Foot Many Toes One HoofOne Hoof One HoofWings No NoYes Yes

CharactersTaxa

Fur/ManeOne Hoof

Wings

Wings4 Steps (with wings developing convergently)

Wings

Loss of Wings

4 Steps (with wings developing in ancestral pony, and lost in PP)

OR

Outgroup PP RDPCFur/Mane No Yes YesYesToes/Foot Many Toes One Hoof One HoofOne HoofWings No No YesYes

CharactersTaxa

Fur/ManeOne Hoof

Wings

Wings4 Steps (with wings developing convergently)

Wings

Loss of Wings

4 Steps (with wings developing in ancestral pony, and lost in PP)

OR

3 Steps

4 Steps

4 Steps

The preferred cladogram is the simplest! (Least

number of assumptions)

So, which cladogram is the best description of the evolution of these little ponies?