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Mechanisms of Evolution
Microevolution
Population Genetics
Key Concepts
• 23.1: Population genetics provides a foundation
for studying evolution
• 23.2: Mutation and sexual recombination
produce the variation that makes evolution
possible
• 23.3: Natural selection, genetic drift, and gene
flow can alter a population’s genetic composition
• 23.4: Natural selection is the primary
mechanism of adaptive evolution
• Concept 23.2: Mutation and sexual
recombination produce the variation that
makes evolution possible
• Two processes, mutation and sexual
recombination produce the variation in
gene pools that contributes to differences
among individuals
Mutation
• Mutations
– Are changes in the nucleotide sequence of DNA
– Cause new genes and alleles to arise
Figure 23.6
Point Mutations
• A point mutation
– Is a change in one base in a gene
– Can have a significant impact on phenotype
– Is usually harmless, but may have an
adaptive impact
Mutations That Alter Gene
Number or Sequence
• Chromosomal mutations that affect many
loci
– Are almost certain to be harmful
– May be neutral and even beneficial
• Gene duplication
– Duplicates chromosome segments
Mutation Rates
• Mutation rates
– Tend to be low in animals and plants
– Average about one mutation in every 100,000
genes per generation
• Mutations are spread more rapidly in
microorganisms because of short
generation times
Mutations
• Can only be passed on to offspring only if they
occur in the germ line
• Are the ultimate source of genetic variation (new
genes and alleles)
• But are NOT considered a significant source of
genetic change, especially in slowly reproducing
plants and animals
Sexual Recombination
• In sexually reproducing populations, sexual
recombination is far more important than
mutation in producing the genetic differences
that make adaptation possible
• Most variation is produced by genetic
differences that result from recombination of
existing alleles
• Recombination may affect genotype frequencies
but usually has no effect on allele frequencies
• Concept 23.3: Natural selection, genetic
drift, and gene flow can alter a
population’s genetic composition
• Three major factors alter allele frequencies
and bring about most evolutionary change
– Natural selection
– Genetic drift
– Gene flow
Natural Selection
• Differential success in reproduction
– Results in certain alleles being passed to the
next generation in greater proportions
Genetic Drift
• Genetic drift is chance changes in the
gene pool
• Chance changes have more of an effect
on a small gene pool
• Statistically, the smaller a sample the
greater the chance of deviation from a
predicted result
• Genetic drift
– Describes how allele frequencies can
fluctuate unpredictably from one generation to
the next
– Tends to reduce genetic variation
CWCW
CRCR
CRCR
Only 5 of
10 plants
leave
offspring
CRCW
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CWCW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.7
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect
• In the bottleneck effect
– A sudden change in the environment may
drastically reduce the size of a population
– The gene pool
may no longer be
reflective of the
original
population’s gene
pool
(a) Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
Figure 23.8 A
Original
population
Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect
– Can increase understanding of how human
activity affects other species
(b) Similarly, bottlenecking a population
of organisms tends to reduce genetic
variation, as in these northern
elephant seals in California that were
once hunted nearly to extinction.
Figure 23.8 B
The Founder Effect
• The founder effect
– Occurs when a few individuals become
isolated from a larger population
– Can affect allele frequencies in a population
Gene Flow
• Gene flow
– Causes a population to gain or lose alleles
– Results from the movement of fertile individuals
or gametes
– Tends to reduce differences between
populations over time
• Concept 23.4: Natural selection is the
primary mechanism of adaptive evolution
• Natural selection
– Accumulates and maintains favorable genotypes
in a population
– Is the only deviation from the Hardy-Weinberg
that leads to adaptation (of the population to the
environment)
– Requires genetic variation
Variation
• Phenotypic variation
– Occurs between individuals in populations of all species
– Is not always heritable
– Only the genetic component can have evolutionary
consequences
(a) Map butterflies that
emerge in spring:
orange and brown
(b) Map butterflies that
emerge in late summer:
black and white
Figure 23.9 A, B
Variation Within a
Population
• Both discrete and quantitative characters
contribute to variation within a population
• Discrete characters
– Can be classified on an either-or basis
• Quantitative characters
– Vary along a continuum within a population
Polymorphisms
• Phenotypic polymorphism
– Describes a population in which two or more
distinct morphs for a character are each
represented in high enough frequencies to be
readily noticeable
• Genetic polymorphisms
– Are the heritable components of characters that
occur along a continuum in a population
Measuring Genetic Variation
• Population geneticists
– Measure the number of polymorphisms in a
population by determining the amount of
heterozygosity at the gene level and the
molecular level
• Average heterozygosity
– Measures the average percent of loci that are
heterozygous in a population
Variation Between Populations
• Most species exhibit geographic variation
in the gene pools of separate populations
or population subgroups
Figure 23.10
1
2.4
3.14
5.18
8.11
9.12
10.16
13.17
1
2.19
3.8
4.16
9.10
11.12
13.17
15.18
6
7.15
19
XX
5.14
6.7
XX
Cline
• Some geographic variations occur as a cline,
which is a graded change in a trait along a
geographic axis
Heights of yarrow plants grown in common garden
EXPERIMENT
Researchers observed that the average size
Mean height (cm)
of yarrow plants (Achillea) growing on the slopes of the Sierra
Nevada mountains gradually decreases with increasing
elevation. To eliminate the effect of environmental differences
at different elevations, researchers collected seeds
from various altitudes and planted them in a common
garden. They then measured the heights of the
resulting plants.
Atitude (m)
RESULTS The average plant sizes in the common
garden were inversely correlated with the altitudes at
which the seeds were collected, although the height
differences were less than in the plants’ natural
environments.
CONCLUSION The lesser but still measurable clinal variation
in yarrow plants grown at a common elevation demonstrates the
role of genetic as well as environmental differences.
Figure 23.11
Sierra Nevada
Range
Great Basin
Plateau
Seed collection sites
Five Causes of Microevolution
• Two processes, mutation and sexual
recombination produce the variation in
gene pools that contributes to differences
among individuals and that makes
evolution possible
• Three major factors alter allele frequencies
and bring about most evolutionary change
– Natural selection
– Genetic drift
– Gene flow