Evolution, 4th Edition

PHylogENy: THE UNITy ANd dIvERSITy of lIfE 403

lineages—an instance of convergent evolution. The character of bipedalism contra-

dicts our supposition that each trait arises only once. But if we place the bird on our

tree as the closest relative of the human (as in Figure 16.2B), we must postulate that

the four features that the human shares with other mammals all underwent evolu-

tionary reversal in the bird lineage, in which these features are the same as in the

crocodile and other reptiles. These four “extra” evolutionary changes contradict our

supposition that each trait arises only once and does not change. We must choose

between a tree in which one character (bipedalism) violates our supposition and a

tree in which at least four characters do so. A method called parsimony that is used

for estimating phylogenies follows the simple rule of choosing the tree that requires

the fewest evolutionary changes, namely the tree in which the fewest traits arise

more than once or undergo reversal. By that rule, we accept the tree in which the

human and bird are not closely related. (The bird is actually related to the crocodile.)

Notice that the method of parsimony uses derived traits as evidence for com-

mon ancestry; it does not use shared traits that are ancestral. Animals without a

backbone are sometimes called “invertebrates.” But invertebrates are not a clade. If

they were, all invertebrates would be more closely related (share a MRCA) to each

other than to vertebrates. Abundant evidence from fossils, morphology, and DNA

sequences shows this is not the case. Insects and sponges are both invertebrates,

but insects are more closely related to vertebrates than they are to sponges. Hence,

certain similarities among species can give the wrong phylogeny if they are taken

at face value. Among the species in Figure 16.2, all have an external tail except the

human—but the tail is an ancestral character (shared with fishes!). It does not tell us

that all the other species are more closely related to each other than to the human.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_16.02.ai Date 01-17-2017

Rat

Ever-growing incisors

Placenta

Single jawbone

Milk

Hair

Beaver

Human

Kangaroo

Bird

Crocodile

Salamander

Rat

(A)

(B)

Beaver

Human

Bird

Kangaroo

Crocodile

Salamander

FIGURE 16.2 A simple example of how phylogenetic

relationships are based on shared derived characters

(shown by black crossbars). (A) Features such as hair,

mammary glands, and a single jawbone unite the

mammals. Their absence in the outgroups (represented

by crocodile, bird, and salamander) shows that they

are derived characters. Similarly, placenta and ever-

growing incisors are derived characters of groups of

mammal species. The red crossbars show the origin

of bipedalism, which has evolved independently in

the ancestors of human and of birds. (B) The dashed

branch leading to the bird shows a convergent char-

acter with the human, bipedalism (red crossbar). If we

supposed that the bird and human are close relatives

because both are bipedal, the four derived characters

that the human shares with some or all of the other

mammals would have to have been lost (reversed) in

the evolution of the bird. These losses are shown by

the four blue crossbars on the bird branch. The result-

ing tree, with its four extra changes, is less parsimoni-

ous than the tree in (A).

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