Biology (Holt)

There are several types of barriers that may isolate two or more

closely related groups. For example, groups may be geographically

isolated or may reproduce at different times. Physical differences

may also prevent mating, or they may not be attracted to one another

for mating. The hybrid offspring may not be fertile or suited to the

environment of either parent.

Biologists have seen the stages of speciation in many different

organisms. Thus, the way that natural selection leads to the formation

of new species has been thoroughly documented. As changes continue

to build up over time, living species may become very different from

their ancestors and from other species that evolved from the same

recent common ancestor, leading to the appearance of new species.

292 CHAPTER 13The Theory of Evolution

Analysis

1. Interpreting Graphics
   How did the average hind-limb
   length of each island’s lizard
   population change from that of
   the original population?
   2. Predict what would happen
   to a population of lizards with
   short hind limbs if they were
   placed on an island with a
   larger average perch diameter
   than from where they came.
   3. Justify the argument that
   this experiment supports the
   theory of evolution by natural
   selection.

Analyzing Change in

Lizard Populations

Background

In 1991, Jonathan Losos, an American sci-

entist, measured hind-limb length of lizards

from several islands and the average perch

diameter of the island plants. The lizards

were descended from a common popula-

tion 20 years earlier, and the islands had

different kinds of plants on which the

lizards perched. Examine the graph at right

and answer the following questions:

x + 6x - 7 - 0^2 <

8

493

0

5

2

Increasing

perch diameter

Increasing hind-limb length

Each island's

lizard population

Original lizard

population

Hind-Limb Length Variation

Listfour elements of natural selection. 7B

Describethe mechanism that causes

population changes in antibiotic-resistant

bacteria. 7B

Identifywhat caused the change in the finch’s

beaks as seen in the Grants’ study. 7A

Describehow speciation takes place. 7A

Critical ThinkingEvaluating ResultsBased

on the results of David Lack’s study and the

Grants’ study of finches, what conclusion can

you make about the length of time required for

evolution of a new species to take place?

The beaks of finches on the

Galápagos Islands enlarged over generations in

response to 7B

Aisolation. Crain.

Bpollution. Dlimited food supply.

TAKS Test PrepTAKS Test Prep

Section 3 Review

2C 2D 7A

http://www.scilinks.org

Topic: Species Formation

Keyword: HX4167

3F 7B