Evolution, 4th Edition

T HE TREE of LifE 39

including humans, includes at least a few genes that have been horizontally

acquired during their ancestry [4].

Among prokaryotes, both during the early evolution of life and among living bac-

teria, HGT has played a major evolutionary role [1, 6]. Bacteria acquire genes from

other species by many mechanisms, including transfer of plasmids and other mobile

genetic elements, and natural transformation: uptake of DNA that has been released

into the environment by the death of other bacterial cells. HGT among some bacteria

can be so frequent that their relationships may look more netlike than treelike. HGT

has enabled some bacteria to metabolize new nutritional substrates and to adapt to

toxic environments, including antibiotics. Methicillin-resistant Staphylococcus aureus

(MRSA) is a dangerous pathogen, especially prevalent in hospitals, that has evolved

resistance to almost all antibiotics, partly by HGT from other bacteria. Pathogenic

Clostridium difficile and Escherichia coli are among the other species that have acquired

antibiotic resistance by HGT [17]. A major concern is that human pathogens can

acquire antibiotic resistance from the bacteria that inhabit cattle and other livestock

that are routinely treated with antibiotics in order to promote rapid growth [25].

Not only organisms have “phylogenies”

So far we have been concerned with inferring phylogenetic trees of species. But the

same methods can shed light on the history of any diverse objects that have arisen

by a history of divergence from common ancestors. For example, different copies of

a gene, whether within a single species or in more than one species, have a history

of descent from common ancestral genes. (We already have encountered this notion

in using hypothetical squirrel genes to understand a basic phylogenetic method.) A

branching tree that portrays the history of DNA sequences of a gene (haplotypes) is

often called a gene tree or a gene genealogy (see Chapter 7). For example, the tree

that portrays a gene acquired by aphids from fungi (see Figure 2.12) is a gene tree. A

more usual kind of gene tree is illustrated for the mitochondrial cytochrome b gene in

MacGillivray’s warbler (FIGURE 2.13). It shows that most of the haplotypes in Mexi-

can populations of this species are more closely related to one another than they are

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_02.13.ai Date 11-02-2016

M P C D O N Q E L F K H G J A I B

G. philadelphia (outgroup)

Mexico

USA

FIGURE 2.13 A gene tree showing the relation-

ships among haplotypes (different sequences) of

the mitochondrial cytochrome b gene in MacGilli-

vray’s warbler (Geothlypis tolmiei), using a se-

quence from the mourning warbler (G. philadel-

phia) as an outgroup. Haplotypes are more closely

related within a region than between regions,

implying that there is little mixture between war-

bler populations in Mexico and the United States.

(After [26].)

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