Evolution, 4th Edition

34 CHAPTER 2

but she is still more closely related to her sister. Likewise, two closely related spe-

cies may be less similar to each other than one is to a more distantly related species

(FIGURE 2.7). For instance, dolphins are more closely related to hippopotamuses and

humans than they are to sharks, even though they resemble sharks in some ways.

Dolphins have independently evolved fins and a body form adapted for swimming.

Crocodiles and lizards are superficially more similar to each other than they are to

birds, but crocodiles are more closely related to birds than they are to lizards. The

MRCA of lizards, crocodiles, and birds certainly had a lizardlike body form, but

birds evolved more differences from that ancestral form than crocodiles did. Even

though certain aspects of similarity may be used as data to determine the relationships

among species, a phylogeny portrays relationship (common ancestry), not similarity.

A phylogenetic tree may be drawn in any of several equivalent ways. The

junctions may be angular (FIGURE 2.8A) or rectangular (FIGURE 2.8B). Fig ure

2.8B illustrates three equivalent trees that differ in the orientation of the implied

time axis. Figure 2.8A shows that the clades arising from a branch point may

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Au:

Q:

(A) (B) (C)

FIGURE 2.7 Similarity versus relationship.

(A) Chimpanzees and monkeys are more

closely related to each other than to ro-

dents, and they are also more similar.

(B) Dolphins are closely related to hip-

popotamuses and other mammals, even

though they superficially look more like

sharks. (C) Crocodiles and birds share a

more recent common ancestor than either

does with lizards, but birds look very differ-

ent because they have undergone more

evolutionary changes than crocodiles.

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(A) (B) (C)

1 2 1 2 3

1 3

3

2

3 2 1

3 1 2

2 1 3

1 2 3

3

2

FIGURE 2.8 A) These three trees are (^1

equivalent: they all show that species 2

and 3 are the closest relatives. (B) These

three trees are equivalent to each other,

but they differ in the direction in which

the flow of time is shown (upward, to

the right, and down). These trees are

also equivalent to those in (A) since

again species 2 and 3 are the closest

relatives. The trees in (A) and (B) differ in

how the shapes of the branching events

are shown. (C) These two trees are not

equivalent to each other, or to the trees

in (A) and (B), because they show dif-

ferent species as each other’s closest

relatives.

02_EVOL4E_CH02.indd 34 3/23/17 8:59 AM