chapter 23: the evolution of populations. important point:

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Chapter 23: The Evolution of Populations

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Page 1: Chapter 23: The Evolution of Populations. Important Point:

Chapter 23:The Evolution of

Populations

Page 2: Chapter 23: The Evolution of Populations. Important Point:

Important Point:

Page 3: 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

Page 4: Chapter 23: The Evolution of Populations. Important Point:

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

Page 5: Chapter 23: The Evolution of Populations. Important Point:

Men

del m

eets

H.W

. Recall Mendelian genetics

Hardy-Weinberg Equilibrium

means genotype frequencies stay

the same

Page 9: Chapter 23: The Evolution of Populations. Important Point:

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)

Page 10: Chapter 23: The Evolution of Populations. Important Point:

H.W

. Equ

ilibr

ium

Hardy-Weinberg

means that both

genotype and allele

frequencies stay the

same over time

Page 11: Chapter 23: The Evolution of Populations. Important Point:

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

Page 12: Chapter 23: The Evolution of Populations. Important Point:

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

Page 13: Chapter 23: The Evolution of Populations. Important Point:

Egg

s &

Milt

(S

perm

) in

Buc

ket

http://wdfw.wa.gov/wildwatch/salmoncam/

hatchery.html

Page 14: Chapter 23: The Evolution of Populations. Important Point:

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

Page 15: Chapter 23: The Evolution of Populations. Important Point:

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

Page 16: Chapter 23: The Evolution of Populations. Important Point:

Chalk discussion of H.W. theorem, including, especially,

p2 + 2pq + q2 = 1

Page 17: Chapter 23: The Evolution of Populations. Important Point:

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!

Page 18: Chapter 23: The Evolution of Populations. Important Point:

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???

Page 19: Chapter 23: The Evolution of Populations. Important Point:

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

Page 20: Chapter 23: The Evolution of Populations. Important Point:

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

Page 21: Chapter 23: The Evolution of Populations. Important Point:

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)]

Page 22: Chapter 23: The Evolution of Populations. Important Point:

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?)

Page 23: Chapter 23: The Evolution of Populations. Important Point:

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

Page 24: Chapter 23: The Evolution of Populations. Important Point:

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

Page 25: Chapter 23: The Evolution of Populations. Important Point:

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)

Page 26: Chapter 23: The Evolution of Populations. Important Point:

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

Page 27: Chapter 23: The Evolution of Populations. Important Point:

Sampling Error: Genetic DriftErrors get bigger (as fraction of sample) as

samples get smaller!

Page 28: Chapter 23: The Evolution of Populations. Important Point:

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

Page 29: Chapter 23: The Evolution of Populations. Important Point:

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

Page 30: Chapter 23: The Evolution of Populations. Important Point:

Cheetah, Product of Bottleneck

The longer a population

remains at a reduced size, the greater the effect of genetic drift on allele frequency

Page 31: Chapter 23: The Evolution of Populations. Important Point:

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

Page 32: Chapter 23: The Evolution of Populations. Important Point:

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

Page 33: Chapter 23: The Evolution of Populations. Important Point:

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

Page 34: Chapter 23: The Evolution of Populations. Important Point:

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

Page 35: Chapter 23: The Evolution of Populations. Important Point:

Mutation & Neutral Variation

Note change in allele frequencies

Page 36: Chapter 23: The Evolution of Populations. Important Point:

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)

Page 37: Chapter 23: The Evolution of Populations. Important Point:

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

Page 38: Chapter 23: The Evolution of Populations. Important Point:

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

Page 39: Chapter 23: The Evolution of Populations. Important Point:

Migration (Gene Flow)

Migration (movement of individuals) makes allele

frequencies become more similar

Page 40: Chapter 23: The Evolution of Populations. Important Point:

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

Page 41: Chapter 23: The Evolution of Populations. Important Point:

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

Page 42: Chapter 23: The Evolution of Populations. Important Point:

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

Page 43: Chapter 23: The Evolution of Populations. Important Point:

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

Page 44: Chapter 23: The Evolution of Populations. Important Point:

Chalk discussion of effect of natural

selection on H.W. frequencies

Page 45: Chapter 23: The Evolution of Populations. Important Point:

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

Page 46: Chapter 23: The Evolution of Populations. Important Point:

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

Page 47: Chapter 23: The Evolution of Populations. Important Point:

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?

Page 48: Chapter 23: The Evolution of Populations. Important Point:

Modes of Selection

Page 49: Chapter 23: The Evolution of Populations. Important Point:

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

Page 50: Chapter 23: The Evolution of Populations. Important Point:

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

Page 51: Chapter 23: The Evolution of Populations. Important Point:

Disruptive Selection

In disruptive selection the

intermediate is selected against

Disruptive selection can result in

balanced polymorphisms

Page 52: Chapter 23: The Evolution of Populations. Important Point:

Sic

kle-

Cel

l Pre

vale

nce

Selection by malaria

exposure

Page 53: Chapter 23: The Evolution of Populations. Important Point:

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

Page 54: Chapter 23: The Evolution of Populations. Important Point:

Sexual Selection

Page 55: Chapter 23: The Evolution of Populations. Important Point:

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

Page 56: Chapter 23: The Evolution of Populations. Important Point:

Sex

ual S

elec

tion

Page 57: Chapter 23: The Evolution of Populations. Important Point:

Cost of Sex (Why bother?)

Page 58: Chapter 23: The Evolution of Populations. Important Point:

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

Page 59: Chapter 23: The Evolution of Populations. Important Point:

Sexual Dimorphism (elephant seals)

Hey, I was bottlenecked, too!

Page 60: Chapter 23: The Evolution of Populations. Important Point:

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

Page 61: Chapter 23: The Evolution of Populations. Important Point:

Gen

etic

Pol

ymor

phis

m

Page 62: Chapter 23: The Evolution of Populations. Important Point:

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

Page 63: Chapter 23: The Evolution of Populations. Important Point:

Env

ironm

enta

l Var

iatio

n: A

Clin

e

Page 64: Chapter 23: The Evolution of Populations. Important Point:

Temporal Phenotypic Variation

Page 65: Chapter 23: The Evolution of Populations. Important Point:

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

Page 66: Chapter 23: The Evolution of Populations. Important Point:

The End