A Bio-Blueprints Primer: Paraphyly and Last Common Ancestors

There are a couple of papers that I want to discuss soon that involve the topic of paraphyly.  Instead of trying to cram background and a paper review into one post, I am trying a new method, where I present a quick primer of a concept that can be referred back to in later posts. Perhaps that will make this post none too interesting, but I hope it will be useful in the future.

It’s hard not to call the jellyfish that I study “primitive”.  Certainly jellyfish are much simpler than most animals; they lack organs, a central nervous system, or a middle layer of cells.  Living jellyfish also look similar to ones in the fossil record, so it is probably fair to say they haven’t changed much during the course of evolution.  However, the reason I try to stop myself from calling them “primitive” is that it gives the false impression that our ancestors looked like a jellyfish. 

For example, let’s take a simple evolutionary tree (or phylogeny), showing the relationship between a jellyfish and a human, and ask ourselves what the common ancestor of these two animals looked like:

Given the information I presented about jellyfish, you might think that the last common ancestor looked like the simple jellyfish.  However, both the jellyfish and the human have been evolving for an equal amount of time, so the last common ancestor of the two animals might look nothing like either species:

But let’s say we add a second species of jellyfish to our phylogeny, and we get this result:

 

In this scenario, some jellyfish are more closely related to humans than they are to other jellyfish.  This means the last common ancestor of the creatures we call “jellyfish” would also include the ancestor of human beings.  If this tree were correct, scientists would say that “jellyfish” are a paraphyletic group, meaning that you cannot capture the last common ancestor of jellyfish without also including other animals that we would not normally call jellyfish.

The nice thing about paraphyletic groups is that they give you a much better idea of what the last common ancestor looked like.  In my jellyfish example, the last common ancestor of these three animals either looked like a jellyfish, or both “jellyfish” lineages independently evolved all of the characteristics that jellyfish have in common (which is unlikely).

There is no scientific evidence that the phylogeny I presented is true.  It was just an example of the principle.  However, one example of a paraphyletic group that scientists do think is real is the “reptiles”:

For a long time, some scientists have argued that crocodiles are more closely related to birds then they are to other reptiles (lizards, snakes, or turtles).   This is because fossil evidence suggests that crocodiles are evolutionary cousins of the dinosaurs, and birds evolved from dinosaurs.  DNA evidence supports this hypothesis, meaning that the term “reptile” is paraphyletic.

This phylogeny provides good evidence that the last ancestor of birds and crocodiles probably was cold-blooded, and had four legs, scaly skin, and a tail.  Of course, if we only looked at living animals, we would have no idea of the bizarre creatures that connect the two lineages, which is why paleontology is so important:

In coming posts, I will discuss the growing body of genetic evidence that suggests that many groups of animals are paraphyletic, and how this provides a better understanding of the direction of evolutionary change.