Penguin Tree of Life

Today I would like to introduce the penguin evolutionary tree.  This tree serves as the framework to almost every avenue of research in fossil penguin evolution,  because we need to know where a species belongs on the tree before we can quantitatively study patterns of changes in features like bone density, flipper shape, geographic range, and body size.

Systematists construct these trees by searching for shared evolutionary novelties (synapomorphies) that support recent common ancestry between groups of species.  These features may be morphological – for example the shared novel feature of having  flippers (instead of wings, the primitive state for birds) indicates all penguins share a more recent common ancestor with one another than they do with any other species of bird.  At a finer scale, the King and Emperor penguins both have an opening in the mandible (lower jaw bone) that indicates these two species share a more recent common ancestor with each other than either does with any other penguin species.  Although morphological features are key to deciphering the evolutionary relationships of extinct species, molecular data (DNA) provide another line of evidence for grouping living species (and in some cases, extinct species too – we’ll explore that in a later post).

So with that brief introduction, here is the penguin tree:

Some of the names on the branches may be familiar from previous posts.  The pattern of the branches represents the evolutionary relationships of the species.  This is somewhat like a family tree, except that branches can only split (as two taxa split off from their common ancestor) but not reticulate  – i.e., they never join back together.

Rooting the Tree: It is intuitive to use tree terms to talk about the parts and shape of these evolutionary trees, and that is just what we do.   At the very base of the tree is the root.  This is the portion of the tree that represents the earliest part of the evolutionary history of a group.  We root trees so that the bottom of the tree represents the evolutionary split between the group pf interest and its relatives – in the case of penguins, we would root the tree to the point where penguins split off from the procellariform seabirds (albatrosses and petrels).

Here is what the tree would look like if we turned it on its side and “planted” it by turning it on its side.

Stem Taxa: Fossil species that are lower down the tree are called stem taxa.  Essentially, these species represent dead branches sticking off the stem or trunk of the tree.  They split off before the most recent common ancestor shared by the living species evolved.  Each branch from the stem part of the tree represents a lineage of penguins that evolved along their own path for some amount of time, and then died out, leaving no descendents.  Stem taxa are by definition extinct – each can be thought of as a short, leafless twig from a once growing, but now dead, part of the tree.

Crown Taxa: At the top of the tree are the crown taxa, or “modern” penguins. This is analogous to the crown of a tree – the leafy part at the top.  Each living species can be thought of as a green leaf sitting on the end of its own branch.  The crown clade includes all the living species of penguins plus all fossil relatives that share their most recent common area.  Thus, while stem taxa are always extinct, crown taxa may be living or extinct.  These fossil crown taxa can be envisioned as brown leaves on live branches.

The trees for different clades may have very different shapes. Those shapes depend both on the number of fossils and living species, and their pattern of evolutionary relationships.  Long, sparse branches characterize the trees for groups where deep evolutionary splits have left a handful of highly unique species  – like the monotremes  (the platypus, echidnas and their handful of fossil relatives).  Dense, leafy bushes represent clades where recent explosive radiations led to hundreds or thousands of closely species – groups like bats.

To provide a visual example, the penguin family tree is most similar to certain types of pine. There is a very long stem leading to the crown.  That is, there is a very long series of now completely extinct penguins leading from the root of the tree (where penguins first split off from related birds) to the modern radiation of the 19 living penguin species. The stem of the tree is “tall” because so many extinct species are represented. In fact, in penguins the fossil species outnumber the living species.

Building the evolutionary tree, and updating it as new fossils are found is a major endeavor.  How we collect the data that yields the shape of the tree will be the focus of a future post.