Evolution, 4th Edition

28 CHAPTER 2

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_02.01.ai Date 11-02-2016

(A) (B)

the equivalent fly gene. This is only one of many systems of interacting genes that

are now known to be shared by insects and vertebrates—and indeed by all animals.

In a more extreme variation on this theme, more than one-third of the genes

in the genome of yeast (a fungus) have recognizable similarity to human genes.

In a recent experiment, biologists studied 414 genes that are necessary for yeast

to survive, replacing them with the equivalent human genes [18]. Almost half of

the human gene replacements enabled the yeast to survive!

Only one natural cause can explain these amazing results: these organ-

isms share fundamental characteristics because they—and their genes—have

descended from a common ancestor in the distant past. Throughout biology and

health science, common ancestry between humans and all other living things is

a fundamental principle that explains countless facts. Indeed, it is the basis for

using nonhuman species as models in human biomedical research. Descent from

common ancestors is one of the two great themes in Darwin’s On the Origin of

Species. It is one of the two greatest principles of evolution.

The Tree of Life, from Darwin to Today

When Darwin recounted his visit to the Galápagos Islands in The Voyage of the Bea-

gle (1845), he wrote about the finches (FIGURE 2.2) that, “Seeing this gradation and

variety of structure in one small, intimately related group of birds, one might really

fancy that from an original paucity of birds in this archipelago, one species has been

taken and modified for different ends.” Likewise, he came to suspect that the dif-

ferent forms of mockingbirds and of tortoises had descended from a single ancestor.

But this thought, logically extended, suggested that those mockingbirds’ ancestor

itself had descended from an older ancestor that could have given rise to yet other

descendants—the South American mockingbirds, for instance. In 1837, a year after

he returned from the voyage, Darwin sketched a branching diagram representing

the idea of descent from common ancestors (FIGURE 2.3). By the time he published

On the Origin of Species (1859, [5]), he could write, “I doubt not that the theory of

descent with modification embraces all the members of the same class.” Extending

the logic, he went on: “Analogy would lead me one step further, namely to the belief

that all animals and plants have descended from some one prototype.” And finally,

in one of the most daring thoughts anyone has ever had: “I should infer from anal-

ogy that probably all the organic beings which have ever lived on this earth have

descended from some one primordial form.”

Extending his early sketch (see Figure 2.3) into a great metaphor, Darwin pro-

posed that all species, extant and extinct, form a great “Tree of Life,” or phylogenetic

FIGURE 2.1 Eyes were induced to de-

velop in abnormal places in fruit flies by

inserting extra copies of either the fly eye-

less gene (A) or the corresponding mouse

gene, Pax6 (B). Photo (A) shows extra eyes

on the fly’s mouthparts (arrows). Photo (B)

shows eye induced by expression of the

mouse gene in the fly’s leg region. (From

[12]. Photos courtesy of W. Gehring and G.

Halder.)

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