Lecture 8Molecular phylogenetics

• The goal of phylogeny is simply to reconstruct the historical relationships between a group of taxa

• NB: Phylogenetic trees are hypotheses

• A tree may have very strong support, or it may have very little support - a large number of characters supporting a specific topology

• Weak support for a topology may arise due to – many possible alternatives which cannot be excluded

• species trees illustrate the evolutionary histories of a group of related species

- i.e., species trees record the details of speciation for the group• gene trees show the evolutionary relationships among DNA sequences

for a locus • gene trees may not be the same as species trees for one main reason

– the existence of ancestral polymorphism

• if this ancestral polymorphism is lost in some taxa but not in others, then one sequence isolated in species A may be more closely related to one in species B than to any other conspecific sequence

• the gene tree will thus be different from the true species tree

• the best way to guarantee that this will not occur is to use information provided by multiple independent loci!

NB Gene trees are not the same as species trees

Terminology• Trees may be Rooted and Unrooted• By an Outgroup – Which is a taxon assumed

to have diverged before the taxa studied• Topology – refers to the branching pattern• You can have internal and external

branches• Any type of data can be used to

reconstruct phylogenetic trees • Historically morphological data was used• Molecular data sources are:– Allozymes, immunological distance, DNA-DNA

hybridization, Restriction site data, Amino acid sequences, DNA sequence data

Characters• Characters may be binary (i.e., presence or absence

of an isozyme allele) or multistate (i.e., ACGT)• characters may also be ordered or unordered • when characters are ordered a certain directionality

is implied among changes

The important stuff

• Two important assumptions about the characters used to build trees:

• 1. the characters are independent • 2. the characters are homologous • a homologous character is one shared by

two species because it was inherited from a common ancestor

• homoplasy

How do we construct trees? • There are four major types of phylogenetic

methods: 1. Distance methods (e.g. UPGMA, NJ) 2. Maximum parsimony methods (MP) 3. Maximum likelihood methods (ML)4. Bayesian tree-building methods (such as that used

by the computer program MrBayes)

• MP and ML are both called “cladistic” methods• All these methods can use an Outgroup or Root

Distance

Methods

• The general strategy behind distance methods is to cluster taxa (or OTUs) so that the most similar ones are found close together in the tree

• This strategy is called a phenetic approach • The best tree, according to this

approach, is to minimize the total distance among all taxa

• The branch lengths in phenograms carry important information

• The closer two taxa resemble one another the higher they are positioned in the phenogram

Maximum parsimony (MP)

• according to the MP approach, the best tree minimizes the number of evolutionary steps (i.e., changes among characters)

• this is the principle of parsimony - the least number of changes, required the better the tree

• evolutionary change does not always obey laws of parsimony but it is a reasonable starting point

• MP trees are based exclusively on synapomorphies • shared by two or more taxa that are derived from

some ancestral state • essential to use one or more outgroups

Maximum Likelihood (ML)

• given a certain model of base substitution and a specific tree, what is the probability of obtaining this set of DNA sequences?

• this probability is estimated by a tree’s likelihood score

• the best tree is that which has the highest likelihood, or probability of being produced

• ML methods are computationally intensive and thus have not been easy to use until recently

How do you select the correct tree

• Correlation between distance, MP, and ML topologies is found – Reconstruction is GOOD

• Common approach is to use a statistical technique called bootstrapping

• Bootstrapping procedure works by randomly re-sampling the nucleotide sequence data (with replacement), constructing a tree from this data and counting the number of times a particular branch is found out of say, 100, 500, or 1,000 replicate pseudosamples

So?• Trees based on genetic markers will

accurately reflect evolutionary relationships only if the– rates of evolution are constant– markers are neutral – foundation population is monomorphic (or there

are many independently inherited markers)

• If starting population is polymorphic, fixation of different initial sequences in different lineages (termed lineage sorting) may lead to incorrect inferred phylogenies

Outbreeding depression

• Outbreeding depression is the reduction in reproductive fitness resulting from crossing of populations

• Occurs when: adaptation to a certain area occurred; crossing different sub-species

Diagnosing genetic problems

1. How large is the population (Ne)?2. Has it experienced significant bottlenecks

in the past?3. Has it lost genetic diversity?4. Is it suffering from inbreeding

depression?5. Is it genetically fragmented?• How do you recover a small population?• outbreed (if available), or• inbred but genetically differentiated from the

population to which they are being introduced• DL - Eldridge et al. 1999 – Black-footed Rock

wallaby

Genetically viable populations

• How large must populations be, to be genetically viable in the long term?

• minimum viable population size (MVP)

• the minimum size required to retain reproductive fitness and evolutionary potential over thousands of years

How large?

• Three genetic components must be considered in answering this question:

• Is the population size large enough to avoid inbreeding depression?

• Is there sufficient genetic diversity for evolution to occur in response to environmental change?

• Is the population large enough to avoid accumulating new deleterious mutations?