Download Chapter 23 Hardy Weinberg only

Chapter 23
The Evolution of Populations
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: The Smallest Unit of Evolution
• One common misconception about evolution is
that individual organisms evolve, in the
Darwinian sense, during their lifetimes
• Natural selection acts on individuals, but
populations evolve
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• Genetic variations in populations
– Contribute to evolution
Figure 23.1
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• Concept 23.1: Population genetics provides a
foundation for studying evolution
• Microevolution
– Is change in the genetic makeup of a
population from generation to generation
Figure 23.2
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The Modern Synthesis
• Population genetics
– Is the study of how populations change
genetically over time
– Reconciled Darwin’s and Mendel’s ideas
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• The modern synthesis
– Integrates Mendelian genetics with the
Darwinian theory of evolution by natural
selection
– Focuses on populations as units of evolution
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Gene Pools and Allele Frequencies
• A population
– Is a localized group of individuals that are capable of
interbreeding and producing fertile offspring
MAP
AREA
•
Fairbanks
Fortymile
herd range
•
Whitehorse
Figure 23.3
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• The gene pool
– Is the total aggregate of genes in a population
at any one time
– Consists of all gene loci in all individuals of the
population
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The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem
– Describes a population that is not evolving
– States that the frequencies of alleles and
genotypes in a population’s gene pool remain
constant from generation to generation
provided that only Mendelian segregation and
recombination of alleles are at work
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• Mendelian inheritance
– Preserves genetic variation in a population
Generation
1
CW CW
CRCR
genotype
genotype
Plants mate
Generation
2
All CRCW
(all pink flowers)
50% CR
gametes
50% CW
gametes
Come together at random
Generation
3
25% CRCR
50% CRCW
50% CR
gametes
25% CWCW
50% CW
gametes
Come together at random
Generation
4
25% CRCR 50% CRCW 25% CWCW
Figure 23.4
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Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions
Preservation of Allele Frequencies
• In a given population where gametes contribute
to the next generation randomly, allele
frequencies will not change
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Hardy-Weinberg Equilibrium
• Hardy-Weinberg equilibrium
– Describes a population in which random
mating occurs
– Describes a population where allele
frequencies do not change
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• A population in Hardy-Weinberg equilibrium
Gametes for each generation are drawn at random from
the gene pool of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
(80%)
CW
(20%)
CR
(80%)
pq
p2
64%
CRCR
CW
(20%)
Eggs
p2
16%
CRCW
16%
CRCW
qp
4%
CW CW
q2
If the gametes come together at random, the genotype
frequencies of this generation are in Hardy-Weinberg equilibrium:
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of the next generation:
16% CR from
64% CR from
+
CRCW homozygotes
CRCR homozygotes
4% CW from
homozygotes
CWCW
+
16% CW from
CRCW heterozygotes
=
80% CR = 0.8 = p
=
20% CW = 0.2 = q
With random mating, these gametes will result in the same
mix of plants in the next generation:
Figure 23.5
64% CRCR, 32% CRCW and 4% CWCW plants
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• If p and q represent the relative frequencies of
the only two possible alleles in a population at
a particular locus, then
– p2 + 2pq + q2 = 1
– And p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents
the frequency of the heterozygous genotype
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Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem
– Describes a hypothetical population
• In real populations
– Allele and genotype frequencies do change
over time
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• The five conditions for non-evolving
populations are rarely met in nature
– Extremely large population size
– No gene flow
– No mutations
– Random mating
– No natural selection
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Population Genetics and Human Health
• We can use the Hardy-Weinberg equation
– To estimate the percentage of the human
population carrying the allele for an inherited
disease
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• 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
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Mutation
• Mutations
– Are changes in the nucleotide sequence of DNA
– Cause new genes and alleles to arise
Figure 23.6
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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
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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
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• Gene duplication
– Duplicates chromosome segments
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Mutation Rates
• Mutation rates
– Tend to be low in animals and plants
– Average about one mutation in every 100,000
genes per generation
– Are more rapid in microorganisms
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Sexual Recombination
• In sexually reproducing populations, sexual
recombination
– Is far more important than mutation in
producing the genetic differences that make
adaptation possible
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• 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
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Natural Selection
• Differential success in reproduction
– Results in certain alleles being passed to the
next generation in greater proportions
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Genetic Drift
• Statistically, the smaller a sample
– The greater the chance of deviation from a
predicted result
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• 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
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.7
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Only 2 of
10 plants
leave
offspring
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.
Original
population
Figure 23.8 A
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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
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The Founder Effect
• The founder effect
– Occurs when a few individuals become
isolated from a larger population
– Can affect allele frequencies in a population
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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
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