Phylogeny Phylogeny Evolutionary history of a species or a group of - - PowerPoint PPT Presentation

Phylogeny

Phylogeny

• Evolutionary history of a species or a group of

species

• Goal: Resulting phylogeny should match

taxonomy (classification of an organism)

Phylogeny and Classification

Phylogenetic Tree

• Constructed using morphological similarities

(homologies) of living or fossil species, DNA and protein sequences

• Show evolutionary history and relationships

among groups of organisms (hypothesis)

• Trees are dynamic (constantly being revised)

Understanding Phylogenies

• Click on the hyperlink above for an

introduction to phylogenetic trees

• Read the first page and then click “next” at

the bottom of the page for page 2 of Understanding Phylogenies

• Write down 3 things in your notes that you

learned about phylogeny by exploring this link

Tree Terminology

Tree Terminology

• Sister taxa – groups of organisms that share

an immediate common ancestor

• Node (branch point) – represents the most

recent common ancestor of a group

• Root – single branch point from which all

branches originate in the tree

Rooted versus Unrooted Trees

Clades

• Clade: a group of organisms that consists of a

common ancestor and all of its descendants

Monophyletic Group

• A clade is also known as a monophyletic

group

Phylogenetic Trees vs Cladograms

• Sometimes used interchangeably
• Branch lengths of trees may show

evolutionary time and amount of genetic change

Character

• Characters are heritable traits that can be

compared across organisms, such as physical characteristics (morphology), genetic sequences, and behavioral traits

• Example: wings shown below

Derived versus Ancestral Characters

• A derived character is one that evolved in one

group but not in the other group (a new trait)

• An ancestral character is thought to have

evolved in a common ancestor of both groups

Derived versus Ancestral Characters

• Identify a derived character for mammals
• Identify an ancestral character for mammals

Example of a Derived Trait

• Number of heart chambers in animals
• Tbx5 protein influences the formation of two

ventricles in bird and mammalian hearts

Trees Show Speciation Events and Relatedness

• Examine common ancestry in order to

determine relatedness

• Who is species 5 most closely related to?

Outgroup

• An outgroup is a group of organisms that

serve as a reference group when determining the evolutionary relationship among a monophyletic group of organisms

• Used as a point of comparison for the ingroup
• Example:
• Chordates – Ingroup
• Echinodermata - Outgroup

Chordate Evolution

Another Outgroup Example

Trees May Show Time and Extinct Lineages

Extinct versus Extant

Phylogenetic Trees Based on Sequence Data

• Use of molecular (i.e. DNA, proteins) genetics

to determine evolutionary relationships

• Disadvantages

–Need common genes –Gene sequences need to be “aligned” first

Sequence Alignment

• Goal of Sequence Alignment: Maximize the

number of matching nucleotides in all compared sequences

• Compare SNPs and Indels

SNPs and Indels

• SNPs – single nucleotide polymorphisms
• Indels – insertions and deletions

Maximum Parsimony

• Choosing a tree that

requires the fewest evolutionary events (fewest amount of molecular changes)

• The simplest

explanation that is consistent with the facts

Which is the most parsimonious tree?

Constructing a Phylogenetic Tree

• May require the use of a character table
• + or 1 indicates the presence of the character,
• or 0 indicates the absence of the character

Now You Try It

• Based on the shared characteristics in the

table below, build a tree of the most likely evolutionary history of these organisms

• + (present), - (absent)