Download The Evolution of Populations

Chapter 23:
The Evolution of
Populations
Important Point:
Gene Pools
A gene pool is the sum
of alleles at all loci
within a population
One species, but members
are more likely to mate within
their herd than the other
Polymorphism
A 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
Mendel meets H.W.
Recall
Mendelian
genetics
Hardy-Weinberg
Equilibrium
means genotype
frequencies stay
the same
Hardy-Weinberg Theorem
x2
320
20
Hardy-Weinberg Theorem
Hardy-Weinberg Theorem
Hardy-Weinberg Theorem
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. Equilibrium
HardyWeinberg
means that
both
genotype
and allele
frequencies
stay the
same over
time
H.-W. Frequencies (2 alleles)
Calculated H.W.
frequencies,1 locus, 2
alleles
“Fixed”
allele
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)
H.-W. Assumptions
 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 HardyWeinberg 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
Eggs & Milt (Sperm) in Bucket
http://wdfw.wa.gov/wildwatch/sa
lmoncam/hatchery.html
Non-Random Mating
Nonrandom mating
results in deviations from
a Hardy-Weinberg
generation of genotypes
from a given frequency of
alleles
Anything that
interferes with the
random mating
between individuals
is nonrandom mating
H.-W. Equilibrium
 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
Solving H.-W. Problems
 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!
Working with Decimals
 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???
Phenotype  Genotype
 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 dominantrecessive 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-toone mapping of genotype to phenotype
 That is, for the latter (& only for the latter)
genotype frequencies will be the same as
phenotype frequencies
Dominant Genotypes
 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
Genotype  Allele
 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)]
Allele  Genotype
 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-Darwinian Evolution
 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-Darwinian Evolution
 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-Random Mating
 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-Darwinian Evolution
 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 Drift
Errors get bigger (as fraction of sample) as
samples get smaller!
Non-Darwinian Evolution
 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: Bottleneck
When 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-Darwinian Evolution
 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 Effect
Note that the alleles lost are not necessarily the
same alleles as may have been lost due to natural
selection
New
population
Genetic drift is
sampling error
Products of Genetic Drift
A locus for which
only a single
allele exists for
an entire gene
pool is
considered to be
fixed, i.e., a fixed
locus
Isolated
populations by
chance “fixed”
different
karyotypes
Non-Darwinian Evolution
 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
Mutation (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)
Mutation (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-Darwinian Evolution
 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-Darwinian Evolution
 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
Natural Selection (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
Natural Selection (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
Incorporating Selection
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
"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
Seeds that drift
onto mine tailings
die unless they
are genetically
predisposed
toward heavymetal resistant
Darwinian Fitness
 “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
Relative Fitness
 “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
Stabilizing selection eliminates
phenotypic extremes within a
population, thus increasing the
frequency of genotypes underlying
intermediate phenotypes
Stabilized populations tend to be
reasonably well adapted to their
environments
Directional Selection
Directional selection
is natural selection
against only one
phenotypic extreme
Directional 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
Sickle-Cell Prevalence
Selection by
malaria
exposure
Directional Selection (in macroevolution)
"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
Note: This example is Macroevolutionary, not Micro…
Sexual Selection
Sexual Selection
 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
Sexual Selection
Cost of Sex (Why bother?)
Sexual Dimorphism
Nyala sexual dimorphism
In sexual dimorphism,
males and females differ
phenotypically in addition
to their possessing different
sexual organs
Ammonite sexual dimorphism
Sexual Dimorphism (elephant seals)
Hey, I was
bottlenecked, too!
Genetic Polymorphism
 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
Genetic Polymorphism
Balanced Polymorphism
 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
Environmental Variation: A Cline
Temporal Phenotypic Variation
Why no Perfect Organisms?
 "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