How to read a phylogenetic tree
Remember there are multiple different ways to depict relationship s in a phylogenetic tree.Any node in a phylogenetic tree can be rotated without altering the relationships between taxa.
15 Figure 4.7 Rotating around any node leaves a phylogeny unchanged
Imagine that a phylogenetic tree was constructed of balls for nodes and sticks for branches. One could rotate any node 180° in space without changing the structure of the tree itself. The tree may look different, but notice that the relationships between nodes remain unaltered by the rotation.Evolution, 1st EditionCopyright © W.W. Norton & CompanyFigure 4.7 Rotating around any node leaves a phylogeny unchanged
16 Figure 4.8 Rotating phylogenetic trees
Whether a phylogeny is represented as a tree (A) or ladder (B), one can rotate at any node—or any combination of nodes—without changing the structure of the tree. Thus the leftmost tree shown in each row is identical from a phylogenetic perspective to the trees shown to the right. The colors indicate the nodes that were rotated in each case.Evolution, 1st EditionCopyright © W.W. Norton & CompanyFigure 4.8 Rotating phylogenetic trees
17 How to read a phylogenetic tree
We build phylogenetic trees to use to figure out evolutionary relationships between taxa and to identify “natural” groupings among taxa, those that reflect their true evolutionary relationships.A key idea is that natural groupings called clades are monophyletic groups.
18 How to read a phylogenetic tree
Clade: a group of taxa that share a common ancestor.Monophyletic group: consists of an ancestor and all of the taxa that are descendants of that ancestor.
19 Taxonomic units legitimate only if they represent a clade
Clades are monophyletic groups
21 How to read a phylogenetic tree
In the next slides elephants, manatees and hyraxes plus their common ancestor form a monophyletic group.Similarly tapirs, rhinoceroses and horses plus their common ancestor form another monophyletic group.
22 Figure 4.11 Monophyletic clades of mammals
A partial phylogenetic tree of the mammals shows examples of monophyletic groups. Elephants, manatees, and hyraxes form one monophyletic group, tapirs and rhinoceroses form another, and the group of tapirs, rhinoceroses, and horses form a third. However, pachyderms– elephants, rhinoceroses, and hippopotamuses—are not a monophyletic group. Adapted from Murphy et al. (2001).Evolution, 1st EditionCopyright © W.W. Norton & CompanyFigure 4.11 Monophyletic clades of mammals
23 Polyphyletic groupA taxon is polyphyletic if it does not contain the most recent common ancestor of all members of the group.
25 Polyphyletic groupA polyphyletic group requires the group members to have each had an independent evolutionary origin of some diagnostic feature.“poly” means many. Hence many origins in this case.
26 E.g. Referring to Elephants, rhinos, and hippos as“pachyderms.”
Pachyderms are a polyphyleticgroup because each groupevolved thick skin separately.A partial phylogenetic tree of the mammals shows examples of monophyletic groups. Elephants, manatees, and hyraxes form one monophyletic group, tapirs and rhinoceroses form another, and the group of tapirs, rhinoceroses, and horses form a third. However, pachyderms– elephants, rhinoceroses, and hippopotamuses—are not a monophyletic group. Adapted from Murphy et al. (2001).Evolution, 1st EditionCopyright © W.W. Norton & CompanyElephants, rhinos and hippos would formpolyphyletic group
27 Paraphyletic groupA taxon is paraphyletic if it includes the most recent common ancestor of a group and some but not all of its descendents.
29 Paraphyletic groupAn example of a paraphyletic group among vertebrates is“fish.”All tetrapods (four-legged animals) are descended from lobe-finned fish ancestors, but are not considered “fish” hence “fish” is a paraphyletic group because the tetrapods are excluded.
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