Evolution, 4th Edition

402 CHAPTER 16

is critically important for interpreting fossils: it provides a basis for tell-

ing us that a mammalian skull is relevant to hominid evolution, and

that there is an evolutionary connection between dinosaurs and birds.

A phylogenetic perspective on evolution is so important that we

introduced the topic in Chapter 2. There, we provided an example

of the reasoning by which one might infer relationships among spe-

cies from very simple data (see Fig ure 2.10). That example was greatly

oversimplified, because real phylogenetic analyses use far more data

and take into account many complicating factors (which are inter-

esting in themselves). In this chapter we describe how phylogenetic

studies are actually done, and some of what we can learn from a phy-

logenetic history. Phylogenies help us describe the history of life, they

are the basis of a meaningful classification of organisms, and they are

necessary for testing many hypotheses about how evolution works.

Inferring Phylogenies

Phylogenetic relationships among species are estimated from simi-

larities and differences. For example, biologists since Darwin have

agreed that all animals with vertebrae form a single phylogenetic

branch of species that share a common ancestor. This conclusion

is based on the supposition that vertebrae were not present in the

ancestor of all animals, and instead represent a derived character

state, in contrast to the ancestral state, absence of vertebrae. Further-

more, we suppose that the evolution of a structure as complicated as

a vertebra would be so rare that it is unlikely to have happened more

than once within the animal kingdom. So a derived character that is

shared by a group of species (a synapomorphy) is evidence that the

species evolved from a common ancestor (FIGURE 16.2A). It is strong

evidence if we can feel confident that the character evolved only once

in the evolutionary history of the organisms we are studying. The set

of species that have descended from a common ancestor is called a

monophyletic group or clade. Mammals are a clade of species with synapomorphic

characteristics that distinguish them from reptiles and amphibians, such as hair,

milk, and a lower jaw consisting of only a single bone on each side. If we are cor-

rect in supposing that these and other unique features evolved only once, we must

conclude that mammals shared a most recent common ancestor (MRCA) that was

not the ancestor of other vertebrates. Similarly, among the four mammals shown in

Figure 16.2A, the rat, human, and beaver share complex derived features not found

in the kangaroo, of which the most striking is the placenta. Among our three pla-

cental mammals, the rat and beaver share a highly unusual feature: incisors that

grow throughout life. These synapomorphies imply the phylogeny shown in the

figure. In this figure, the reptile (crocodile) and the amphibian (salamander) are

outgroups, a term that refers to species more distantly related to the members of a

certain clade (mammals, in this case) than species within the clade are to each other.

Outgroups help us determine the direction of evolutionary change. For example,

these outgroups lack milk and hair, which implies that these were “new” features

that evolved in the ancestor of the mammals.

If we now add a bird to our study, we notice that it resembles the human in one

interesting respect: both are bipedal, walking on only their hind legs (FIGURE

16.2B). But we would not conclude that the human and bird are each other’s clos-

est relatives, because the bird has none of the unique characters that the human

shares with other mammals. Hair and the other mammalian traits outweigh the

single character, bipedalism, that must have evolved independently in the two

Futuyma Kirkpatrick Evolution, 4e

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Hummingbird

Theropods

Falcon

Archaeopteryx

Velociraptor

Tyrannosaurus

Triceratops

Crocodile

FIGURE 16.1 Birds are living dinosaurs, falling within the

theropod dinosaurs, a group that includes Tyrannosau-

rus. (Their most recent common ancestor is shown by the

red circle.) The theropods are distantly related to ornith-

ischian dinosaurs such as Triceratops (see Figure 17.26).

Extinct dinosaurs, birds, and crocodiles descended from

a more ancient ancestor and are included in a group

called the archosaurs. (After [3].)

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