This document contains slides from a lecture on natural selection. It discusses different types of natural selection such as directional selection, stabilizing selection, disruptive selection, and balancing selection. It defines key terms like reproductive fitness, adaptive traits, and mean fitness of a population. The slides include examples and diagrams to illustrate natural selection patterns.

1. CHAPTER 24
LECTURE
SLIDES
Prepared by
Brenda Leady
University of Toledo
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2. Overview
 Natural Selection
 Reproductive Fitness
 Types of Natural Selection
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3. Natural selection
 Process in which beneficial traits that are
heritable become more common in
successive generations
 Over time, natural selection results in
adaptations
 Changes in populations of living organisms
that promote their survival and reproduction in
a particular environment
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4.  Reproductive success
 Likelihood of an individual contributing fertile
offspring to the next generation
 Attributed to 2 categories of traits
 Certain characteristics make organisms better
adapted to their environment and more likely to
survive to reproductive age
 Traits that are directly associated with
reproduction, such as the ability to find a mate and
the ability to produce viable gametes and offspring
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5. Modern description of natural selection
1. Within a population, allelic variation arises from random
mutations that cause differences in DNA sequences
2. Some alleles encode proteins that enhance an
individual’s survival or reproductive capability compared
to other members of the population
3. Individuals with beneficial alleles are more likely to
survive and contribute their alleles to the gene pool of
the next generation
4. Over the course of many generations, allele frequencies
of many different genes may change through natural
selection, thereby significantly altering the
characteristics of a population
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6. Fitness
 Relative likelihood that a genotype will
contribute to the gene pool of the next
generation as compared with other
genotypes
 Measure of reproductive success
 Hypothetical gene with alleles A and a
 AA, Aa, aa
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7.  Suppose average reproductive successes
are…
 AA produces 5 offspring
 Aa produces 4 offspring
 aa produces 1 offspring
 Fitness is W and maximum is 1.0 for
genotype with highest reproductive ability
 Fitness of AA: WAA = 5/5 = 1.0
 Fitness of Aa: WAa = 4/5 = 0.8
 Fitness of aa: Waa = 1/5 = 0.2
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8. Mean fitness of population
 Average reproductive success of
members of a population
 As individuals with higher fitness values
become more prevalent, natural selection
increases the mean fitness of the
population
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9. Natural selection patterns
 Directional selection
 Stabilizing selection
 Disruptive/Diversifying selection
 Balancing selection
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10. Directional selection
 Individuals at one extreme of a phenotypic
range have greater reproductive success
in a particular environment
 Initiators
 New allele with higher fitness introduced
 Prolonged environmental change
10

11. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Population of mice in a dimly lit forest
Number of
individuals
Light fur Dark fur
Many generations
Many generations
Number of
individuals
Light fur Dark fur
(a) An example of directional selection (b) Graphical representation of directional selection
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12. Stabilizing selection
 Favors the survival of individuals with
intermediate phenotypes
 Extreme values of a trait are selected
against
 Clutch size
 Too many eggs and offspring die due to lack
of care and food
 Too few eggs does not contribute enough to
next generation 12

13. 13

14. Disruptive/Diversifying selection
 Favors the survival of two or more
different genotypes that produce different
phenotypes
 Likely to occur in populations that occupy
heterogeneous environments
 Members of the populations can freely
interbreed
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15. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Contaminated soil
Number of individuals
Agrostis capillaris
Metal sensitive Metal resistant
(a) Growth of Agrostis capillaris on contaminated soil Many generations
Number of individuals
Metal sensitive Metal resistant
(b) Graphical representation of disruptive selection
a: © Courtesy Mark McNair/University of Exeter
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16. Balancing selection
 Maintains genetic diversity
 Balanced polymorphism
 Two or more alleles are kept in balance, and
therefore are maintained in a population over
the course of many generations
 2 common ways
 For a single gene, heterozygote favored
 Heterozygote advantage – HS allele
 Negative frequency-dependent selection
 Rare individuals have a higher fitness
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