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PowerPoint Lectures forBiology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 23
he E!olution o" Populations
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• Overview: The Smallest Unit of Evolution
• One common misconception about evolution is
that individual organisms evolve, in the
arwinian sense, during their lifetimes
• !atural selection acts on individuals, but
populations evolve
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• "enetic variations in populations
# #ontribute to evolution
Figure 23.1
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• #oncept $%&': Population genetics provides a
foundation for stud(ing evolution• )icroevolution
# *s change in the genetic ma+eup of a
population from generation to generation
Figure 23.2
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The Modern Synthesis
• Population genetics
# *s the stud( of how populations changegeneticall( over time
# econciled arwin-s and )endel-s ideas
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• The modern s(nthesis
# *ntegrates )endelian genetics with thearwinian theor( of evolution b( natural
selection
# .ocuses on populations as units of evolution
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Gene Pools and Allele Frequencies
• / population
# *s a locali0ed group of individuals that are capable ofinterbreeding and producing fertile offspring
)/P
/E/
/ L / S 1 /
# / ! / /
2eaufor t Sea
Porcupine
herd r ange
•
.airban+s
•
3hitehorse
.ort(mile
herd range
! O , T 4 3
E S T
T E , , * T O , * E S
/ L / S 1 /
5 U 1 O !
Figure 23.3
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• The gene pool
# *s the total aggregate of genes in a populationat an( one time
# #onsists of all gene loci in all individuals of the
population
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The Hardy-Weinber Theorem
• The 4ard(63einberg theorem
# escribes a population that is not evolving
# States that the fre7uencies of alleles and
genot(pes in a population-s gene pool remain
constant from generation to generationprovided that onl( )endelian segregation and
recombination of alleles are at wor+
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• )endelian inheritance
# Preserves genetic variation in a population
Figure 23.4
"eneration
'
C R C R
genot(pe
C W C W
genot(pePlants mate
/ll C R C W
8all pin+ flowers9
;< C R
gametes;< C W
gametes
#ome together at random
"eneration
$
"eneration
%
"eneration
=
$< C R C R ;< C R C W $< C W C W
;< C R
gametes;< C W
gametes
#ome together at random
$< C R C R ;< C R C W $< C W C W
/lleles segregate, and subse7uent
generations also have three t(pes
of flowers in the same proportions
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Preservation of Allele Frequencies
• *n a given population where gametes contribute
to the ne>t generation randoml(, allelefre7uencies will not change
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Hardy-Weinberg quilibrium
• 4ard(63einberg e7uilibrium
# escribes a population in which randommating occurs
# escribes a population where allele
fre7uencies do not change
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• / population in 4ard(63einberg e7uilibrium
Figure 23.5
"ametes for each generation are drawn at random from
the gene pool of the previous generation:
?;< C R 8 p @ ;&?9 $;< C W 8q @ ;&$9
SpermC R
8?;<9C W
8$;<9
p2
A=<C R C R
'A<C R C W
'A<C R C W
=<C W C W
qp
C R
8 ? ; < 9
E g g s
C W
8 $ ; < 9
pq
*f the gametes come together at random, the genot(pe
fre7uencies of this generation are in 4ard(63einberg e7uilibrium:
q$
A=< C R C R , %$< C R C W , and =< C W C W
"ametes of the ne>t generation:
A=< C R from
C R C R homo0(gotes
'A< C R from
C R C W homo0(gotesB @ ?;< C R @ ;&? @ p
'A< C W from
C R C W hetero0(gotesB @ $;< C W @ ;&$ @ q
3ith random mating, these gametes will result in the same
mi> of plants in the ne>t generation:
A=< C R
C R
, %$< C R
C W
and =< C W
C W
plants
p2
=< C W from
C W C W homo0(gotes
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• *f p and q represent the relative fre7uencies of
the onl( two possible alleles in a population ata particular locus, then
# p$ B $ pq B q$ @ '
# /nd p$ and q2 represent the fre7uencies of thehomo0(gous genot(pes and $ pq represents
the fre7uenc( of the hetero0(gous genot(pe
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Conditions for Hardy-Weinberg quilibrium
• The 4ard(63einberg theorem
# escribes a h(pothetical population
• *n real populations
# /llele and genot(pe fre7uencies do changeover time
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• The five conditions for non6evolving
populations are rarel( met in nature # E>tremel( large population si0e
# !o gene flow
# !o mutations
# andom mating
# !o natural selection
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Population !enetics and Human Healt"
• 3e can use the 4ard(63einberg e7uation
# To estimate the percentage of the humanpopulation carr(ing the allele for an inherited
disease
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• #oncept $%&$: )utation and se>ual
recombination produce the variation thatma+es evolution possible
• Two processes, mutation and se>ual
recombination # Produce the variation in gene pools that
contributes to differences among individuals
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Mutation
• )utations
# /re changes in the nucleotide se7uence of !/
# #ause new genes and alleles to arise
Figure 23.6
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Point #utations
• / point mutation
# *s a change in one base in a gene
# #an have a significant impact on phenot(pe
# *s usuall( harmless, but ma( have an adaptiveimpact
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#utations $"at Alter !ene Number or %equence
• #hromosomal mutations that affect man( loci
# /re almost certain to be harmful
# )a( be neutral and even beneficial
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• "ene duplication
# uplicates chromosome segments
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#utation Rates
• )utation rates
# Tend to be low in animals and plants
# /verage about one mutation in ever( ';;,;;;
genes per generation
# /re more rapid in microorganisms
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Se!ual Recombination
• *n se>uall( reproducing populations, se>ual
recombination # *s far more important than mutation in
producing the genetic differences that ma+e
adaptation possible
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• #oncept $%&%: !atural selection, genetic drift,
and gene flow can alter a population-s geneticcomposition
• Three maCor factors alter allele fre7uencies and
bring about most evolutionar( change # !atural selection
# "enetic drift
# "ene flow
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"atural Selection
• ifferential success in reproduction
# esults in certain alleles being passed to thene>t generation in greater proportions
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Genetic #ri$t
• Statisticall(, the smaller a sample
# The greater the chance of deviation from apredicted result
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• "enetic drift
# escribes how allele fre7uencies can fluctuateunpredictabl( from one generation to the ne>t
# Tends to reduce genetic variation
Figure 23.7
C R C R
C R C W
C R C R
C W C W C R C R
C R C W
C R C W
C R C W C R C R
C R C R
Onl( of
'; plants
leave
offspring
C W C W C R C R
C R C W
C R C R C W C W
C R C W
C W C W C R C R
C R C W C R C W
Onl( $ of
'; plants
leave
offspring
C R C R
C R C R C R C R
C R C R C R C R
C R C R
C R C R
C R C R
C R C R C R C R
Generation 2
p @ ;&
q @ ;&
Generation 3
p @ '&;
q @ ;&;
Generation 1
p 8fre7uenc( of C R 9 @ ;&D
q 8fre7uenc( of C W 9 @ ;&%
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$"e &ottlenec' ffect
• *n the bottlenec+ effect
# / sudden change in the environment ma(drasticall( reduce the si0e of a population
# The gene pool ma( no longer be reflective of
the original population-s gene pool
Original
population
2ottlenec+ing
eventSurviving
population
Figure 23.8 A
(a) $ha%ing just a "e& marbles through the
narro& nec% o" a bottle is analogous to a
drastic reduction in the si'e o" a population
a"ter some en!ironmental disaster. By chance,
blue marbles are o!er(represented in the ne&
population and gold marbles are absent.
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• Understanding the bottlenec+ effect
# #an increase understanding of how humanactivit( affects other species
Figure 23.8 B
(b) $imilarly, bottlenec%ing a population
o" organisms tends to reduce genetic
!ariation, as in these northern
elephant seals in Cali"ornia that &ereonce hunted nearly to e)tinction.
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$"e Founder ffect
• The founder effect
# Occurs when a few individuals becomeisolated from a larger population
# #an affect allele fre7uencies in a population
G Fl
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Gene Flo%
• "ene flow
# #auses a population to gain or lose alleles
# esults from the movement of fertile
individuals or gametes
# Tends to reduce differences between
populations over time
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• #oncept $%&=: !atural selection is the primar(
mechanism of adaptive evolution
• !atural selection
# /ccumulates and maintains favorable
genot(pes in a population
G ti & i ti
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Genetic &ariation
• "enetic variation
# Occurs in individuals in populations of allspecies
# *s not alwa(s heritable
Figure 23.9 A, B
8a9 )ap butterflies that
emerge in spring:
orange and brown
8b9 )ap butterflies that
emerge in late summer:
blac+ and white
( i i Wi "i P l i
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(ariation Wit"in a Population
• 2oth discrete and 7uantitative characters
# #ontribute to variation within a population
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• iscrete characters
# #an be classified on an either6or basis
• uantitative characters
# Far( along a continuum within a population
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• Pol(morphism
• Phenot(pic pol(morphism
# escribes a population in which two or more
distinct morphs for a character are each
represented in high enough fre7uencies to bereadil( noticeable
• "enetic pol(morphisms
# /re the heritable components of characters
that occur along a continuum in a population
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• )easuring "enetic Fariation
• Population geneticists
# )easure the number of pol(morphisms in a
population b( determining the amount of
hetero0(gosit( at the gene level and themolecular level
• /verage hetero0(gosit(
# )easures the average percent of loci that are
hetero0(gous in a population
( i ti & t P l ti
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(ariation &et)een Populations
• )ost species e>hibit geographic variation
# ifferences between gene pools of separatepopulations or population subgroups
' $&= %&'= &'? A D&'
GG'H'%&'D';&'AH&'$?&''
' $&'H %&? =&'A &'= A&D
GG'&'?'%&'D''&'$H&';Figure 23.1
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• Some e>amples of geographic variation occur
as a cline, which is a graded change in a trait
along a geographic a>is
Figure 23.11
!"#!$%&!' esearchers observed that the average si0e
of (arrow plants 8 Achillea9 growing on the slopes of the Sierra
!evada mountains graduall( 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& The( then measured the heights of theresulting plants&
$!*+ The average plant si0es in the common
garden were inversel( correlated with the altitudes at
which the seeds were collected, although the height
differences were less than in the plants- natural
environments&
C'C+*%' The lesser but still measurable clinal variation
in (arrow plants grown at a common elevation demonstrates the
role of genetic as well as environmental differences&
) e a n h e i g h t 8 c m 9
/ t i t u d e 8 m 9
4eights of (arrow plants grown in common garden
Seed collection sites
Sierra !evada
ange"reat 2asin
Plateau
A Closer Loo' at "atural Selection
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A Closer Loo' at "atural Selection
• .rom the range of variations available in a
population
# !atural selection increases the fre7uencies of
certain genot(pes, fitting organisms to their
environment over generations
volutionary Fitness
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volutionary Fitness
• The phrases Istruggle for e>istenceJ and
Isurvival of the fittestJ
# /re commonl( used to describe natural
selection
# #an be misleading
• eproductive success
# *s generall( more subtle and depends on man(factors
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• .itness
# *s the contribution an individual ma+es to thegene pool of the ne>t generation, relative to
the contributions of other individuals
• elative fitness
# *s the contribution of a genot(pe to the ne>t
generation as compared to the contributions of
alternative genot(pes for the same locus
*irectional *isruptive and %tabili,ing %election
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*irectional+ *isruptive+ and %tabili,ing %election
• Selection
# .avors certain genot(pes b( acting on thephenot(pes of certain organisms
• Three modes of selection are
# irectional
# isruptive
# Stabili0ing
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• irectional selection
# .avors individuals at one end of thephenot(pic range
• isruptive selection
# .avors individuals at both e>tremes of the
phenot(pic range
• Stabili0ing selection
# .avors intermediate variants and acts against
e>treme phenot(pes
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• The three modes of selection
Fig 23.12 A-C
(a) ire/tiona0 e0e/tion shifts the overall
ma+eup of the population b( favoring
variants at one e>treme of the
distribution& *n this case, dar+er mice are
favored because the( live among dar+
roc+s and a dar+er fur color conceals them
from predators&
(b) iruptie e0e/tion favors variants
at both ends of the distribution& These
mice have coloni0ed a patch( habitat
made up of light and dar+ roc+s, with the
result that mice of an intermediate color are
at a disadvantage&
(/) tabi0iing e0e/tion removes
e>treme variants from the population
and preserves intermediate t(pes& *f
the environment consists of roc+s of
an intermediate color, both light and
dar+ mice will be selected against&
Phenot(pes 8fur color9
Original population
Original
population
Evolved
population
. r e 7 u e n c (
o f i n d i v i d u a l s
The Preser(ation o$ Genetic &ariation
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The Preser(ation o$ Genetic &ariation
• Farious mechanisms help to preserve genetic
variation in a population
#i)loidy
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#i)loidy
• iploid(
# )aintains genetic variation in the form ofhidden recessive alleles
*alancin Selection
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*alancin Selection
• 2alancing selection
# Occurs when natural selection maintainsstable fre7uencies of two or more phenot(pic
forms in a population
# Leads to a state called balanced pol(morphism
Hetero+yote Ad(antae
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Hetero+yote Ad(antae
• Some individuals who are hetero0(gous at a
particular locus
# 4ave greater fitness than homo0(gotes
• !atural selection
# 3ill tend to maintain two or more alleles at that
locus
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• The sic+le6cell allele
# #auses mutations in hemoglobin but alsoconfers malaria resistance
# E>emplifies the hetero0(gote advantage
Figure 23.13
.re7uencies of thesic+le6cell allele
;K$&<
$&K&;<
&;KD&<
D&K';&;<
';&;K'$&<
'$&<
istribution of
malaria caused b(
Plasmodium falciparum
8a proto0oan9
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• .re7uenc(6ependent Selection
• *n fre7uenc(6dependent selection
# The fitness of an( morph declines if it becomes
too common in the population
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• /n e>ample of fre7uenc(6dependent selection
P h e n o t ( p i c
d i v e r s i t (
Figure 23.14
#arenta0 popu0ation ap0e
!perienta0 group ap0e
Plain bac+ground Patterned bac+ground
On pec+ing a moth image
the blue Ca( receives a
food reward& *f the bird does not detect a moth
on either screen, it pec+s
the green circle to continue
to a new set of images 8a
new feeding opportunit(9&
;&;A
;&;
;&;=
;&;%
;&;$
; $; =; A; ?; ';;"eneration number
.re7uenc(6
independent control
Neutral (ariation
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Neutral (ariation
• !eutral variation
# *s genetic variation that appears to confer noselective advantage
Se!ual Selection
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Se!ual Selection
• Se>ual selection
# *s natural selection for mating success
# #an result in se>ual dimorphism, mar+ed
differences between the se>es in secondar(
se>ual characteristics
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• *ntrase>ual selection
# *s a direct competition among individuals ofone se> for mates of the opposite se>
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• *nterse>ual selection
# Occurs when individuals of one se> 8usuall(females9 are choos( in selecting their mates
from individuals of the other se>
# )a( depend on the showiness of the male-sappearance
Figure 23.15
The ,(olutionary ,nima o$ Se!ual Re)roduction
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The ,(olutionary ,nima o$ Se!ual Re)roduction
• Se>ual reproduction
# Produces fewer reproductive offspring than ase>ualreproduction, a so6called reproductive handicap
Figure 23.16
Aeua0 reprou/tion
.emale
eua0 reprou/tion
.emale
)ale
"eneration '
"eneration $
"eneration %
"eneration =
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• *f se>ual reproduction is a handicap, wh( has it
persistedM
# *t produces genetic variation that ma( aid in
disease resistance
Why "atural Selection Cannot Fashion Per$ect ranisms
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y
• Evolution is limited b( historical constraints
• /daptations are often compromises
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• #hance and natural selection interact
• Selection can onl( edit e>isting variations