How are phylogenetic trees read?

1 Answer
Jan 3, 2018

See below.


Phylogenetic trees show the evolutionary relationships between organisms or taxa. The root is the ancestral lineage and the tips of the tree are the descendants. The descendants may be living or extinct. The descendants may be individual species but they do not have to be.

Node -Represent common ancestors. Speciation events happen at nodes*
Branches -Represent the evolutionary history or lineage. In some instances, but not all, the branch length represents the amount of time
Clade -An ancestor and all of its descendants (see fourth image)
Root -The ancestral lineage
Sister group -Two descendants that split from the same node are considered in one sister group
Outgroup -A lineage that falls outside of the clade or sister groups of interest

Basic example of an evolutionary tree with five descendants:
Created by KM

What can be said about last common ancestors and evolutionary relationships when reading trees:
Created by KM
A is more closely related to B than it is to C because it shares a more recent common ancestor with B when compared to C.

Phylogenetic trees do not need to be read from top to bottom. The same descendants can be displayed in different orders as long as their positions in relation to nodes is correct. Both of the trees below show the exact same information and are both correct:
Created by KM

Examples of what clades are and what a clade is not:
Created by KM

*While in phylogenetic trees speciation events happen at nodes, the reality is that speciation may not be an abrupt, obvious change that can be narrowed down to a single event or year. Speciation may occur very slowly.

For more information, see another explanation and example here and learn about the pitchfork specifically here.