Evolution, 4th Edition

T HE TREE of LifE 41

during the ancestry of the vertebrates, all of which have both genes. The α- and

β-hemoglobins arose by gene duplication in the ancestor of jawed vertebrates,

all of which have a functional hemoglobin composed of both α- and β-chains,

whereas the jawless vertebrates (e.g., lampreys) have only a single hemoglobin

chain. The origin of the other globin genes can be similarly traced based on their

sequences and phylogenetic distribution. A more extended description of the evo-

lution of gene families, and of genome size, is provided in C hapter 14.

Cells give rise to lineages of cells by division, and these lineages can be traced

by the somatic mutations that arise and are inherited by descendant cells. Biolo-

gists are beginning to use the “phylogeny” of cells to trace the developmental

history of the brain and other organs [24], and phylogenies of tumor cells are

important for studying the source and spread of metastatic cancers [28]. And the

applications of phylogenetic methods extend beyond biology. French, Spanish, and

the other Romance languages evolved from Latin, an example of nongenetic cul-

tural evolution. Students of cultural evolution are increasingly using phylogenetic

methods, borrowed from evolutionary biology, to analyze the history of languages

and other cultural traits (see Chapter 16).

Phylogenetic insights into Evolutionary History

Phylogenetic studies, sometimes in concert with information from the fossil record,

enable biologists to piece together the evolutionary history of organisms and their

characteristics, ranging from DNA sequences to geographic distributions. They doc-

ument patterns of evolution—aspects of change that are common to many groups of

organisms. Some of these patterns were already known to Darwin and his followers,

but have been studied in depth using phylogenetic and other methods.

Inferring the history of character evolution

One of the most important uses of phylogenetic information is to reconstruct the

history of evolutionary change in interesting characteristics by “mapping” character

states on the phylogeny and inferring the state in each common ancestor, right back to the

root of the entire tree. In the simplest methods, we assign to ancestors those character

states that require us to postulate the fewest evolutionary changes for which we lack

independent evidence. This method enables us to infer when (i.e., on which branch

or segment of a phylogeny) changes in characters occurred, and thus to trace their

h i stor y.

Humans, for example, have nonopposable first (great) toes, while the orangutan,

gorilla, and chimpanzee have opposable first toes (like our thumbs). In FIGURE 2.16

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_02.16.ai Date 03-05-2017

O

A 3

A 2

A 1

A 2

A 1

G C H O G C H

A 3

O N

N O O N

FIGURE 2.16 Inferring ancestral charac-

ter states. Two possible histories of the

evolution of opposable (O) versus nonop-

posable (N) toes in the Hominoidea (O,

orangutan; G, gorilla; C, chimpanzee; H,

human) are shown. At left, if nonopposable

toes (open circles) are hypothesized for

A 3 , the common ancestor of chimpanzee

and human, two state changes must be

postulated. At right, opposable toes are

hypothesized for A 3 , and only one change

need be postulated. Assuming that charac-

ter state changes are rare, the more likely

hypothesis is that humans evolved from an

ancestor with opposable toes.

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