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Chapter 23:The Evolution of
Populations
Important Point:
Gen
e P
ools
One species, but members are more likely to mate
within their herd than the other
A gene pool is the sum of alleles at all loci within a population
Pol
ymor
phis
mA polymorphism is
more than one allele present at a given
locus within a single population of
organisms
Population genetics is essentially the study
of allele and genotype frequencies within
populations of organisms
Men
del m
eets
H.W
. Recall Mendelian genetics
Hardy-Weinberg Equilibrium
means genotype frequencies stay
the same
Har
dy-W
einb
erg
The
orem
320 20
x2
Har
dy-W
einb
erg
The
orem
Har
dy-W
einb
erg
The
orem
Har
dy-W
einb
erg
The
orem Note same
The triumph of Darwinism
occurred with the ‘Modern
synthesis’, the integration of the
mechanics of Darwinian
evolution with those of
Mendelian genetics (1930s)
H.W
. Equ
ilibr
ium
Hardy-Weinberg
means that both
genotype and allele
frequencies stay the
same over time
H.-
W. F
requ
enci
es (
2 al
lele
s)
Note how genotype frequencies are 100% a function of previous-generation allele frequencies.
This is precisely what the H.W. equation tells us.It is the default evolutionary assumption (i.e., no evolution is occurring)
Calculated H.W. frequencies,1 locus,
2 alleles
“Fixed” allele
H.-
W. A
ssum
ptio
ns To assume Hardy-Weinberg equilibrium all of the
following must be true:
1. The population must be very large (no sampling error/genetic drift)
2. There must be no net mutation
3. There must be no natural selection (though as we will see that this assumption can be temporarily suspended in the course of using the Hardy-Weinberg theorem)
4. No migration between populations
5. Random mating (equivalent to mixing all sperm and eggs in population into a common bucket to foster fertilization)
In other words, no mechanisms that can affect genetic structure—i.e., allele or genotype frequencies—may be operating
Egg
s &
Milt
(S
perm
) in
Buc
ket
http://wdfw.wa.gov/wildwatch/salmoncam/
hatchery.html
Non-Random Mating
Anything that interferes with the
random mating between individuals
is nonrandom mating
Nonrandom mating results in deviations from
a Hardy-Weinberg generation of genotypes from a given frequency
of alleles
H.-
W. E
quili
briu
m If no mechanisms that can affect genetic
structure are operating, then
• Hardy-Weinberg genotype frequencies will be established in a single generation…
• And these frequencies will persist indefinitely
• (I.e., so long as there are no mechanisms operating that can affect genetic structure)
Remember that an organism can be homozygous for a given allele even if within the population is polymorphic (meaning that more than one allele exists)
Indeed, three alleles can exist within a population, even if only at best two can exist within a single individual
Chalk discussion of H.W. theorem, including, especially,
p2 + 2pq + q2 = 1
Sol
ving
H.-
W. P
robl
ems Work with Decimals, not percentages, not
fractions, not absolute numbers Convert Phenotypes to Genotypes, whenever you
are given phenotype information you should be pondering (i) how can I convert phenotypes to genotypes? and (ii) how can I convert known phenotype frequencies to genotype frequencies?
Convert Genotypes to Alleles, once you know genotype frequencies it should be trivial to convert to allele frequencies: don’t let this step trip you up
Convert Alleles to Genotypes, if you know allele frequencies, but not genotype frequencies, then chances are you will need to figure out the latter
Incorporating Selection, usually selection only operates at the diploid stage make sure frequencies always add up to one
Practice, Practice, Practice, Practice, Practice!
Wor
king
with
Dec
imal
s Convert percentages to decimals (I.e., by dividing by 100): 25% 0.25
Convert fractions to decimals (I.e., by dividing by the denominator): ¼ 0.25
Convert absolute numbers to decimals (I.e., by dividing number by total): 60/240 0.25
Many a Hardy-Weinberg solution has been tripped up by not employing decimals, i.e., by not employing frequencies
E.g., 25% x 25% = 625%! (which is incorrect)
E.g., 0.25 x 0.25 = 0.0625! (which is correct)
Yes, 25/100 x 25/100 = 625/100/100 = 0.0625
But isn’t that absurdly complicating???
Phe
noty
pe
Gen
otyp
e Phenotype to Genotype conversions are going to depend on the genetics of your locus
Always in these problems genotypes will be diploid
If alleles have a dominance-recessive relationship, then the heterozygote will have the same phenotype as the dominant homozygote
Therefore, if the relationship is dominant-recessive you will know with certainty only the genotypes of recessive homozygotes
If the relationship is codominant or incomplete dominant, however, then there will be a one-to-one mapping of genotype to phenotype
That is, for the latter (& only for the latter) genotype frequencies will be the same as phenotype frequencies
Dom
inan
t Gen
otyp
es If a population is in Hardy-Weinberg equilibrium
then the frequency of all genotypes, even dominant genotypes, may be estimated
Start with the frequency of the recessive homozygote this equals q2
q therefore is equal to the square root of the frequency of the recessive homozygote
p, the frequency of the dominant allele, therefore (if 2 alleles) can be assumed to be equal to 1 – q
The dominant homozygote therefore can be assumed to have a frequency of (1 – q)2
The heterozygote therefore can be assumed to have a frequency simply of 2*p*q
Always assume Hardy-Weinberg equilibrium unless you have a compelling reason not to
Gen
otyp
e
A
llele
Once you know genotype frequencies, going from genotype frequencies to allele frequencies is easy
Don’t let it trip you up! There are two formulas one can use and which one
you use depends on whether you are working with absolute numbers versus genotype frequencies
f(A) = [2*f(AA) + 1*f(Aa) + 0*f(aa)] / 2 [note that 2 = 2*f(AA) + 2*f(Aa) + 2*f(aa) since all
frequencies should add up to 1] Note that this is just a ratio of number of alleles of a
one type to total number of alleles present in a population
Alternatively, with X= # AA, Y= # Aa, & Z= # aa: f(A) = (2*X + 1*Y + 0*Z) / 2*(X + Y + Z) Note also that f(A) = 1 – f(a) (for 2 allele system) [for ABO (3-allele) system, f(IA) = 1 - f(IB) - f(i)]
Alle
le
G
enot
ype
Genotype frequencies can be estimated from allele frequencies
First, you must assume Hardy-Weinberg equilibrium
Then simply calculate genotype frequencies from allelic frequencies using the Hardy-Weinberg theorem
(recall that p and q are allele frequencies) If you had 70 A alleles and 120 a alleles, then
what are the expected frequencies of AA, Aa, and aa?
f(A) = 70 / (70 + 120) = 0.37 f(a) = 0.63 f(AA) = 0.372 = 0.14; f(aa) = 0.632 = 0.40; f(Aa) = 2
* 0.37 * 0.63 = 0.47; Check your answer 0.14 + 0.40 + 0.47 = 1.01,
which is pretty close to 1.0 (rounding error?)
Non
-Dar
win
ian
Evo
lutio
n Generally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)
Keep in mind, though, that selection cannot operate without genetic variation
Genetic variation, in turn, ultimately is a consequence of mutation
Non-Darwinian mechanisms generally are not adaptive and include:
1. Genetic drift
2. Mutation
3. Migration
4. Non-Random mating
Non
-Dar
win
ian
Evo
lutio
n Generally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)
Keep in mind, though, that selection cannot operate without genetic
Genetic variation, in turn, ultimately is a consequence of mutation
Non-Darwinian mechanisms generally are not adaptive and include:
1. Genetic drift
2. Mutation
3. Migration
4. Non-Random mating
Non
-Ran
dom
Mat
ing
Random mating violates statistical independence, which would complicate our math
Recall the “Rule of Multiplication” from Chapter 14 “How do we determine the chance that two or more
independent events will occur together in some specific combination? The solution is in computing the probability for each independent event, then multiplying these individual probabilities to obtain the overall probability of the two events occurring together.” (p. 254 C & R, 2002)
It is because matings are random that the odds, e.g., of one A allele (from mom) being paired with another A allele (from dad) is p * p or p2
If matings were not random then the probability of the above pairing could be >p2 or <p2, depending on whether “opposites” repel or “opposites” attract (respectively)
Non
-Dar
win
ian
Evo
lutio
n Generally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)
Keep in mind, though, that selection cannot operate without genetic
Genetic variation, in turn, ultimately is a consequence of mutation
Non-Darwinian mechanisms generally are not adaptive and include:
1. Genetic drift
2. Mutation
3. Migration
4. Non-Random mating
Sampling Error: Genetic DriftErrors get bigger (as fraction of sample) as
samples get smaller!
Non
-Dar
win
ian
Evo
lutio
n Generally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)
Keep in mind, though, that selection cannot operate without genetic variation
Genetic variation, in turn, ultimately is a consequence of mutation
Non-Darwinian mechanisms generally are not adaptive and include:
1. Genetic drift — Bottleneck
2. Mutation
3. Migration
4. Non-Random mating
Sampling Error: BottleneckWhen a population is
reduced in size randomly, sampling error results in the allele frequencies of the new population not
likely matching what were the allele frequencies in
the old population
Cheetah, Product of Bottleneck
The longer a population
remains at a reduced size, the greater the effect of genetic drift on allele frequency
Non
-Dar
win
ian
Evo
lutio
n Generally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)
Keep in mind, though, that selection cannot operate without genetic variation
Genetic variation, in turn, ultimately is a consequence of mutation
Non-Darwinian mechanisms generally are not adaptive and include:
1. Genetic drift — Founder effect
2. Mutation
3. Migration
4. Non-Random mating
Sampling Error: Founder EffectNote that the alleles lost are not necessarily the
same alleles as may have been lost due to natural selection
Genetic drift is sampling error
New population
Pro
duct
s of
Gen
etic
Drif
t
Isolated populations by chance “fixed”
different karyotypes
A locus for which only a single allele exists for an entire
gene pool is considered to be fixed, i.e., a fixed
locus
Non
-Dar
win
ian
Evo
lutio
n Generally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)
Keep in mind, though, that selection cannot operate without genetic variation
Genetic variation, in turn, ultimately is a consequence of mutation
Non-Darwinian mechanisms generally are not adaptive and include:
1. Genetic drift
2. Mutation
3. Migration
4. Non-Random mating
Mutation & Neutral Variation
Note change in allele frequencies
Mut
atio
n (1
/2)
Mutation (or, at least, net mutation) also automatically changes allele frequency
For example, a mutation involves the conversion of one allele into another allele
Typically mutation does not play a big, direct role in changing allele frequency because mutation rates per locus tend to be low
However, indirectly mutation is absolutely essential to microevolutionary processes because all allelic variation ultimately has a mutational origin
Mutations represent random changes in highly evolved (i.e., information laden) nucleotide sequences, so often give rise to losses in gene function (thus most mutations are recessive)
Mut
atio
n (2
/2)
"Organisms are the refined products of thousands of generations of past selection, and a random change is not likely to improve the genome any more than firing a gunshot blindly through the hood of a car is likely to improve engine performance.“
Every now and then, though, a mutational change is adaptive (and even less often, both adaptive and dominant or codominant), i.e., novel functions or novel expression of old functions
"On rare occasions, however, a mutant allele may actually fit its bearer to the environment better and enhance the reproductive success of the individual. This is not especially likely in a stable environment, but becomes more probable when the environment is changing and mutations that were once selected against are now favorable under the new conditions." your text
Non
-Dar
win
ian
Evo
lutio
n Generally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)
Keep in mind, though, that selection cannot operate without genetic variation
Genetic variation, in turn, ultimately is a consequence of mutation
Non-Darwinian mechanisms generally are not adaptive and include:
1. Genetic drift
2. Mutation
3. Migration
4. Non-Random mating
Migration (Gene Flow)
Migration (movement of individuals) makes allele
frequencies become more similar
Non
-Dar
win
ian
Evo
lutio
n Generally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)
Keep in mind, though, that selection cannot operate without genetic variation
Genetic variation, in turn, ultimately is a consequence of mutation
Non-Darwinian mechanisms generally are not adaptive and include:
1. Genetic drift
2. Mutation
3. Migration
4. Non-Random mating
Nat
ural
Sel
ectio
n (1
/2)
Make sure that you understand that…
• Natural selection acts on phenotypes
• Genotypes underlie phenotypes
• Alleles underlie genotypes
• Therefore, natural selection ultimately acts on allele frequencies, though selection occurs through the filter of both phenotype and genotype
"An organism exposes its phenotype—its physical traits, metabolism, physiology, and behavior—not its genotype, to the environment. Acting on phenotypes, selection indirectly adapts a population to its environment by increasing or maintaining favorable genotypes in the gene pool." your text
Nat
ural
Sel
ectio
n (2
/2)
Natural selection can act during the haploid or diploid stage
The effect of natural selection is to reduce (not to increase) the absolute number of genotypes or alleles
That is, mutation places alleles into a gene pool, other microevolutionary forces can serve to increase the frequency of the allele, but selection acts to selectively remove maladaptive alleles (mutation in, selection out)
In the absence of natural selection an organism contributes x gametes to the next generation; in the presence of natural selection an organism contributes <x gametes to the next generation
Natural selection is differential reproductive success
Natural selection serves to increase the information content found within genomes
Inco
rpor
atin
g S
elec
tion
Recall, for example, that we are diploid, and
assume that natural selection is acting only at the diploid stage
Chalk discussion of effect of natural
selection on H.W. frequencies
Selection for Toxin Resistance
Seeds that drift onto mine tailings
die unless they are genetically predisposed
toward heavy-metal resistant
"The modern synthesis emphasizes the importance of populations as the units
of evolution, the central role of natural selection as the
most important mechanism of evolution, and the idea of gradualism to explain how large changes can evolve
as an accumulation of small changes occurring over long
periods of time." your text
Dar
win
ian
Fitn
ess
“Darwinian fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals.” p. 457, Campbell & Reece, 2002
Darwinian fitness is the allelic contribution an individual makes to the next generation
Darwinian fitness is a quantity equal to the average reproductive output associated with a given genotype
The more likely an individual is to survive and reproduce (i.e., to contributes its alleles to the next generation), the higher that individual's Darwinian fitness
Darwinian fitness is often simply called fitness
People typically consider Darwinian fitness on a locus-by-locus basis
Rel
ativ
e F
itnes
s “In a more quantitative approach to natural selection,
population geneticists define relative fitness as the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus… The relative fitness of the most reproductively successful variants is set at 1 as a basis for comparison.” pp. 458-459, Campbell & Reece, 2002
Restatement: Typically the genotype with the highest Darwinian fitness is given a relative fitness of 1.0
All other genotypes, i.e., those with lower than the highest Darwinian fitness, then have relative fitness values of less than 1.0
If one genotype produces on average 4 progeny per generation and another produces on average 1 progeny per generation, then what is the relative fitness of the latter genotype? The former?
Modes of Selection
Stabilizing Selection
Stabilized populations tend to be reasonably well adapted to their
environments
Stabilizing selection eliminates phenotypic extremes within a
population, thus increasing the frequency of genotypes underlying
intermediate phenotypes
Directional Selection
Directional selection is natural selection against only one
phenotypic extremeDirectional selection
is what people typically think of
when they think of natural selection
Disruptive Selection
In disruptive selection the
intermediate is selected against
Disruptive selection can result in
balanced polymorphisms
Sic
kle-
Cel
l Pre
vale
nce
Selection by malaria
exposure
Dire
ctio
nal S
elec
tion
(in m
acro
evol
utio
n)
Note: This example is Macroevolutionary, not Micro…
"Of all the causes of microevolution, only natural selection generally
adapts a population to its environment. The other agents of
microevolution are sometimes called non-Darwinian because of their
usually non-adaptive nature." your text
Sexual Selection
Sex
ual S
elec
tion
Sexual selection are forces that impact on mate procurement
If you don’t mate, you don’t make babies
Mate procurement involves competing with same gender individuals (e.g., other males) and attracting other-gender individuals
Intrasexual selection is a consequence of direct competition (e.g., fighting) with one’s own gender
Intersexual selection (mate choice) is competition for the other gender’s “eye”
How these mechanisms operate can differ greatly from gender to gender
Basically, for some species (e.g., us), procuring a mate can be a very complicated experience
Sex
ual S
elec
tion
Cost of Sex (Why bother?)
Sex
ual D
imor
phis
m
Ammonite sexual dimorphismNyala sexual dimorphism
In sexual dimorphism, males and females differ phenotypically in addition
to their possessing different sexual organs
Sexual Dimorphism (elephant seals)
Hey, I was bottlenecked, too!
Gen
etic
Pol
ymor
phis
m Genetic polymorphism is the presence of
multiple alleles at a given locus within a gene pool
In general, there is a lot more genetic polymorphism in populations than “meets the eye”
This in part is because of hidden recessive alleles, and also because different alleles do not necessarily give rise to different phenotypes
Heritable variation within a population is synonymous with polymorphism
Therefore, the raw material of natural selection are polymorphisms
Gen
etic
Pol
ymor
phis
m
Bal
ance
d P
olym
orph
ism
Balanced polymorphisms are stably maintained multiple alleles at a given locus
Heterozygous advantage, a.k.a., balancing selection
E.g., Sickle cell anemia but otherwise probably not too important
Hybrid Vigor a product of heterozygous advantage and the masking of deleterious alleles
E.g., Hybrid corn, but can this maintain polymorphisms in the wild?
Frequency-dependent selection selection for alleles because they are rare, e.g., Major Histocompatibility Complex
Neutral variation selection not strong enough to remove alleles (unless environment changes)
There is more neutral variation in larger populations due reduced strength of genetic drift
Env
ironm
enta
l Var
iatio
n: A
Clin
e
Temporal Phenotypic Variation
Why
no
Per
fect
Org
anis
ms? "An organism's phenotype is constrained by
its evolutionary history“ "Adaptations are often compromises“ "Not all evolution is adaptive“• It takes too much energy to optimize
everything so much of most organisms is simply good enough to get the job done (a.k.a., the principle of allocation)
"Selection can only edit variations that exist“ Even if a perfect organism existed, it would
only remain perfect so long as its environment remained unchanged
To make matters worse, environments even change over single individual's life spans
The End