Evolution, 4th Edition

T HE TREE of LifE 37

least three evolutionary changes. (Convince yourself by trying it out!) So the most

likely hypothesis is that the ancestor had the sequence shown at the root, and that

species then gave rise to four living species by the phylogeny shown in tree 1.

This example touches on two points that we will explore in detail in Chapter

16. First, the logic behind our approach here is to find the most likely tree. In this
    example, it is the tree in which a change at any given base happens only once.
    With other cases, however, that is no longer true. (The same mutation is likely
    to happen more than once when mutation rates are very high, the evolutionary
    time scale is very long, or there are many species in the phylogeny.) Second, the
    evolutionary tree of the hemoglobin gene probably reflects the evolutionary tree
    of the squirrel species, but there are situations in which it will not. For example,
    if two distantly related squirrel species hybridized in the past, the hemoglobin
    gene from one species might have spread through the other. That would cause
    the sequences of their hemoglobin genes to make the species seem more closely
    related than they really are.
    Futuyma Kirkpatrick Evolution, 4e
    Sinauer Associates
    Troutt Visual Services
    Evolution4e_02.10.ai Date 11-02-2016

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_02.10.ai Date 11-02-2016

Ground Fox

Eastern gray

Tree 1

Western gray

(A) (B)

(C)

1

1

2

2

3

3

4

4

Ground

Site:

Fox

Eastern gray

Western gray

ATT

123

ATT

ATA

ATA

A AT

456

A AT

AAA

A AT

GAA

ATT A ATGAA

789

GAT

GAA

ATT A AT GAA

GAA

1

2

3

4

Tree 2

1 3 2 4

Tree 3

1 4 2 3

ATT A ATGAA

FIGURE 2.10 A simple example illustrates the logic of one method of inferring phy-

logenies. (A) Four species of squirrels. The aim is to determine relationships among

three species of Sciurus (species 2, 3, and 4). The ground squirrel (Spermophilus citellus;

species 1) is an outgroup. (B) Hypothetical sequences of a small part of a hemoglobin

gene in the four species. Note the differences among the species at sites 3 and 9. (C)

There are three possible relationships (trees 1, 2, and 3) among the three ingroup taxa

(fox, eastern gray, and western gray squirrels). In tree 1, the red bar indicates the single

evolutionary change at site 3 from T to A in the ancestor of species 3 and 4 (eastern and

western gray squirrels). The blue bar in the species 2 lineage marks evolution from A to

T at site 9. Trees 2 and 3 would require us to suppose that evolutionary changes hap-

pened twice at site 3, shown by two red bars. Based on the assumption that each base

pair difference among the species evolved only once, tree 1 is the correct tree.

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