Evolution, 4th Edition

580 CHAPTER 22

tetrapods (see Chapter 17), birds (see Chapter 16), and mammals (see Chapter 20).

Critically important intermediates are still being found, such as ancestors of mod-

ern turtles (see Chapter 20). The fossil record, moreover, documents two impor-

tant aspects of character evolution: mosaic evolution and gradual change of indi-

vidual features (both illustrated by hominin evolution; see Chapter 21).

Many discoveries in the fossil record fit predictions based on phylogenetic or

other evidence. For example, the age of groups estimated from the fossil record often

matches phylogenetically predicted sequences (see Chapters 17 and 19). Prokaryotes

precede eukaryotes in the fossil record, wingless insects precede winged insects,

fishes precede tetrapods, ferns and gymnosperms precede flowering plants.

Phylogenetic and comparative studies

Even if we had no fossil record at all, many other kinds of information would pro-

vide incontrovertible evidence for evolution. Common ancestry of, for example,

birds and crocodiles, is implied by both anatomical characteristics and DNA

sequences. Molecular phylogenetic trees support many relationships that have

long been implied by entirely independent morphological data (see Chapters 2

and 17).

We are confident today that all known living things stem from a single ancestor

because of the many features that are universally shared (see Chapter 17), such as

the genetic code, the mechanisms of transcription and translation, and proteins

composed only of “left-handed” (l isomer) amino acids. Many genes are shared

among all organisms, including the three major domains (Bacteria, Archaea,

and Eucarya), and these genes have been successfully used to infer the deepest

branches in the tree of life. Systematists have demonstrated the common origin,

or homology, of characteristics that may differ greatly among taxa (see Chapters 2,

15, and 20). Hox genes and other developmental mechanisms are shared among

animal phyla that diverged from common ancestors more than a half-billion years

ago (see Chapters 15 and 20).

Genes and genomes

Molecular biology and genomics show the extraordinary commonality of all living

things. Common ancestry is the only scientific rationale for learning about human

biology by studying yeast, flies, rats, or monkeys (FIGURE 22.2).

Molecular studies show that the genomes of most organisms have similar ele-

ments, such as a great abundance of noncoding pseudogenes and satellite DNA

and a plethora of “selfish” transposable elements that generally provide no advan-

tage to the organism. These and other features are readily understandable under

evolutionary theory, but lack any evidence of intelligent design [7]. Some DNA

polymorphisms are shared among species, so that, for example, some major his-

tocompatibility sequences of humans are more similar and more closely related to

chimpanzee sequences than to other human sequences (FIGURE 22.3). What more

striking evidence of common ancestry could there be?

Biogeography

We noted in Chapter 18 that the geographic distributions of organisms provided

Darwin with abundant evidence of evolution, and they have continued to do so.

For example, the distributions of many taxa correspond to geological events such

as the formation and dissolution of connections between land masses. We saw

that the phylogenies of Hawaiian species match the sequence by which the islands

came into existence. We saw, as did Darwin, that an isolated region such as an

island commonly lacks whole groups of organisms, and that human-introduced

species often come to dominate.

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