HUMAN BIOLOGY

448 Chapter 23

How species Come and go

Comparing genes

Figure 23.11 Comparing the structure of

an amino acid in different organisms can

shed light on evolutionary relationships. The

human version of the protein cytochrome c is

built from 104 amino acids. This graph shows

the amino acid count for the same protein in

some other organisms. (© Cengage Learning)

23.6

the kinds and numbers of outward traits species do (or

don’t) share are clues to how closely they are related. the

same holds true for genes and proteins. remember, the

Dna of each species contains instructions for making rnas

and then proteins. this means that comparisons of Dna,

rna, and proteins from different species are additional

ways of evaluating evolutionary relationships.

if you didn’t already know that apes, monkeys, humans,

and chimpanzees are all primates (section 1.2), you might

guess that they are related. We can test this idea by looking

for differences in the amino acid sequence of a protein,

such as hemoglobin, that occurs in all primates. We also

can check to see whether the nucleotide sequences in their

Dna match closely or not much at all. logically, the species

n The history of life on Earth is marked by extinction of some

species and by the evolution of new ones.

In extinction, species are lost forever

Extinction is the permanent loss of a species. Overall,

species disappear at a fairly steady rate of “background

extinction.” A mass extinction is a sudden, widespread

rise in extinctions above the background level (Figures

23.12 and 23.13). Major groups are wiped out simultane-

ously and the overall number of living species plummets.

About 65 million years ago dinosaurs and many marine

groups died out during a mass extinction, possibly due to

environmental changes that occurred after one or more

large meteorites struck Earth in a short time. The fossil

that are most similar in their biochemistry are the most

closely related.

over time, mutations crop up in most genes. When

two species both have the same gene and its nucleotide

sequence is the same or nearly so in both, they must be

closely related. otherwise there would be more genetic

difference. if, on the other hand, the sequences are quite

different, many neutral mutations must have occured in

them. a very long time must have passed since the species

shared a common ancestor.

for instance, many organisms produce cytochrome c, a

protein of electron transport chains. studies show that the

gene coding for the protein has changed very little over

vast spans of time. human cytochrome c has a primary

structure of 104 amino acids. Chimps have

the identical amino acid sequence. it differs

by one amino acid in rhesus monkeys, by

thirty-one in turtles, and fifty-six in yeasts,

a type of fungus (figure 23.11). Given

this biochemical information, would you

assume humans are more closely related

in evolutionary time to a chimpanzee, a

rhesus monkey, or a turtle?

23.7

record indicates that it may take

as long as 100 million years for

the overall number of species to

recover after a mass extinction.

You may be aware that in the

past few hundred years human

activities have caused the extinc-

tion of thousands of species. The

extinction rate is speeding up

as we cut down forests, fill in

wetlands, and otherwise destroy

habitats of other animals and plants with which we share

Earth. Global climate change is another factor that has

contributed to patterns of extinctions. We will delve more

deeply into these concerns in Chapter 25.

SCIENCE COMES TO LIFE

Chimpanzee

Rhesus monkey

Domestic dog

Domestic chicken

Rattlesnake

Tu rtle

Yeast

0

1

13

18

20

31

56

Species

Number of amino acids that differ from the human sequence

adaptive radiation The

movement of new species

of a lineage into a wide

range of habitats.

extinction Permanent loss

of a species.

mass extinction A sudden,

widespread rise in extinc-

tions above the typical

background level.

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