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Maximum Parsimony & Likelihood Methods in Phylogeny

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  • 00:00 Phylogeny
  • 1:33 Maximum Parsimony
  • 3:45 Maximum Likelihood
  • 5:30 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

How are living things related? That's the question behind phylogeny. In this lesson, we are going to explore the concept of phylogeny as well as scientists create phylogenetic trees of life and test their accuracy.

Phylogeny

In biology, we like trees. Trees are nice. Well today we are going to be examining a very specific kind of tree. This is a phylogenetic tree; some people call it a tree of life. You see, in biology, phylogeny is the evolutionary history of a group of organisms. Basically, it's the study of how all things are genetically related based on the fundamental assumption that species descend from a common ancestor. I say that this is an assumption because there is no such thing as a truly complete phylogenetic tree. There are always gaps in the fossil record. Do we know exactly what the common ancestor of humans and apes was? No, there's a missing link that we've never found but we assume exists based on shared traits between us and other primates.

Now, this one here is a smaller phylogenetic tree. It just covers the major primates with each branch being a distinct lineage and the trunk being the common ancestor. Some phylogenetic trees are very specific, but we can also grow some that are pretty big. Theoretically, you can create a major tree of all life starting with the common ancestor of all living things.

So, how do we make phylogenetic trees? Well, let's take a walk through the greenhouse.

Maximum Parsimony

So, say we want to make a phylogenetic tree. How do we do this? There are several potential methods, but the most common one is through the use of parsimony, a principle that within a set of possible explanations, the simplest is most likely to be true. Basically, we can look at the various relationships between organisms, but the simplest explanation is the most likely. In terms of phylogeny, this means that the tree with the fewest branches - the fewest common ancestors - is most likely correct. We can then say that the phylogeny with the fewest common ancestors has maximum parsimony. Let's see what this means in terms of an actual phylogenetic tree.

We'll start with a simple tree. Here, we've got four species of birds: a hawk, a blue jay, a robin, and a hummingbird. Okay, so using maximum parsimony we are going to infer that the simplest explanation for the relationships between these species is the most accurate. Since three of the birds have similar wings - the robin, jay, and hawk - we can assume that this is a trait from a common ancestor, not something that was individually adopted. Since hummingbirds do not have this trait, we can give them a separate branch. So, from our theoretical common ancestor to all birds we now have two branches, one with hummingbirds and one with the other three. Of those other three, hawks are raptors while jays and robins are both perching birds, so we can create another junction here. Finally, we can tell that jays and robins are different so we'll put a junction there with a branch to each individual species. Now we have a phylogenetic tree and can read it to see how closely everything is related. Jays and robins share one common ancestor, so they are most closely related. Next, they share a common ancestor with hawks. The common ancestor with hummingbirds is the most distant. And that's our phylogenetic tree.

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