The Evolution of Population: The Mechanisms of Microevolution

I. Evolution (What actually changes?)

the amount of a particular allele as found in the gene pool

Time

Gene Pool A

Gene Pool C

Gene Pool B

B. Gene Pool: sum total of all the alleles in a population

A. Gene frequency:

Microevolution: The change in gene frequency in a population over time

Gene Pool A 100 Organisms Gene Pool B 100 Organisms

Phenotype PhenotypeGenotype Genotype

DarkDark

Medium Medium

Light Light

# #

DDDD

Dd Dd

dddd

25

50

25

10

30

60

Gene Frequency = allele

total alleles

Gene Frequency D =(25 x 2) + 50

200= .5

Gene Frequency d =(25 x 2) + 50

200= .5

Gene Frequency D =

Gene Frequency d =

(10 x 2) + 30= .25

200

(60 x 2) + 30

200= .75

GF of D = .5 GF of d = .5 GF of D = .25 GF of d = .75

Time

Slide 4

The Hardy – Weinberg Theory

A. States that gene frequencies will not change in a population only due to sexual reproduction (Skip to III. Math Theory)

B. Gene frequencies will not change (stay in Hardy-Weinberg equilibrium) unless one or more of the following is taking place:

1. Genetic drift (and/or small population size)

2. Migration of genes from other populations

3. Mutation

4. Selective mating

5. Natural selection

Implication:

The Hardy-Weinberg mathematically proves that microevolution will not take place unless one or more of the above is occurring

II. The Summary

23_08EvolutionaryChanges_A.swf

IV. The Equations

Assumption: Genes in gene pool interact independently

p + q = 1

p = gene frequency of dominant allele

q = gene frequency of recessive allele

1 = total genes in gene pool (100%)

Equation #2

Assumption: Genes found as pairs and interact within organism

Equation #1

p + 2pq + q = 12 2

p = % homozygous dominant2

q = % homozygous recessive2

2pq = % heterozygous

1 = 100% of individuals

Taste Lab and Application Worksheet

V. Concepts Relating to the 5 Hardy-Weinberg Conditions

A. Large Populations vs Small Populations

(Genetic Drift)

1. Small populations are more susceptible to “genetic drift” (random events that change gene frequencies due to sample size)

a) Bottleneck effect

A random “bottlenecking event” reduces the population number that result in new gene frequencies

b) Founder effect

When a new habitat is colonized, the genotypes of the original colonist will influence the gene frequencies as the population grows

1. Genetic drift (and/or small population size)

2. Migration of genes from other populations

3. Mutation

4. Selective mating

5. Natural selection

B. Migration (Gene Flow)

1. Tends to reduce differences between populations

2. Extensive gene flow will cause 2 populations to interact as 1 population

C. Mutations

1. Can immediately affect gene pool by substituting one allele for another

2. Rare; usually harmful

3. Becomes the source of new variations

D. Natural Selection interactions on genetic variations (Phenotypes)1. Types of genetic variations

a) Polymorphism (morphs)

Two or more phenotypes found in a population. Allows for natural selection to “pick” the most fit

d) Balanced Polymorphism

1) Heterozygous Advantage

2) Frequency Dependent Selection

3) Neutral Variation

b) Geographic VariationRegional differences in gene frequencies in isolated populations. Example: Island mice

c) Clines

Graded variations within a population along a geographic axis. Environmental gradient may lead to genetic variations Example: Yarrow

Natural selection stabilizes gene frequencies of 2 or more phenotypes

Natural selection selects against the homozygous dominant and homozygous recessive. Example Sickle cell anemia

The Survival and reproduction of a particular phenotype declines as the phenotype becomes more common. Example: parasite/host relationships

Variations of no apparent selective advantage. Most variations are probably neutral

2. What is Meant by “Fitness”

a) Darwinian Fitness

b) Relative Fitness

The contribution an individual makes to the gene pool of the next generation as compared to other individuals

A “super” phenotype in a sterile organism has no fitness value

Evolutionary impact of a gene is only measured by the continued success of offspring

A quantitative value that compares the Darwinian fitness of phenotypes found in a population Example Frogs (page 1)

3. Patterns (Modes) of Natural Selection

# of mice

PhenotypesLightest Darkest

Directional Selection Diversifying Selection Stabilizing Selection

Selection favors one extreme Selection favors both extremes Selection favors heterozygous “Heterozygous advantage”

E. Non-Random Mating

2. Sexual Dimorphism

a. Intrasexual Selection

b. Intersexual Selection

1. The disadvantage of sexual reproduction

Males do not directly produce offspring. Is maleness an unfit phenotype?

a. Asexual reproduction produces more offspring more efficiently (less energy) then sexual reproduction. Is sexual reproduction an unfit phenotype?

Phenotypic differences between males and females resulting from non-random mating. Example: peacocks and peahens

“within the same sex”

Males compete with their own sex for mates. Males defeat other males for possession of females.

“mate choice”

One sex (female) chooses over individuals of the other sexExamples

Although grebes compete for mates why are the males and females so similar?

III. The Math Theory

D

D

D D

DD

D

D

D

Gene Pool A

d

d

d d

d

d d

d

d

d

1. What possible genotypes can results with two random frogs producing offspring from this gene pool?

Gene Frequency D = .5

Gene Frequency d = .5

Frog #1 Chance of picking D = .5

Chance Of picking d = .5

Frog #2 Chance of picking D = .5 Chance Of picking d = .5

Probability of genotypes of Possible Offspring

.5

.5 .5

.5

DD

.25

Dd

.25

Dd

.25

dd

.25

As sexual reproduction takes place over time, will the gene frequencies ever change? NO!

Slide 3But… Gene Frequencies do change…. WHY?

D

Genetic Drift due to Small Population Size

Slide 6

An Example of a Bottleneck Event

Gene Frequency

p = .7 q = .3

Gene Frequency

p = 1 q = 0

Gene Frequency

p =.83 q = .17

“earthquake hits flower island”

Slide 5

Flower Island Guano Island

Gene Frequency

P = .7 q = .3

Gene Frequency

P = .5 q = .5

Flower Island “The Sequel”

Founder Effect

Colonization

Slide 6

Slide 7

Morphological Differences in Yarrow at Different Altitudes

Slide 7

Density Dependent Selection

Slide 8

Polymorphic Expressions of the Common Garter Snake

Slide 6

The distribution of the sickle cell gene and the distribution of malaria parasite

Slide 7

Normal cells Sickle cells

Gene Pool B 100 Organisms

Phenotype Genotype

Dark

Medium

Light

#

DD

Dd

dd

10

30

60

GF of D = .25GF of d = .75

2. Which is the most “fit” phenotype?

3. What is the “relative fitness” of the light colored phenotype?

Most abundant phenotype is always set at a relative fitness of “1”

Light colored frog

4. What is the relative fitness of the medium color phenotype?

The medium colored frogs produce ½ the amount of surviving offspring ( 30/60) so its relative fitness is .5

5. What is the relative fitness of the dark phenotype?

The dark phenotype produces 1/6 the amount of surviving offspring (10/60). Its relative fitness is .17

1. Which is the most fit gene?

“d” is the most fit

Slide 9

Thicker beaks in the dry years are more common due to the abundance of hard seeds and the lack of soft seeds

Slide 9

Directional Selection

Black Bellied Seed Crackers

Smaller beaks feed on soft seeds best

Larger beaks feed on hard seeds best

Medium beaked birds have a hard time feeding on either hard or soft seed

Slide 9

Many offspring; more fitness

Fewer offspring; less fitness

Male

Male

Male

The “Unfit” Nature of Sexual Reproduction

Slide 10

Slide 10

Intrasexual selection or Intersexual selection?

Intrasexual

IntrasexualIntersexual

Intersexual

Slide 10