If this population is in genetic equilibrium, chances of an individual in the next generation having genotype AA would be 16% (p2 = 0.402 = 0.16 or 16%). The probability of genotype aa would be 36% (q2 = 0.602 = 0.36). The probability of genotype Aa would be 48% (2pq = 2 (0.40) (0.60) = 0.48). If a population doesn't show these predicted phenotype frequencies, evolution is taking place. The Hardy-Weinberg principle predicts that five conditions can disturb genetic equilibrium and cause evolution to occur: (1) nonrandom mating; (2) small population size; and (3) immigration or emigration; (4) mutations; or (5) natural selection.

▸ Nonrandom Mating In genetic equilibrium, individuals must mate with other individuals at random. But in many species, individuals select mates based on heritable traits, such as size, strength, or coloration, a practice known as sexual selection. When sexual selection is at work, genes for the traits selected for or against are not in equilibrium.

▸ Small Population Size Genetic drift does not usually have major effects in large populations, but can affect small populations strongly. Evolutionary change due to genetic drift thus happens more easily in small populations.

▸ Immigration or Emigration Individuals who join a population may introduce new alleles into the gene pool, and individuals who leave may remove alleles. Thus, any movement of individuals into (immigration) or out of (emigration) a population can disrupt genetic equilibrium, a process called gene flow.

▸ Mutations Mutations can introduce new alleles into a gene pool, thereby changing allele frequencies and causing evolution to occur.

▸ Natural Selection If different genotypes have different fitness, genetic equilibrium will be disrupted, and evolution will occur.

One or more of these conditions usually holds for real populations. So, most of the time, in most species, evolution happens.

A peacock dancing in front of a peahen.

FIGURE 17–10 Choosing a Mate Random mating is one condition required to maintain genetic equilibrium in a population. However, in many species, mating is not random. Female peacocks, for example, choose mates on the basis of physical characteristics such as brightly patterned tail feathers. This is a classic example of sexual selection.

17.2 Assessment

Review Key Concepts

    1. Review How does natural selection affect a single-gene trait?

    2. Compare and Contrast Compare directional selection and disruptive selection.

    1. Review Define genetic drift.

    2. Relate Cause and Effect How can the founder effect lead to changes in a gene pool?

    1. Review What five conditions are necessary to maintain genetic equilibrium?

    2. Infer Why is genetic equilibrium uncommon in actual populations?

Apply the Big idea

Evolution

  1. Do you think populations stay in genetic equilibrium after the environment has changed significantly? Explain your answer.

End ofPage 492

Table of Contents

Miller & Levine Biology
About the Authors
Changing the World of Biology Education
Contents
Diversity of Life: A Visual Guide DOL•1–DOL•64
Labs and Activities
Features
UNIT 1 The Nature of Life
1 The Science of Biology
2 The Chemistry of Life
Unit Project
UNIT 2 Ecology
3 The Biosphere
4 Ecosystems and Communities
5 Populations
6 Humans in the Biosphere
Unit Project
UNIT 3 Cells
7 Cell Structure and Function
8 Photosynthesis
9 Cellular Respiration and Fermentation
10 Cell Growth and Division
Unit Project
UNIT 4 Genetics
11 Introduction to Genetics
12 DNA
13 RNA and Protein Synthesis
14 Human Heredity
15 Genetic Engineering
Unit Project
UNIT 5 Evolution
16 Darwin's Theory of Evolution
17 Evolution of Populations
18 Classification
19 History of Life
Unit Project
UNIT 6 From Microorganisms to Plants
20 Viruses and Prokaryotes
21 Protists and Fungi
22 Introduction to Plants
23 Plant Structure and Function
24 Plant Reproduction and Response
Unit Project
UNIT 7 Animals
25 Introduction to Animals
26 Animal Evolution and Diversity
27 Animal Systems I
28 Animal Systems II
29 Animal Behavior
Unit Project
UNIT 8 The Human Body
30 Digestive and Excretory Systems
31 Nervous System
32 Skeletal, Muscular, and Integumentary Systems
33 Circulatory and Respiratory Systems
34 Endocrine and Reproductive Systems
35 Immune System and Disease
Unit Project
A Visual Guide to The Diversity of Life
HOW TO USE THIS GUIDE
THE TREE OF LIFE
Bacteria
Protists
Fungi
Plants
Animals
Appendices
Appendix A Science Skills
Appendix B Lab Skills
Appendix C Technology & Design
Appendix D Math Skills
Appendix E Periodic Table
Glossary
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q, R
S
T
U
V
W
X, Y, Z
Index
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Credits
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