Reading Trees

Phylogenetic trees convey quite a bit of information. To figure out what that information is, you have to be able to read the tree — that is, to understand the relationships that the tree illustrates. This section explains what you need to know.

Knowing your nodes

A phylogenetic tree is comprised of branches and nodes — places where branches connect — that represent ancestral species (species that give rise to the species at the tips of the branches). Figure 9-3 shows the same tree as does Figure 9-2, except that the nodes are circled. Note that, among the many connections, you can see that crocodiles and birds share a common ancestor. Go a bit farther back in time, and you can see another ancestral species that they share with lizards and snakes.

Scientists refer to nodes as taxa. All the organisms that appear at the tips of the tree (in Figure 9-3, the birds, crocodiles, lizards, snakes, mammals, etc.) are terminal taxa.

Common Ancestor All Anguid Lizards

Getting oriented: Up, down, or round and round

As you read phylogenetic trees, keep in mind that the important thing is the positions of the groups relative to one another. Changes in how the diagram is drawn that don't change these relative positions are unimportant. Exactly the same information is conveyed either way.

Figure 9-4 shows the simple human-dolphin-goldfish tree from Figure 9-1 drawn four different ways, but — and this part is the important part — all four diagrams in this figure represent exactly the same relationships among these organisms. In each diagram, dolphins and humans have a common ancestor more recent than the ancestor they share with goldfish. The differences among the four trees can be described as the results of rotating the branches around the nodes.

Figure 9-4:

Four versions of the same tree, all showing the same relationships.

Figure 9-4:

Four versions of the same tree, all showing the same relationships.

From each of the diagrams in this figure, you can see that at some point in the past, a speciation event led to the goldfish lineage and to the lineage that subsequently diverged into dolphins and humans. Note that the dolphin and human (both of which are mammals and have a fair bit in common) share a most recent common ancestor that neither shares with the goldfish. But all three species have more distant common ancestor: All three are vertebrates.

The left-to-right order of the diagram conveys absolutely no information. You can draw exactly the same tree with humans in the left position or the middle position. The branching pattern relative to the nodes is what's important.

Figure 9-5 shows another, more complex tree with a section that's been rotated. Again, the branches for a particular node can be rotated around a node without changing the information in the tree. Both sections convey exactly the same evolutionary relationships: Lizards and sharks are just as related to each other as they were before, and the combined group of mammals/lizards/ sharks/crocodiles/birds is in exactly the same position relative to the other branches of the tree as it was before the rotation. The horizontal order of the tips of the tree is different, but the positions of the groups relative to one another haven't changed.

Figure 9-5:

Both diagrams show the same relationships despite their different appearance.

Understanding groups

When you look at a tree, one of the key things you notice is that the branching pattern creates groupings of species. This section explains the two types of groups: monophyletic and paraphyletic:

^ Monophyletic groups represent all the descendants of a common ancestor.

^ Paraphyletic groups also represent shared ancestry, but of only part of the group.

Why do we even care about these groups? Because they're "real" — they indicate an actual past connection. Mammals are a monophyletic group, for example, and so are bats. The members may have diverged in different ways, but they all started the same.

Monophyletic group

A monophyletic group is a group of species that (1) has a common ancestor and (2) includes all the descendants of that ancestor. Figure 9-6, for example, includes three monophyletic groups:

^ All the vertebrates in Figure 9-6 have a common ancestor, indicated by the number 1.

^ All the mammals comprise a monophyletic group (the bigger box) that includes all the descendants of the common ancestor indicated by the number 2.

^ Dolphins and chimps comprise a monophyletic group (the smaller box) descended from the ancestor indicated by the number 3.

Peixes Evolu

A monophyletic group is also referred to as a clade, as in "Aren't you clade a monophyletic group has another, equally hard-to-remember name?"

Monophyletic groups are important in a phylogenetic classification system because they're based on the evolutionary process, not on some arbitrarily selected character.

Paraphyletic group

A group of organisms that has a common ancestor but doesn't include all the descendents of that ancestor is called a paraphyletic group. Figure 9-7 shows two paraphyletic groups, the first of which is fish. Although the figure includes several monophyletic groups of fish (sturgeon, lungfish, and teleost), fish as a whole don't make up a monophyletic group. The common ancestor of the fish, indicated by the number 1, gave rise to many other vertebrate groups.

Another paraphyletic group in the vertebrates is the group commonly referred to as the reptiles, represented in Figure 9-7 by the snake, lizard, and crocodile. The reptiles are a paraphyletic group rather than a monophyletic group because the common ancestor of snakes, lizards, and crocodiles also gave rise to birds.

Fish Reptiles

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Tesla Coil Diagram

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