ap bio chapter 20: Concept 20.3: Shared characters are used to construct phylogenetic trees

What do systematists do after identifying homologous characters?

They use them to infer a phylogeny.

What does cladistics classify organisms by?

Common descent.

What is a clade?

A group of species that includes an ancestral species and all of its descendants.

Can clades be nested within other clades?

Yes.

What makes a grouping a valid clade?

It must include the ancestor and all its descendants (monophyletic group).

What is a monophyletic group?

A group that includes an ancestral species and all of its descendants.

What is a paraphyletic group?

A group with an ancestor but missing one or more descendants.

What is a polyphyletic group?

A group that includes species with different ancestors.

What’s the problem with a polyphyletic group?

It includes organisms from different evolutionary lineages, which don’t belong in the same clade.

What is a shared ancestral character?

A trait that originated in an ancestor of the taxon (older trait).

What is a shared derived character?

A new trait unique to a specific clade (evolved more recently).

Can a character be both ancestral and derived?

Yes, depending on the context and placement on the tree.

Why are derived characters important in phylogeny?

They help determine when traits appeared and identify clades.

What is an outgroup?

A group of species that is closely related to but not part of the ingroup.

What is the ingroup?

The group of species being studied.

What does comparing an outgroup to an ingroup reveal?

Which traits are ancestral vs. derived.

What kind of traits are shared by both outgroup and ingroup?

Ancestral traits.

What does branch length represent in some phylogenetic trees?

The number of genetic changes in DNA sequences.

What else can branch length represent?

Chronological time based on fossil evidence.

What determines branching points in time-based trees?

The fossil record.What is maximum parsimony?

What is maximum parsimony?

The simplest explanation—tree with the fewest evolutionary changes.

Why use maximum parsimony?

To narrow down the most likely evolutionary tree when data is large or complex.

What tools are used to find parsimonious trees?

Computer programs that analyze evolutionary data.

Are phylogenetic trees permanent facts?

No, they are hypotheses that can change with new evidence.

What kinds of data support a phylogenetic hypothesis?

Morphological, molecular (DNA), and fossil data.

What is phylogenetic bracketing?

Using features of living descendants to infer traits of extinct ancestors.

What does phylogenetic bracketing rely on heavily?

The fossil record.

What features do birds and crocodiles share?

Four-chambered hearts, song, nest building, and brooding.

What do shared traits in birds and crocodiles suggest about dinosaurs?

Dinosaurs likely had these traits too, due to shared ancestry.

What supports nest building and brooding in dinosaurs?

Fossil evidence.