The evolution of populations

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

MAP

AREA

CANADA

ALASKA

– Is a localized group of individuals that are capable of
interbreeding and producing fertile offspring

Beaufort Sea

Porcupine
herd range

N
TE OR
RR TH
IT WE
O S
RI T
ES

Fortymile
herd range

ALASKA
YUKON

•
Fairbanks

•
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
CWCW
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

Figure 23.4
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50% CRCW

25% CWCW

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%)

pq

CR
(80%)

p2
64%
CRCR
CW
(20%)

Eggs

p2

16%
CRCW

16%
CRCW

qp

4%
CWCW

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
CWCW homozygotes

+

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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings


• 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings