Evidence for Evolution

• Direct Observation• Fossil Record• Homology• Biogeography

Application of Information

1. Graph the data found in the table

2. Examine the graph and hypothesize why the percentage of mosquitoes resistant to DDT rose rapidly

3. Suggest an explanation for the global spread of DDT resistance

Individuals do not evolve, populations do.

• There was a population of 1200 ground finches that was wiped to 180 during a period of drought. Those that survived had larger, deeper beaks than those that died. Scientists observed that soft seeds, which finches normally eat, were in short supply, whereas hard ones were more plentiful. In the next generation of finches, the average beak size was bigger.

Fig. 23-1

Individuals do not evolve, populations do.

• In a paragraph, describe the statement above. Use examples to help support your answer. You may use the finch example if you choose, or you may come up with another one.

Chapter 23: The Evolution of Populations

Ms. KlinkhachornMarch 21, 2011

AP Biology

Microevolution

• Microevolution = the change in allele frequencies over generations– Example:

• The allele for larger beak became more frequent after the drought selected for finches with this type of beak

• The allele for smaller beak became less frequent since these finches would have a hard time surviving.

Fig. 23-4

1.0

0.8

0.6

0.4

0.2

046 44 42 40 38 36 34 32 30

GeorgiaWarm (21°C)

Latitude (°N)

MaineCold (6°C)

Ldh-

B b

alle

le f

req

uen

cy

What makes evolution possible?

• Genetic variation!– If there are no differences in the gene pool, there

is nothing for natural selection to pick

What causes genetic variation?

1. Mutation (chromosomal and DNA)– Source of new alleles

• More harmful, but can occasionally be beneficial

2. Sexual Reproduction– Every individual gets a unique combination

• Crossing over• Independent assortment• Fertilization

Background Info

• Population = a group of individuals of the same species that live in the same area and mate with one another

• Gene Pool = all of the genes of a population of organisms– Fixed population

Hardy – Weinberg Principle

• HW Principle says that the frequencies of alleles and genotypes in a population will remain constant from generation to generation– A gene pool in this state is said to be in Hardy-

Weinberg equilibrium

• Use this principle to help us figure out if a population is evolving or not

Conditions for Hardy – Weinberg

1. No mutations2. Random mating3. No natural selection4. Large population size5. No gene flow (no immigration and emigration)

• If these conditions change, evolution can occur and you are no longer in HW equilibrium

Hardy-Weinberg Equation

• If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then– p2 + 2pq + q2 = 1– where p2 and q2 represent the frequencies of the

homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

– p is usually the dominant allele, q is recessive

Fig. 23-6

Frequencies of allelesAlleles in the population

Gametes produced

Each egg: Each sperm:

80%chance

80%chance

20%chance

20%chance

q = frequency of

p = frequency ofCR allele = 0.8

CW allele = 0.2

Fig. 23-7-1

SpermCR

(80%)

CW

(2

0%)

80% CR ( p = 0.8)

CW (20%)

20% CW (q = 0.2)

16% ( pq) CRCW

4% (q2) CW CW

CR

(80%

)

64% ( p2) CRCR

16% (qp) CRCW

E gg s

Example Problem

• Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

Application of HW

• The occurrence of PKU is 1 per 10,000 births– q2 = 0.0001– q = 0.01

• The frequency of normal alleles is– p = 1 – q = 1 – 0.01 = 0.99

• The frequency of carriers is– 2pq = 2 x 0.99 x 0.01 = 0.0198– or approximately 2% of the U.S. population

Practice Problem

• Within a population of butterflies, the color brown (B) is dominant over the color white (b). And, 40% of all butterflies are white.

• Calculate the following:– The percentage of butterflies in the population

that are heterozygous.– The frequency of homozygous dominant

individuals– If there are 200 butterflies, how many are brown?

Practice Problem

• After graduation, you and 19 of your friends charter a plane to go on a round-the-world tour. Unfortunately, you all crash land (safely) on a deserted island. No one finds you and you start a new population totally isolated from the rest of the world. Two of your friends are carriers for cystic fibrosis. – Assuming that the frequency of this allele does not

change as the population grows, what will be the incidence of cystic fibrosis on your island?

Allelic Frequencies Can Be Altered

• Three major factors alter allele frequencies and bring about most evolutionary change:– Natural selection– Genetic drift– Gene flow

Natural Selection

Genetic Drift

• Genetic drift = chance events can cause allele frequencies to fluctuate unpredictably, especially in small populations

Fig. 23-8-1

Generation 1p (frequency of CR) = 0.7q (frequency of CW ) = 0.3

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

Fig. 23-8-2

Generation 1p (frequency of CR) = 0.7q (frequency of CW ) = 0.3

Generation 2p = 0.5q = 0.5

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

Fig. 23-8-3

Generation 1

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

p (frequency of CR) = 0.7q (frequency of CW ) = 0.3

Generation 2

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR CR CR

CR CR

CR CR CR CR

Types of Genetic Drift

1. Founder Effect– A few individuals in a population become

isolated from the rest of the population and FOUND a new population

2. Bottleneck Effect– Caused by drastic reduction in population size

because of fire or flood (catastrophe)

Fig. 23-9

Originalpopulation

Bottleneckingevent

Survivingpopulation

Gene Flow

• Gene flow = transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes– Reduce the genetic differences across different

population• Could end up combining the populations and make a

common gene pool

Fig. 23-11

Natural Selection and Relative Fitness

• Relative fitness = the contribution an individual makes to the gene pool of the next generation, relative to the contribution of other individuals– Depends on adaptations– More adaptations there are, the more fit you are

Mechanism of Natural Selection

Normal Distribution of Traits

Types of Natural Selection

• Natural selection changes the allelic frequency, depending on how the phenotypes are favored in the environment

1. Directional2. Disruptive3. Stabilizing

Fig. 23-13

Original population

(c) Stabilizing selection(b) Disruptive selection(a) Directional selection

Phenotypes (fur color)F

req

uen

cy o

f in

div

idu

als

Originalpopulation

Evolvedpopulation

Directional Selection

• Conditions favor individuals exhibiting an extreme of a phenotypic range (shift the curve to the right or the left

• Most commonly occurs when:– The environment changes (drought, fire)– Migration

Directional Selection

Fig. 23-13a

Original population

(a) Directional selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Original population

Evolved population

Examples: Directional Selection

• Peppered Moths and the Industrial Revolution– When the trees were light

colored (before), the moths that were light colored were favored

– When the trees were darkened (by the pollution), the moths that were darker were favored

• See regular shifts

Examples: Directional Selection

• Bird beaks on an island

Disruptive Selection

• Conditions favor individuals at BOTH extremes of the phenotype range

• Example: – Birds with small beaks and birds with really big

beaks are specialized• Birds with medium beaks are inefficient

Fig. 23-13b

Original population

(b) Disruptive selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Evolved population

Examples: Disruptive Selection

• Harbor Seals– Smaller seals are faster

and more agile. They can hunt small, quick prey

– Larger seals are powerful and can get bigger prey

– Medium sized can’t do either well

Stabilizing Selection

• Conditions favor the intermediate phenotypes• Most common type of selection

Fig. 23-13c

Original population

(c) Stabilizing selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Evolved population

Example: Stabilizing Selection

• Weight of newborn babies is typically between 6 and 9 lbs– Too small: lose heat, get

sick more easily– Too big: harder to deliver

during childbirth

Example: Stabilizing Selection

• Mountain Goats and Leg Length– Short legs: not as agile,

can’t move as easily– Long legs: don’t have

good balance

Quick Write

• What is evolution? – Describe the mechanism and how, depending on

the environment, the selection can vary. – Give examples of how genetic variation can enter

a population at the molecular level AND at the population level.