Biology Now, 2e

200 ■ CHAPTER 11 Evidence for Evolution

EVOLUTION

behaviorally mimic the plants they live on—an

adaptation that helps them avoid detection by

predators (Figure 11.10).

The Ultimate Family Tree

Thick bones are not restricted to just the

water-loving ancestors of whales, as we noted,

but can be seen in many other animals, such

as hippos. This similarity across organisms is

another type of evidence for evolution—shared

characteristics among species. Many shared

characteristics—such as thick bones for animals

that take to shallow water, or sexual reproduc-

tion via egg and sperm, or eukaryotic cells—

result from organisms sharing traits that evolved

from a common ancestor, an organism from

which many species have evolved. A group of

organisms have common descent if they share

a common ancestor.

When one species splits into two, the two

resulting species share similar features, or

homologous traits, because they have common

descent—though these features may begin

to look different from one another over time

like Indohyus’s bones, to behaviors, to the func-

tions of individual proteins. Echolocation in

bats, for example, is an adaptation for catching

insects in the dark. Stick insects physically and

Pakicetus, an extinct

water-dwelling whale

ancestor

Manatee, a water-

dwelling mammal

Indohyus, an extinct

water-dwelling

whale cousin

Hippopotamus, a

land- and water-

dwelling animal

Polar bear, a land-

dwelling mammal

Rat, a land-dwelling

mammal

Figure 11.10

Stick insects avoid detection by predators

Stick insects are well adapted to their environment; they move slowly and

look just like the branches they live on.

Figure 11.9

Cross sections of the femurs of

Indohyus, water-dwelling animals, and

land-dwelling animals

Aquatic organisms have thick, dense bone

around a narrow marrow space, while terrestrial

organisms have thin bone and a large marrow

space. Indohyus bone structure is a trait that is

shared with other water-dwelling animals.

Q1: Why do water-dwelling animals have

thicker bones than land-dwelling animals?

Q2: Why does this thick-bone adaptation

suggest a water-dwelling lifestyle?

Q3: How did this adaptation likely increase

survival or reproduction in Indohyus?