Phylogenetic Trees: Quick Review

Darwin: life on Earth shows common descent; lineages branch over time to form a Tree of Life.
Phylogenetic trees: visual representation of evolutionary history; a hypothesis about relationships among populations, species, or genes.
Tree anatomy basics:
- Internal nodes = hypothetical ancestors.
- Clade = a node and all its descendants.
- Trees can be drawn in different shapes but represent the same relationships.

Data source: traits/characters (morphology, behavior, DNA, etc.).
Taxa are grouped by shared traits; more traits in common implies closer relationship.
Character states:
- Binary state: $0$ or $1$ .
- Shared vs. derived (synapomorphies) vs. ancestral (plesiomorphies).
Homologous vs. Analogous:
- Homologous: traits present in a common ancestor; reflect shared descent.
- Analogous: traits with similar function but not from a common ancestor (convergent evolution).
Outgroups: distant relative outside the ingroup used to infer ancestral vs derived states.
Data matrix: characters with binary states used to build trees.
Parsimony: best tree is the one with the fewest evolutionary steps (state changes).
- Goal: minimize the total number of state changes across all characters and branches.
- Often represented as: minimize $ext{sum of changes across all branches}$ .

Trees are not linear progressions from simple to complex; they represent branching history.
Branch order can be rearranged around nodes without changing relationships (topology).
Tick marks indicate character-state changes; taxa above a tick share the derived state.
Most Recent Common Ancestor (MRCA): the most recent ancestor shared by a set of taxa.

Rooted vs. unrooted trees:
- Rooted: has a direction of time from root to tips.
- Outgroup helps root the tree.
Outgroup concept:
- An organism outside the ingroup used to determine ancestral vs derived states.
- Provides a reference point to polarize characters.
Example use: choose an organism clearly not part of the ingroup (e.g., kangaroo for a mammal study) to assign ancestral vs derived states.

Monophyletic group (clade): includes an ancestor and all its descendants.
Polyphyletic: group that does not include the most recent common ancestor of its members.
Paraphyletic: group that includes the most recent common ancestor but not all descendants.
In modern taxonomy, groups are preferred to be monophyletic (clades).
Taxonomy uses shared, derived characters to define groups (clades).

Ancestral state (plesiomorphy) vs. derived state (apomorphy).
Synapomorphy: a shared derived character helpful for defining a clade.
Homologous traits: derived from a common ancestor; underlying structure often similar.
Analogous traits: similar function but not from a common ancestor.
Outgroup: reference taxon outside the group of interest used to root the tree.
Ingroup: group of taxa under study.
Node: hypothetical ancestor; branching point in the tree.
Branch: lineage segment between nodes.
Clade: node plus all its descendants.
Tree topology: the branching relationships among taxa; can be rearranged without changing relationships.
Parsimony principle: prefer the tree with the fewest changes; the simplest explanation.

How to tell ancestral vs derived states: use outgroup to polarize characters.
Reading a tree: shared derived states define clades; monophyletic groups are preferred.
MRCA concept helps identify the origin of shared traits among taxa.
Common pitfalls: convergent evolution can create similar traits (analogies) that mislead analyses if not identified.

What makes a group monophyletic? (Answer: includes a common ancestor and all its descendants.)
Why is an outgroup useful? (Answer: helps determine ancestral vs derived character states.)
How does parsimony guide tree selection? (Answer: choose the tree with the fewest character-state changes.)
Distinguish homologous vs. analogous traits with an example.