Download Directional Selection

Processes of Evolution
Chapter 12
Part 1
12.1 Impacts/Issues
Rise of the Super Rats
 When humans tried to eradicate rats with
warfarin, natural selection favored individuals
with a mutation for warfarin resistance
Video: Rise of the super rats
12.2 Making Waves in the Gene Pool
 Individuals in a population share the same traits
(phenotype) because they share the same
genes (genotype)
 Gene pool
• All of the genes in a population
Alleles and Traits
 Alleles of the same genes are the main source
of variation in a population
• Traits with two distinct forms are dimorphic
• Traits with several distinct forms are polymorphic
• Traits with continuous variation may have
interactions of several genes or be influence by
environment
 Mutation is the source of new alleles
Sources of Variation in Traits
Phenotypic Variation in Humans
Mutation Revisited
 Mutations are the original source of alleles, but
many are lethal or neutral
 Lethal mutation
• Mutation that drastically alters phenotype; usually
causes death
 Neutral mutation
• A mutation that has no effect on survival or
reproduction
Allele Frequencies
 Microevolution (change in allele frequencies) is
always occurring in natural populations
 Microevolution
• Small-scale change in allele frequencies of a
population or species
 Allele frequency
• Abundance of a particular allele among members
of a population
Genetic Equilibrium
 Genetic equilibrium
• Theoretical state in which a population is not
evolving (allele frequencies do not change)
 Only occurs if five conditions are met:
• Mutations never occur, population is infinitely
large, population is isolated from gene flow,
mating is random, all individuals survive and
reproduce equally
Processes of Microevolution
 Genetic equilibrium does not occur in nature
because processes that drive microevolution are
always in play
•
•
•
•
Mutation
Natural selection
Genetic drift
Gene flow
Animation: Adaptation to what?
Animation: How to find out if a
population is evolving
Animation: Sources of genotype
variation
12.3 Natural Selection Revisited
 Natural selection occurs in different patterns
depending on species and selection pressures
• Directional selection
• Stabilizing selection
• Disruptive selection
Directional Selection
 Directional selection
• Mode of natural selection in which phenotypes at
one end of a range of variation are favored
• Allele frequencies shift in a consistent direction in
response to selection pressure
 Examples: peppered moths, rock pocket mice,
antibiotic-resistant bacteria
Directional Selection
Number of individuals
in population
Time 1
Range of values for the trait
Fig. 12-3a, p. 219
Fig. 12-3b, p. 219
Time 2
Fig. 12-3b, p. 219
Time 3
Fig. 12-3c, p. 219
Animation: Directional selection
Directional Selection
in Peppered Moths
 Predation pressure favors moths that are best
camouflaged when the environment changes
Fig. 12-4, p. 219
Fig. 12-4a, p. 219
Fig. 12-4b, p. 219
Fig. 12-4c, p. 219
Fig. 12-4d, p. 219
Directional Selection
in Rock Pocket Mice
 Mice with coat colors that do not match their
surroundings are more easily seen by predators
Stabilizing Selection
 Stabilizing selection
• Mode of natural selection in which intermediate
phenotypes are favored and extreme forms are
eliminated
 Example: sociable weavers
Stabilizing Selection
Fig. 12-6, p. 221
Fig. 12-6a, p. 221
Number of individuals
in population
Time 1
Range of values for the trait
Fig. 12-6a, p. 221
Fig. 12-6b, p. 221
Time 2
Fig. 12-6b, p. 221
Fig. 12-6c, p. 221
Time 3
Fig. 12-6c, p. 221
Animation: Stabilizing selection
Stabilizing Selection
in Sociable Weavers
 Body weight in sociable weavers is a trade off
between starvation and predation
Fig. 12-7, p. 221
Fig. 12-7a, p. 221
Fig. 12-7b, p. 221
Number of survivors
300
200
100
0
35.5
33.5
31.5
29.5
27.5
25.5
23.5
Body mass (grams)
Fig. 12-7b, p. 221
Disruptive Selection
 Disruptive selection
• Mode of natural selection that favors extreme
phenotypes in a range of variation
• Intermediate forms are selected against
 Example: African seedcrackers
Disruptive Selection
Fig. 12-8, p. 222
Fig. 12-8a, p. 222
Number of individuals
in population
Time 1
Range of values for the trait
Fig. 12-8a, p. 222
Fig. 12-8b, p. 222
Time 2
Fig. 12-8b, p. 222
Fig. 12-8c, p. 222
Time 3
Fig. 12-8c, p. 222
Animation: Disruptive selection
Disruptive Selection
in African Seedcrackers
 African seedcrackers tend to have either a large
bill or a small one – but no sizes between
lower bill 12 mm wide
Fig. 12-9a, p. 222
lower bill 15 mm wide
Fig. 12-9b, p. 222
Animation: Change in moth population
Animation: Disruptive selection among
African finches