Biology Now, 2e

234 ■ CHAPTER 13 Adaptation and Species

EVOLUTION

its environment. In many cases, animals fail to

adapt successfully. In fact, scientists estimate that

99 percent of all species that have ever lived are

now extinct. Every extinct species is a silent testa-

ment to a failure to adapt in the face of adversity.

It’s tempting to label the descendant lizards

as a new species, but Irschick is hesitant to do so

without more testing. Most commonly, the term

species is used to refer to members of a group

that can mate with one another to produce

fertile offspring. According to the biological

species concept, a species is a group of natu-

ral populations that can interbreed to produce

fertile offspring and cannot breed with other

such groups; that is, they are reproductively

isolated from other populations (Figure 13.4).

Irschick and Herrel have yet to test whether the

Pod Mrcˇaru lizards can still mate with their

cousins back on Pod Kopište. According to the

The new traits were the result of the lizards

adapting to a different food source, says Irschick:

on Pod Kopište, they ate insects, but on Pod

Mrcˇaru, they began to feast on plants. Flushing

the stomachs of a few lizards revealed that the

lizards’ diet was made up of two-thirds plant

material. Over time, the lizards that had evolved a

new head shape and digestive structures survived

and reproduced better than those that had not,

because these traits enabled them to take advan-

tage of the new food sources on Pod Mrcˇaru.

The lizards evolved not only physical adapta-

tions, but behavioral ones too. They became less

territorial and less aggressive and were mating

more often, probably because more space and

food were available. Adaptations, whether behav-

ioral, physical, or biochemical, have three import-

ant characteristics. First, they closely match an

organism with its environment; in this case, the

lizards evolved to match the ecosystem on Pod

Mrcˇaru. Second, adaptations are often complex,

such as the lizards’ new gut structure. Finally,

adaptations help the organism accomplish

important functions, such as feeding and mating.

The lizards on Pod Mrcˇaru show that evolu-

tion by natural selection can improve the adap-

tive fit of organisms to their environment not only

over long periods of time, as with the evolution

of whales described in Chapter 11, but also over

surprisingly short periods of time. In just 33 years,

the P. sic ula lizard population evolved both physi-

cal and behavioral traits that helped it flourish on

the new island. “It was really, truly rapid evolu-

tion across multiple facets,” says Irschick.

What Makes a Species?

Despite how impressive the lizards’ quick adap-

tation may be, natural selection does not always

result in a perfect match between an organism and

Figure 13.4

A male and a female South Pacific

rattlesnake confirm that they are the

same species

After performing this mating ritual, these snakes

successfully mated, producing viable offspring.

Q1: How do we know that these

rattlesnakes are members of the same

species?

Q2: How would you design an experiment

to determine whether two populations of

snakes are distinct species according to the

biological species concept?

Q3: For which types of populations does

the biological species concept not work as a

way of determining how they are related?

Anthony Herrel worked as a research fellow in Duncan

Irschick’s lab before starting his own lab at the French

National Centre for Scientific Research in Paris, where

he studies the evolution of feeding and locomotion in

vertebrates.

ANTHONY HERREL