microevolution changing allele frequencies - wikispaces... · evolution •evolution is defined as...

Microevolution

Changing Allele Frequencies

Evolution

• Evolution is defined as a change in

the inherited characteristics of

biological populations over

successive generations.

• Microevolution involves the

change in allele frequencies that

occur over time within a

population.

• This change is due to four different

processes: mutation, selection

(natural and artificial), gene flow,

and genetic drift.

2

Determining Allele Frequency

• Examine the frog population

presented here.

• Their color is determined by a

single gene, which has two alleles

and phenotypically exhibits

incomplete dominance.

• CGCG is green, CG CR is blue, and

CR CR is red

• Calculate the allele frequency of

the gene pool in the diagram.

3

• These frogs are diploid, thus have two

copies of their genes for color.

• If allelic frequencies change, then

evolution is occurring.

• Let’s suppose 4 green frogs enter the

population (immigration). How do the

frequencies change?

4


Allele: CG CR

Green (11) 22 0

Blue (2) 2 2

Red (3) 0 6

Total: 24 8

Frequency: p = 24 ÷ 32

p = ¾ = 0.75

q = 8 ÷ 32

q = ¼ = 0.25

Immigration: Determining Allele Frequency

5

Recall that currently: CG = 0.75 & CR = 0.25

Allele: CG CR

Green (15) 30 0

Blue (2) 2 2

Red (3) 0 6

Total: 32 8


p = 8/10 = 0.80

q = 8 ÷ 40

q = 2/10 = 0.20


6

How do the allelic frequencies

change if 4 green frogs leave the

population instead of enter the

population? (emigration)

Emigration: Determining Allele Frequency

7

Recall that originally: CG = 0.75 & CR = 0.25

Allele: CG CR

Green (7) 14 0

Blue (2) 2 2

Red (3) 0 6

Total: 16 8


p = 2/3 = 0.67

q = 8 ÷ 24

q = 1/3 = 0.33

Impact On Small vs. Large Population

8

Before 4 frogs joined After 4 frogs joined

Compare the effect on the small population to 4 frogs joining a

much larger population.

Impact Large Population

9

Allele: CG CR

Green (22) 44 0

Blue (4) 4 4

Red (6) 0 12

Total: 48 16


p = 3/4

= 0.75

q = 16 ÷ 64

q = 1/4

= 0.25

Allele: CG CR

Green (26) 52 0

Blue (4) 4 4

Red (6) 0 12

Total: 56 16


p = 56/72

= 0.78

q = 16 ÷ 72

q = 16/72

= 0.22

Before 4 frogs joined After 4 green frogs joined

larger population larger population

Impact Small Population

10

Allele: CG CR

Green (11) 22 0

Blue (2) 2 2

Red (3) 0 6

Total: 24 8


p = ¾ = 0.75

q = 8 ÷ 32

q = ¼ = 0.25

Allele: CG CR

Green (15) 30 0

Blue (2) 2 2

Red (3) 0 6

Total: 32 8


p = 8/10 = 0.80

q = 8 ÷ 40

q = 2/10 = 0.20

Before 4 frogs joined After 4 green frogs joined

In both cases the allele frequency for CG increases but it has a

bigger impact on the smaller population.

Genetic Drift

11

Small populations can experience changes in allele frequencies

more dramatically than large populations. In very large populations

the effect can be insignificant. Also in small populations genes can

be lost more easily. When there is only one allele left for a

particular gene in a gene pool, that gene is said to be fixed , thus

there is no genetic diversity.

Genetic Drift

• Genetic drift or allelic drift is the change in the

frequency of a gene variant (allele) in a population

due to random sampling in the absence of a selection

pressure.

• Genetic drift is important when populations are

dramatically reduced. Genes are lost and deleterious

genes can also increase.

• When there are few copies of an allele (small

population), the effect of genetic drift is larger, and

when there are many copies the effect is smaller.

12

Genetic Drift

• Genetic drift can be most

profound in populations that

are dramatically reduced

(bottle neck populations)

usually due to some

environmental catastrophe.

• Also genetic drift occurs when

a small population arrives at a

new habitat such as an island.

13

Bottleneck Example

14

In 1900, the population of prairie

chickens in Illinois was 100

million but by 1995, the

population was reduced to

around 50 in Jasper County due

to over hunting and habitat

destruction which caused the

bottleneck to occur.

A comparison of the DNA from

the 1995 bird population

indicated the birds had lost most

of their genetic diversity.

Bottleneck Example

• Additionally, less than 50%

of the eggs laid actually

hatched in 1993.

• Populations outside IL do

not experience the egg

hatching problem.

• Bottleneck populations

generally experience a

severe reduction in genetic

diversity within the

population.

15

Founder Effect

• The founder effect is the loss of genetic

variation that occurs when a new

population is established by a very small

number of individuals from a larger

population and is a special case of

genetic drift.

• Founder effects are very hard to study!

16

Founder Effect

• Biologist got their chance after a hurricane wiped out all the

lizard species on certain islands in the Bahamas, scientists re-

populated the small islands with two lizard pairs, one having

long limbs and one having short limbs.

17

Founder Effect

• Before the hurricane, these

islands supported populations

of a Caribbean lizard, the

brown anole, Anolis sagrei.

• After the hurricane, seven of

the islands were thoroughly

searched. No lizards were

found.

18

Founder Effect

• In May 2005, the researchers

randomly selected one male

and one female brown anole

from lizards collected on a

nearby larger island to found

new anole populations on

seven small islands.

• They then sat back and

watched how those lizards

evolved to get an up-close

look at the Founder Effect.

19

Founder Effect

• During the next four years, the researchers repeatedly

sampled lizards from the source island, from the seven

experimental founder islands, and from 12 nearby

islands that served as a control.

• The team found that all lizard populations adapted to

their environment, yet retained characteristics from their

founders.

20

A Human Founder Effect Example

• The Amish community was founded

by a small number of colonist.

• The founding group possessed the

gene for polydactyly (extra toes or

fingers).

• The Amish population has increased

in size but has remained genetically

isolated as few outsiders become a

part of the population.

• As a result polydactyly is much

more frequent in the Amish

community than it is in other

communities.

21

Impact of Nonrandom Mating

• Nonrandom mating also changes allele frequency.

• Let’s revisit our adorable frogs and suppose that 4

frogs migrate to a pond a small distance from the main

pond.

• It is likely that these 4 frogs will mate with one

another, leaving the rest of the population in the main

pond behind to also mate with one another.

• Nonrandom mating implies a choice of mates which is

more prevalent in animals.

22

Two Types of Sexual Selection

• Darwin wrote:

“The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.”

23

Sexual Selection

• Sexual selection of mates also

affects allele frequency.

• The peacock provides a

particularly well known example

of intersexual selection, where

ornate males compete to be

chosen by females.

• The result is a stunning feathered

display, which is large and

unwieldy enough to pose a

significant survival disadvantage.

24

Sexual Selection

• Female birds of many

species choose the male.

• Males that are “showier”

will better attract

females.

• These males have a

selective advantage even

though they are more

susceptible to predators.

25

Sexual Selection

• Females that are drab, blend in to their

surroundings and as a result, avoid

predators which giving females a

survival advantage.

• Sexual selection can lead to sexual

dimorphism where there is a distinct

difference between males and females.

26

Hardy-Weinberg Equilibrium

27

So, when is there no change in the allele frequency? When the

population is said to be in Hardy-Weinberg Equilibrium, thus no evolution

is occurring.

FIVE Conditions of Hardy-Weinberg Equilibrium:

1. Population must be large so chance is not a factor. (No genetic drift).

2. Population must be isolated to prevent gene flow. (No immigration or

emigration)

3. No mutations occur.

4. Mating is completely random with respect to time and space.

5. Every offspring has an equal chance of survival without regard to

phenotypes. (No natural selection)


• Condition #1 can be met. It is important to have large

populations in order that the loss or addition of genes is not a

factor. By contrast, small populations experience genetic drift.

Additionally, if a small population moves to another area or

becomes isolated, the gene pool will be different from the

original gene pool. And the founder effect comes into play.

• Condition #2 can only be met if the population is isolated. If

individuals immigrate or emigrate from the population, the allele

frequencies change and evolution occurs.

• Condition #3 cannot ever be met since mutations always occur.

Thus mutational equilibrium can never be met.

28


29

Condition #4 can never be met. Mating is never random. Pollen

from an apple tree in Ohio is more likely to pollinate a tree in

Ohio than one in Washington state.

Condition #5 can never be met. There will always be variation.

Variation can help organisms survive longer and/or reproduce

more effectively.

Since 3 out of the 5 H-W conditions can never be met, evolution

DOES occur and allele frequencies do indeed change.

Applying the H-W Model

30

Here we go with our frogs again! Let’s suppose that in a population of 100 frogs,

36 were green (CGCG), 48 were blue (CGCR) and 16 were red (CRCR) and there

was total random mating.

Thus, it can be assumed that 60% of all the gametes (eggs and sperm) should

carry the CG allele and 40% of the gametes should carry the CR allele.

Allele: CG CR

Green (36) 72 0

Blue (48) 48 48

Red (16) 0 32

Total: 120 80


p = 3/5 = 0.60

q = 80 ÷ 200

q = 2/5 = 0.40


31

A population Punnett square is shown above. It indicates that the next generation should

have the following offspring distribution: 36% green (CGCG), 48% blue(CGCR), 16% red

(CRCR). When the second generation gets ready to reproduce, the results will be the same as

before.

CG 0.60 CR 0.40

CG 0.60

CGCG

0.36 CGCR

0.24

CR 0.40

CGCR

0.24 CRCR

0.16

Allele: CG CR

Green (36) 72 0

Blue (48) 48 48

Red (16) 0 32

Total: 120 80


p = 3/5 = 0.60

q = 80 ÷ 200

q = 2/5 = 0.40


32

So, the allele frequency remains at 0.40 CG and 0.60 CR thus no evolution is taking

place.

Let’s suppose that there is an environmental change that makes red frogs more

obvious to predators. How is the population affected and now the population

consists of 36 green, 48 blue, and 6 red frogs?

Now, allele frequencies are changing and there is an advantage to being green or

blue but NOT red. Evolution is indeed occurring.

Allele: CG CR

Green (36) 72 0

Blue (48) 48 48

Red (6) 0 12

Total: 120 60


p = 2/3 = 0.66

q = 60 ÷ 180

q = 1/3 = 0.33

Deriving the H-W Model

33

Examine this Punnett square again. If p represents the allele frequency of CG (dominant)

and q represents the allele frequency of CR (recessive) then two equations for a

population in Hardy-Weinberg equilibrium can be derived where the following

genotypes are represented by:

CGCG = p2 CRCR = q2 CGCR = 2pq

Mathematically then p + q = 1 (1st H-W equation)

So, the Punnett square effectively crossed (p + q ) (p + q ) which gives

p2 + 2pq + q2 = 1 (2nd H-W equation)

CG 0.60 CR 0.40

CG 0.60

CGCG

0.36 CGCR

0.24

CR 0.40

CGCR

0.24 CRCR

0.16

Natural Selection Natural Selection is the only mechanism that consistently causes

adaptive evolution.

• Evolution by natural selection is a blend of chance and “sorting”.

– Chance in the context of mutations causing new genetic

variations

– Sorting in the context of natural selection favoring some alleles

over others

• This favoring process causes the outcome of natural selection to be

anything but random!

• Natural Selection consistently increases the frequencies of alleles

that provide reproductive advantage and thus leads to adaptive

evolution.

Relative Fitness • There are animal species in which

individuals, usually males, lock

horns or otherwise compete

through combat for mating

privileges.

• Reproductive success is usually far

more subtle!

• Relative fitness is defined as the

contribution an individual makes to

the gene pool of the next

generation relative to the

contributions of other individuals.

Three Modes of Natural Selection

• Natural selection can alter the frequency distribution of

heritable traits in three ways depending on which

phenotype is favored:

– Directional Selection

– Disruptive Selection

– Stabilizing Selection

Directional Selection • Directional selection occurs when conditions favor individuals

exhibiting one extreme of a phenotypic range.

• Commonly occurs when the population’s environment changes or

when members of a population migrate to a new (and different)

habitat.

Possible Effect of Continual Directional Selection

38

Fre

qu

en

cy

Fre

qu

en

cy

Phenotype (trait)

before after

If continued, the variance may decrease.

Fre

qu

en

cy

Phenotype (trait)

before after

Phenotype (trait)

before after

Disruptive or Diversifying Selection

• Disruptive selection occurs when conditions favor

individuals at both extremes of a phenotypic range

over individuals with intermediate phenotypes.

• The “intermediates” in the population have lower

relative fitness.

39

Disruptive or Diversifying Selection

• Disruptive selection occurs when conditions favor

individuals at both extremes of a phenotypic range

over individuals with intermediate phenotypes.

• The “intermediates” in the population have lower

relative fitness.

40

Stabilizing Selection

• Stabilizing selection removes extreme variants from

the population and preserves intermediate types.

• This reduces variation and tends to maintain the

status quo for a particular phenotypic character.

41

Sexual Selection

• A form of selection in which individuals with certain

inherited characteristics are more likely than other

individuals to obtain mates.

• Can result in sexual dimorphism which is a difference

between the two sexes with regard to secondary sexual

characteristics.

42

Intrasexual vs. Intersexual Selection

• How does sexual selection operate?

• Intrasexual—selection within the same sex, individuals

of one sex compete directly for mates of the opposite

sex. Males are famous for this!

43

Intrasexual vs. Intersexual Selection

• Intersexual selection (mate choice)—individuals of one

sex are choosy.

• Often these are females that select mates based on their

showiness.

44

Preserving Genetic Variation

• Some of the genetic variation is populations represents

neutral variation, differences in DNA sequence that do

not confer a selective advantage or disadvantage.

• There are several mechanisms that counter the tendency

for directional and stabilizing selection to reduce

variation:

– Diploidy

– Balancing Selection

– Hererzygote Advantage

– Frequency-Dependent Selection

45

Diploidy

• In diploid eukaryotes each organism has two copies

of every gene and a considerable amount of genetic

variation is hidden from selection in the form of

recessive alleles.

• Often alleles are recessive and less favorable than

their dominant counterparts.

• By contrast, haploid organisms express every gene

that is in their genome. What you see is what you

get. It reduces genetic variability.

46

Diploidy

• Recessive alleles persist by propagation in

heterozygous individuals.

• This latent variation is exposed to natural selection

only when both parents carry the same recessive

allele and two copies end up in the same zygote.

• As you might expect, this happens rarely if the

allelic frequency of the recessive allele is very low.

• Why is heterozygote protection of potentially

negative recessive alleles important to species

survival?

47

Balancing Selection

• Balancing selection occurs when natural selection

maintains two or more forms in a population.

• This type of selection includes heterozygote advantage

and frequency-dependent selection.

• Heterozygote advantage involves an individual who is

heterozygous at a particular gene locus thus has a

greater fitness than a homozygous individual.

48

Heterozygote Advantage

• A well-studied case is that of sickle

cell anemia in humans, a hereditary

disease that damages red blood cells.

• Sickle cell anemia is caused by the

inheritance of a variant hemoglobin

gene (HgbS) from both parents.

• In these individuals, hemoglobin in

red blood cells is extremely sensitive

to oxygen deprivation, and this

causes shorter life expectancy.

49


• A person who inherits the

sickle cell gene from one

parent, and a normal

hemoglobin gene (HgbA)

from the other, has a normal

life expectancy.

• However, these heterozygote

individuals, known as carriers

of the sickle cell trait, may

suffer problems from time to

time.

50


• The heterozygote is resistant to

the malarial parasite which

kills a large number of people

each year in Africa.

• There exists a balancing

selection between fierce

selection against homozygous

sickle-cell sufferers, and

selection against the standard

HgbA homozygotes by malaria.

• The heterozygote has a

permanent advantage (a higher

fitness) wherever malaria

exists. 51

Frequency-Dependent Selection

• The fitness of a phenotype depends on how common it

is in the population.

• In positive frequency-dependent selection the fitness

of a phenotype increases as it becomes more common.

• In negative frequency-dependent selection the fitness

of a phenotype increases as it becomes less common.

• For example in prey switching, rare morphs of prey are

actually fitter due to predators concentrating on the

more frequent morphs.

52

Balanced Polymorphism Balanced polymorphism occurs in a given

population when two distinct types (or

morphs) exists and the allele frequencies do

not change. This may be due to

• Variation in the environment where one

morph may be favored over another.

• One morph may be better adapted to a

certain time of the year over the other.

The lady bird beetle has 2 morphs. The red

variety is more abundant in the spring and

winter, whereas the black morph is more

abundant in the summer and fall.

53

Why Natural Selection Cannot Fashion Perfect Organisms

1. Selection can act only on existing variations.

• Natural selection favors only the fittest phenotypes among

those in the population, which may not be the ideal traits.

New advantageous alleles do not arise on demand.

2. Evolution is limited by historical constraints.

• Each species has a legacy of descent with modification

from ancestral forms. Evolution does not scrap the

ancestral anatomy. For example in birds and bats, an

existing pair of limbs took on new functions for flight as

these organisms evolved from nonflying ancestors.

54

Why Natural Selection Cannot Fashion Perfect Organisms

3. Adaptations are often compromises.

• The loud call that enables a frog to attract mates also

attracts predators.

4. Chance, natural selection and the environment

interact.

• Chance can affect the subsequent evolutionary history of

populations. A storm can blow birds hundreds of

kilometers over an ocean to an island, the wind does not

necessarily transport those individuals that are best suited

to the environment!

55

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