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Ch. 23 The Evolu*on of Popula*ons BIOL 221 Overview: The Smallest Unit of Evolu*on •  Misconcep4on • 
organisms evolve •  in the Darwinian sense, during their life4mes •  Natural selec4on acts on individuals • 
but only popula*ons evolve •  Gene4c varia4ons in popula4ons contribute to evolu4on •  Microevolu*on • 
A change in allele frequencies in a popula4on over genera4ons Sources of gene*c varia*on •  Muta4on and sexual reproduc4on produce the varia4on in gene pools •  contributes to differences among individuals 1
Gene*c Varia*on •  Varia4on in individual genotype • 
Leads to varia4on in individual phenotype •  Not all phenotypic varia4on is heritable • 
Norm of reac4on (a)
(b)
•  Natural selec4on • 
Can only act on varia4on with a gene4c component Varia%on Within a Popula%on •  Both discrete and quan4ta4ve characters • 
contribute to varia4on within a popula4on •  Discrete characters • 
classified on an either-­‐or basis •  Purple flowers or white flowers •  Quan.ta.ve characters • 
vary along a con4nuum within a popula4on •  Polygenic •  Height, skin/eye color Varia*on Within Popula*ons •  Popula4on gene4cists • 
Measure polymorphisms in a popula4on •  By determining the amount of heterozygosity •  At the gene and molecular levels •  Average heterozygosity • 
Measures the average percent of loci that are heterozygous in a popula4on •  Nucleo4de variability • 
Measured by comparing the DNA sequences of pairs of individuals 2
Varia*on Between Popula*ons •  Geographic varia*on • 
1
2.4
8.11
9.12
3.14
5.18
10.16 13.17
6
7.15
19
XX
Differences between gene pools of separate 1
popula4ons or 2.19
3.8
4.16 5.14
9.10 11.12 13.17 15.18
6.7
XX
popula4on subgroups 1.0
•  Cline Ldh-B b allele
frequency
• 
0.8
Graded change in a trait along a geographic axis 0.6
0.4
0.2
0
46
44
Maine
Cold (6°C)
42
40
38
36
Latitude (°N)
34
32
30
Georgia
Warm (21°C)
Muta*on •  Muta*ons • 
Changes in the nucleo4de sequence of DNA • 
Only source of novel genes and alleles •  In order to be passed to the next genera4on •  Must occur in germ cells that create gametes • 
Point muta4on •  A change in one base in one gene Point Muta*on •  In noncoding regions of DNA • 
oTen harmless •  In a gene • 
Might not affect protein produc4on due to redundancy in the gene4c code •  Result in a change in protein produc4on • 
OTen harmful • 
But can some4mes increase fit •  between organism and environment 3
Muta*ons That Alter Gene Number or Sequence •  Chromosomal muta4ons that delete, disrupt, or rearrange many loci are typically harmful •  Duplica4on •  Usually harmful •  Small pieces some4mes less harmful •  Increases the genome size •  Can take on new func4ons by further muta4on Muta*on Rates •  Muta4on rates • 
Low in eukaryotes •  Average about one muta4on in every 100,000 genes per genera4on •  DNA repair • 
In prokaryotes •  Muta4ons rates can be lower or higher •  DNA repair doesn’t func4on as well or at all • 
Very high in viruses •  Especially RNA viruses Gene Pools and Allele Frequencies localized group of individuals capable of interbreeding Beaufort Sea
Porcupine
herd range
MAP
ARE
A
T
ES S
HW RIE
RT ITO
NO RR
TE
•  and producing fer4le offspring Porcupine herd
ALASKA
• 
CANADA
•  Popula*on •  Gene pool Consists of all the alleles for all loci in a popula4on • 
A locus is fixed •  If all individuals in a popula4on are homozygous for the same allele Fortymile
herd range
ALASKA
YUKON
• 
Fortymile herd
4
Allele Frequencies •  The frequency of an allele in a popula4on Can be calculated • 
•  In diploid organisms the total number of alleles at a locus •  Is the total number of individuals x 2 •  The total number of dominant alleles at a locus •  Is 2 alleles for each homozygous dominant individual •  Plus 1 allele for each heterozygous individual •  Same logic applies for recessive alleles Allele Frequencies •  If there are 2 alleles at a locus p and q are used to represent their frequencies • 
•  The frequency of all alleles in a popula4on will add up to 1 For example, p + q = 1 • 
•  The Hardy-­‐Weinberg principle describes a popula4on that is not evolving • 
•  If a popula4on does not meet the criteria of the Hardy-­‐Weinberg principle it can be concluded that the popula4on is evolving • 
•  At that par.cular locus Frequencies of
alleles
p = frequency of
CR allele
= 0.8
Alleles in the
population
q = frequency of
CW allele
= 0.2
Gametes
produced
Each egg:
80%
20%
chance chance
Each sperm:
80%
20%
chance chance
Condi*ons for Hardy-­‐Weinberg Equilibrium •  The Hardy-­‐Weinberg theorem • 
describes a hypothe4cal popula4on •  In real popula4ons, allele and genotype frequencies do change over 4me •  The five condi4ons for non-­‐evolving popula4ons are rarely met in nature: • 
No muta4ons • 
Random ma4ng • 
No natural selec4on • 
Extremely large popula4on size • 
No gene flow 5
Hardy-­‐Weinberg Equilibrium •  Hardy-­‐Weinberg equilibrium • 
Describes the constant frequency of alleles in such a gene pool •  If p and q represent the rela4ve frequencies • 
of the only two possible alleles in a popula4on at a par4cular locus, then • 
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype •  p2 + 2pq + q2 = 1 Fig. 23-­‐7-­‐4 20% CW (q = 0.2)
80% CR ( p = 0.8)
CW
(20%)
CR
(80%)
64% ( p2)
CR CR
CW
(20%)
Eggs
Sperm
CR
(80%)
16% ( pq)
CR CW
16% (qp)
CR CW
4% (q2)
CW CW
64% CR CR, 32% CR CW, and 4% CW CW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW
= 20% CW = 0.2 = q
+ 16% CW
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants
Condi*ons for Hardy-­‐Weinberg Equilibrium •  Natural popula4ons • 
can evolve at some loci •  while being in Hardy-­‐Weinberg equilibrium •  at other loci •  We can assume the locus that causes phenylketonuria (PKU) • 
is in Hardy-­‐Weinberg equilibrium given that: •  The PKU gene muta4on rate is low •  Mate selec4on is random •  with respect to whether or not an individual is a carrier for the PKU allele 6
Applying Hardy-­‐Weinberg Equilibrium •  Natural selec4on • 
can only act on rare homozygous individuals •  who do not follow dietary restric4ons •  The popula4on is large •  Migra4on has no effect • 
as many other popula4ons have similar allele frequencies •  The occurrence of PKU is 1 per 10,000 births • 
q2 = 0.0001 • 
q = 0.01 •  The frequency of normal alleles is • 
p = 1 – q = 1 – 0.01 = 0.99 •  The frequency of carriers is • 
2pq = 2 x 0.99 x 0.01 = 0.0198 • 
or approximately 2% of the U.S. popula4on Natural selec*on, gene*c driS, and gene flow •  Three major factors alter allele frequencies and bring about most evolu4onary change: • 
Natural selec4on • 
Gene4c driT •  Fitness to environment as it effects reproduc4ve success •  Changes in allele frequencies of small popula4ons due to random events • 
Gene flow •  Introduc4on or loss of alleles through migra4on between popula4ons Natural Selec*on •  Natural Selec4on • 
Differen4al reproduc4ve success •  results in certain alleles being passed to the next genera4on •  in greater propor4ons •  Not survival of the figest 7
Gene*c DriS •  The smaller a sample • 
greater the chance of devia4on from a predicted result • 
Outliers have a more dispropor4onate effect CR CW
CR CR
CR CW
CW CW
CR CW
CR CR
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CR CR
CR CR
CW CW
CR CR
CR CW
describes how allele frequencies fluctuate unpredictably CR CR
CR CW
CR CR
CW CW
•  Gene*c driS • 
CW CW
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
•  from one genera4on to the next • 
tends to reduce gene4c varia4on through losses of alleles •  If an allele is lost, the other is “fixed” The Founder Effect •  founder effect – type of gene4c driT • 
occurs when a few individuals become isolated from a larger popula4on •  Founding a new popula4on in a new loca4on •  Allele frequencies • 
in the small founder popula4on •  can be different from those in the larger parent popula4on The Bo3leneck Effect •  boWleneck effect • 
sudden reduc4on in popula4on size •  due to a change in the environment •  Resul4ng gene pool Original
population
Bottlenecking
event
Surviving
population
•  may no longer be reflec4ve of the original popula4on’s gene pool •  If the popula4on remains small • 
it may be further affected by gene4c driT 8
Gene Flow •  Gene flow • 
consists of the movement of alleles among popula4ons •  migra4on •  Alleles can be transferred • 
through the movement of fer4le individuals or gametes • 
Tends to reduce differences between popula4ons over 4me • 
More likely than muta4on to alter allele frequencies directly •  Ie. Pollen Gene Flow •  Gene flow • 
can decrease the fitness of a popula4on •  Example -­‐ Bent grass •  alleles for copper tolerance are beneficial in popula4ons near copper mines •  but harmful to popula4ons in other soils •  Windblown pollen •  moves these alleles between popula4ons •  intro of unfavorable alleles •  results in decreased fitness Gene Flow •  Gene flow • 
can increase the fitness of a • 
Example -­‐ Insec4cides popula4on •  Have been used to target mosquitoes •  that carry West Nile virus and malaria •  Alleles have evolved in some popula4ons •  that confer insec4cide resistance to these mosquitoes •  an increase in fitness 9
Direc*onal, Disrup*ve, and Stabilizing Selec*on •  Three modes of selec4on: • 
Direc*onal selec*on Frequency of
individuals
•  favors individuals at one end of the phenotypic range • 
Disrup*ve selec*on Original Evolved
populati populati
on
on
Original
population
Phenotypes (fur
color)
•  favors individuals at both extremes of the phenotypic range • 
Stabilizing selec*on (a) Directional
selection
(b) Disruptive
selection
(c) Stabilizing
selection
•  favors intermediate variants and acts against extreme phenotypes Role of Natural Selec*on in Adap*ve Evolu*on •  Natural selec4on • 
increases the frequencies of alleles that enhance survival and reproduc4on •  Adap4ve evolu4on • 
occurs as the match between an organism and its environment increases Role of Natural Selec*on in Adap*ve Evolu*on •  Environment can change • 
So adap4ve evolu4on is a con4nuous process •  Red Queen hypothesis •  Gene4c driT and gene flow • 
Do not consistently lead to adap4ve evolu4on •  as they can increase or decrease the match •  between an organism and its environment 10
Sexual Selec*on •  Sexual selec*on • 
natural selec4on for ma4ng • 
can result in sexual success dimorphism •  marked differences between the sexes •  in secondary sexual characteris4cs Sexual Selec*on •  Intrasexual selec*on • 
compe44on among individuals of one sex (oTen males) •  for mates of the opposite sex •  Intersexual selec*on • 
oTen called mate choice •  occurs when individuals of one sex (usually females) •  are choosy in selec4ng their mates •  Male showiness due to mate choice • 
can increase a male’s chances of agrac4ng a female •  while decreasing his chances of survival Sexual Selec*on •  How do female preferences evolve? • 
The “good genes” hypothesis suggests EXPERIMENT
•  if a trait is related to male health Female gray
tree frog
SC male gray
tree frog
•  both the male trait and female preference for that trait LC male gray
tree frog
SC sperm
×
Eggs
Offspring of
SC father
×
LC sperm
Offspring of
LC father
Fitness of these half-sibling offspring compared
RESULTS
•  should be selected for Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
11
Diploidy and the Maintenance of Varia*on •  Various mechanisms • 
help to preserve gene4c varia4on in a popula4on •  Diploidy • 
Maintains gene4c varia4on in the form of hidden recessive alleles •  heterozygosity Balancing Selec*on •  Balancing selec*on • 
Occurs when natural selec4on •  maintains stable frequencies of two or more phenotypic forms in a popula4on •  Heterozygote advantage • 
Occurs when heterozygotes have a higher fitness • 
Natural selec4on •  than do both homozygotes •  tends to maintain two or more alleles at that locus • 
Frequencies of the
sickle-cell allele
Sickle-­‐cell allele •  Causes muta4ons in hemoglobin •  but also confers malaria resistance Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-­‐Dependent Selec*on •  Frequency-­‐dependent selec*on • 
The fitness of a phenotype “Right-mouthed”
declines 1.0
common in the popula4on • 
Selec4on can favor •  whichever phenotype is less common in a popula4on Frequency of
“left-mouthed” individuals
•  if it becomes too “Left-mouthed”
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
12
Neutral Varia*on •  Neutral varia*on • 
Gene4c varia4on •  That appears to confer no selec4ve advantage or disadvantage • 
Example – •  Varia4on in noncoding regions of DNA •  Varia4on in proteins that have ligle effect on protein func4on or reproduc4ve fitness Why Can’t Natural Selec*on Fashion Perfect Organisms? 1. 
Selec4on can act only on exis4ng varia4ons • 
Only build on what is there 2. 
Evolu4on is limited by historical constraints 3. 
Adapta4ons are oTen compromises 4. 
Chance, natural selec4on, and the environment interact • 
Environment is always changing • 
Therefore, perfec4on is a moving target You should now be able to: 1. 
Explain why the majority of point muta4ons are harmless 2. 
Explain how sexual recombina4on generates gene4c variability 3. 
Define the terms popula4on, species, gene pool, rela4ve fitness, and neutral varia4on 4. 
List the five condi4ons of Hardy-­‐Weinberg equilibrium 5. 
Apply the Hardy-­‐Weinberg equa4on to a popula4on gene4cs problem 6. 
Explain why natural selec4on is the only mechanism that consistently produces adap4ve change 7. 
Explain the role of popula4on size in gene4c driT 8. 
Dis4nguish among the following sets of terms: direc4onal, disrup4ve, and stabilizing selec4on; intrasexual and intersexual selec4on 9. 
List four reasons why natural selec4on cannot produce perfect organisms 13