ch 23

(1)

Populations

•

The evolutionary changes of natural selection

are visible in allele changes in a population

over time

(2)

(3)

Polymorphism

•

Phenotypic polymorphism

– Describes a population in which two or more distinct morphs for a character are each

represented in high enough frequencies to be readily noticeable

•

Genetic polymorphisms

(4)

The Modern Synthesis

•

Modern synthesis: a comprehensive theory of

evolution that united Mendelian genetics and

Darwinian natural selection

•

Individual variation often reflects genetic variation

– Quantitative traits: vary along a continuum; more than one gene involved

(5)

Genetic variation

•

WITHIN a species!

– Within a population: measured by average heterozygosity

– Between populations

• Geographic variation

(6)

Sources of variation

•

Mutation

•

Chromosome-level changes

(7)

Gene pools

•

A population: a group of individuals of the

same species that live in the same area and

interbreed

•

The aggregate of genes in a population at any

one time is called the

gene pool

(8)

– What is the frequency of the RED flower color allele in a population where 320 plants have red flowers,160 have pink flowers and the remaining 20 have white?

– Can you determine the frequency of the blue eye allele in this room?

(9)

The Hardy-Weinberg Theorem

•

Describes a population that

is not evolving

•

Frequencies of alleles and of genotypes in a

population’s gene pool

remain constant

from

generation to generation

– _{No selection}

– _{No migration into or out of the population}

– _{Random mating}

– _{A really big population}

(10)

There are two formulas that must be memorized:

p

2

+ 2pq + q

2

= 1

p + q = 1

•

p

and

q

represent the relative frequencies of

the only two possible alleles in a population at

a particular locus

•

p

2

and

q

2

represent the frequencies of the

homozygous genotypes and 2

pq

represents

(11)

MAAAAAATTTTHHH

•

What is the probability of flipping two heads?

(12)

Reaching into that gene pool…..

•

The frequency of rr individuals will be q*q

•

The frequency of Rr individuals will be p*q + q*p

– _{This can be written as 2pq}

•

The frequency of RR individuals is p*p

•

These three terms must add to ONE…why?

(13)

Conditions for Hardy-Weinberg Equilibrium

•

In real populations allele and genotype

frequencies do change over time

•

The

five conditions for non-evolving

populations

are rarely met in nature

– Extremely large population size – No gene flow

– No mutations – _{Random mating}

(14)

•

Using the HW equations to calculate expected

frequency of genotypes. Then compare

expected vs. actual…

•

Why do this? Often done to look for evidence

(15)

Step 1: Determine gene frequencies

(try to calculate the frequencies of p and q)

Phenotype Genotype _Individuals# of

Normal Eyes RR 90

Red Eyes rr 15

(16)

f(R) = [(90*2)+(45] / 300 = 225/300 = 0.75

f(r) = [(15*2)+(45)] / 300 = 75/300 = 0.25

•

There are 90 + 15+ 24= 150 individuals

(17)

Step 2: Determine expected genotype frequencies

Plugging the frequencies of each allele into the Hardy-Weinberg equation, we find the expected numbers of each genotype in the population:

f(RR) = p2 = f(R)*f(R) = 0.5625

f(rr) = q2 = f(r)*f(r) = 0.0625

(18)

Multiplying each of these genotype frequencies with the total population number (150), we find that there should be:

84 normal-eyes flies (AA) 9 red-eyed flies (aa)

(19)

Step 3: Compare with original population numbers Compare the expected with the actual numbers to

determine if populations are in equilibrium. In this example:

Phenotype Genotype Expected # Observed #

Normal Eyes RR 84 90

Red Eyes rr 9 15

Pink Eyes Rr 56 45

(20)

•

Three major factors alter allele frequencies

and bring about most evolutionary change

1. Natural selection

2. Genetic drift

3. Gene flow

(21)

1. Natural Selection

•

Differential success in reproduction

– Results in certain alleles being passed to the next generation in greater proportions

MMMM - Tasty

(22)

2. Genetic Drift

•

Genetic drift

– Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation

– Is an effect of small sample size

(23)

2. Drift: The Bottleneck Effect

•

In the bottleneck effect

– A sudden change in the environment may drastically reduce the size of a population

– The gene pool may no longer be reflective of the original population’s gene pool

Original Bottlenecking Surviving

(24)

2. Drift: The Founder Effect

•

The founder effect

– Occurs when a few individuals become isolated from a larger population

– Can affect allele frequencies in a population End c fri;

(25)

3. Gene Flow (migration)

•

Gene flow

– Causes a population to gain or lose alleles – Results from the movement of fertile

individuals or gametes

(26)

A Closer Look at Natural Selection

•

Natural selection is the

primary mechanism

of adaptive evolution

•

From the range of variations available in a

population

(27)

Evolutionary Fitness

•

The phrases “struggle for existence” and

“survival of the fittest”

– Are commonly used to describe natural

selection; misleading b/c they suggest direct competition

•

Reproductive success is generally more subtle

and depends on many factors



Relative fitness:

the contribution an individual

(28)

Directional, Disruptive, and Stabilizing Selection

•

Selection

– Favors certain genotypes by acting on the phenotypes of certain organisms

•

Three modes of selection are

1. Directional

2. Disruptive

(29)

•

Directional selection

– Favors individuals at one end of the phenotypic range

•

Disruptive selection

– _{Favors individuals at both extremes of the} phenotypic range

•

Stabilizing selection

(30)

•

The three modes of selection

Fig 23.12 A–C

(a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the

distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators.

(b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage.

(c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Phenotypes (fur color)

(31)

Sexual Selection

•

Sexual selection

– Is natural selection for mating success

– Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

•

Intrasexual selection

– _{Is a direct competition among individuals of} one sex for mates of the opposite sex

(32)

End G tue

•

Intersexual selection

– Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex

– May depend on the showiness of the male’s appearance

– http://www.youtube.com/watch?v=gqsMTZQ-p mE&feature=relmfu

(33)

The Preservation of Genetic Variation

•

Various mechanisms help to preserve genetic

variation in a population

1.

Diploidy

– Maintains genetic variation in the form of hidden recessive alleles

2.

Balancing selection

– Occurs when natural selection maintains

stable frequencies of two or more phenotypic forms in a population

(34)

3. Heterozygote Advantage end c wed

•

Some individuals who are heterozygous at a

particular locus

– Have greater fitness than homozygotes

– (Can you give a reason for this, thinking about alleles= instructions for making versions of a protein)

•

Therefore: Natural selection

(35)

•

The sickle-cell allele

– Causes mutations in hemoglobin but also confers malaria resistance

– Exemplifies the heterozygote advantage

Frequencies of the sickle-cell allele

0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% 10.0–12.5% Distribution of

malaria caused by

(36)

•

Frequency-Dependent Selection

•

In frequency-dependent selection

(37)

Why Natural Selection Cannot Fashion Perfect Organisms

1.

Selection can only act on existing variation

2.

Evolution is limited by historical constraints

3.

Adaptations are often compromises

4.

Chance, natural selection, and the

environment interact

(38)

Jeopardy!!!!

Ps and qs Who went before Not in HW

Find q

if frequency of homoz. dom. Is 16%

Classified organisms Small populations lose diversity because of….

Find freq of heteroz.

if freq of homoz. Dom is 16%

Too many people, too few resources

One phenotype is

favored, leading to this

What must one assume to use Hardy Weinberg

equations?

Almost published before Darwin

Name two ways in which genetic diversity can be preserved

Father of paleontology

http://www.biology.arizona.edu/evolution/act/drift/frame.html

Animal Olympians : Weight Lifting : Rhino Beetle - Stock Footage - Google Video

http://www.youtube.com/watch?v=gqsMTZQ-pmE&feature=relmfu