Transcript
Page 1: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Chapter 23: The Evolution of

Populations

Page 2: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Essential Knowledge

1.a.1 – Natural selection is a major mechanism of evolution (23.2).

1.a.2 – Natural selection acts on phenotypic variations in populations (23.1 & 23.4).

1.a.3 – Evolutionary change is also driven by random processes (23.3).

2.c.1 – Changes in genotype can result in changes in phenotype (23.4).

4.c.3 – The level of variation in a population affects population dynamics (23.1 – 23.3).

4.c.4 – The diversity of species within an ecosystem may influence the stability of the ecosystem (23.2).

Page 3: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Question?

Is the unit of evolution the individual or the population?

Answer – while evolution affects individuals, it can only be tracked through time by looking at populations.

Page 4: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

So what do we study?

We need to study populations, not individuals.

We need a method to track the changes in populations over time.

This is the area of Biology called

population genetics.

Page 5: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Population Genetics

The study of genetic variation in populations. How do populations change,

genetically, over time? Represents the reconciliation

of Mendelism and Darwinism.

Page 6: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Population A localized group of individuals

of the same species. Must produce viable offspring

Page 7: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Species

A group of similar organisms. A group of populations that

could interbreed (successfully) Populations are animals of the

same species that are isolated due to geography

Page 8: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Gene Pool

The total aggregate of genes in a population. All alleles at all gene loci in all

individuals If evolution is occurring, then

changes must occur in the gene pool of the population over time.

Page 9: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Microevolution

Changes in the relative frequencies of alleles in the gene pool.

Micro = small Microevolution is how we

study evolution at the genetics level

Page 10: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Hardy-Weinberg Theorem

Developed in 1908. Use as a benchmark to study

evolutionary change in a population

Mathematical model of gene pool changes over time.

Page 11: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

H-W Theorem

States: The frequencies of alleles and

genotypes in a population’s gene pool remain constant (in a population that is NOT evolving)

Page 12: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Basic Equation

p + q = 1 p = %/frequency of dominant

allele q = %/frequency of recessive

allele

Page 13: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Expanded Equation

p + q = 1 (p + q)2 = (1)2

p2 + 2pq + q2 = 1 We expand the equation to “fit”

all three types of genotypes (Ex: AA, Aa, aa)

Page 14: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Genotypes

p2 = Homozygous Dominant frequency2pq = Heterozygous frequencyq2 = Homozygous Recessive frequency

Page 15: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Example Calculation

Let’s look at a population where: A = red flowers a = white flowers

Page 16: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural
Page 17: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Starting Population

N = 500 Red = 480 (320 AA+ 160 Aa) White = 20 Total Genes/Alleles

= 2* x 500 = 1000*2 alleles per genotype

(hence the “2” in the equation)

Page 18: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Dominant Allele

A = (320 x 2) + (160 x 1) = 800

= 800/1000 = 0.8 = 80%

320 = AA pop # (2 = # of dominant alleles in that AA genotype);

160 = Aa pop # (1 = # of dominant alleles in Aa genotype);

1000 = total genes

2 = # of times the dom allele is present in homozy dom genotype

1 = # of times the dom allele is present in heterozy genotype

Page 19: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Recessive Allele

a = (160 x 1) + (20 x 2) = 200

= 200/1000 = .20 = 20%

20 = aa pop # (2 = # of recessive alleles in that aa/white genotype);

160 = Aa pop # (1 = # of recessive alleles in Aa genotype);

1000 = total genes

1 = # of times the rec allele is present in heterozy genotype

2 = # of times the rec allele is present in homozy rec genotype

Page 20: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Importance of Hardy-Weinberg

Yardstick to measure rates of evolution.

Predicts that gene frequencies should NOT change over time as long as the H-W assumptions hold.

Way to calculate gene frequencies through time.

Page 21: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Example

What is the frequency of the PKU allele?

PKU is expressed only if the individual is homozygous recessive (aa).

Page 22: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

PKU Frequency

PKU is found at the rate of 1/10,000 births.

PKU = aa = q2

q2 = .0001

q = .01 (frequency of recessive alleles)

Page 23: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Dominant Allele

p + q = 1

p = 1- q

p = 1- .01

p = .99

Page 24: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Expanded Equation

p2 + 2pq + q2 = 1

(.99)2 + 2(.99x.01) + (.01)2 = 1

.9801 + .0198 + .0001 = 1

Freq of Homozy Dom

genotype

Freq of Heterozy genotype

Freq of Homozy Rec

genotype

Page 25: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Final Results

All we did is convert the frequencies (decimals) to % (by multiplying frequencies by 100%)

Normals (AA) = 98.01% Carriers (Aa) = 1.98% PKU (aa) = .01%

Page 26: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

AP Problems Using Hardy-Weinberg

Solve for q2 (% of total) Solve for q (equation) Solve for p (1- q) H-W is always on the national

AP Bio exam

Page 27: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Hardy-Weinberg Assumptions

1. Large Population

2. Isolation

3. No Net Mutations

4. Random Mating

5. No Natural Selection

Page 28: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

If H-W assumptions hold true:

The gene frequencies will not change over time.

Evolution will not occur. How likely will natural

populations hold to the H-W assumptions?

Page 29: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Microevolution

Caused by violations of the 5 H-W assumptions.

Page 30: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Causes of Microevolution

1. Genetic Drift

2. Gene Flow

3. Mutations

4. Nonrandom Mating

5. Natural Selection

Page 31: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Genetic Drift

Changes in the gene pool of a small population by chance.

Types: 1. Bottleneck Effect 2. Founder's Effect

Page 32: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

By Chance

Page 33: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Bottleneck Effect

Loss of most of the population by disasters.

Surviving population may have a different gene pool than the original population.

Results: Some alleles lost, others are over-represented, genetic variety is decreased

Page 34: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural
Page 35: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Importance

Reduction of population size may reduce gene pool for evolution to work with.

Ex: Cheetahs

Page 36: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Founder's Effect

Genetic drift in a new colony that separates from a parent population.

Ex: Old-Order Amish Results: Genetic variety

reduced, some alleles increase while other lost

Page 37: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Importance

Very common in islands and other groups that don't interbreed.

Page 38: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Gene Flow

Movement of genes in/out of a population.

Ex: Immigration Emigration

Result: change in gene frequency

Page 39: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Mutations

Inherited changes in a gene.

Page 40: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Result

May change gene frequencies (small population).

Source of new alleles for selection.

Often lost by genetic drift.

Page 41: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Nonrandom Mating

Failure to choose mates at random from the population.

Page 42: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Causes

Inbreeding within the same “neighborhood”.

Assortative mating (like with like).

Page 43: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Result

Increases the number of homozygous loci.

Does not in itself alter the overall gene frequencies in the population.

Page 44: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Natural Selection

Differential success in survival and reproduction.

Result - Shifts in gene frequencies.

Page 45: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Comment As the environment changes,

so does natural selection and gene frequencies.

Page 46: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Result

If the environment is "patchy", the population may have many different local populations.

Page 47: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Genetic Basis of Variation

1. Discrete Characters – Mendelian traits with clear phenotypes.

2. Quantitative Characters – Multigene traits with overlapping phenotypes.

Page 48: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Polymorphism

The existence of several contrasting forms of the species in a population.

Usually inherited as Discrete Characteristics.

Page 49: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Examples

Garter SnakesGaillardia

Page 50: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Human Example

ABO Blood Groups Morphs = A, B, AB, O

Page 51: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Quantitative Characters

Allow continuous variation in the population.

Result – Geographical Variation Clines: a change along a

geographical axis

Page 52: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Yarrow and Altitude

Page 53: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Sources of Genetic Variation

Mutations. Meiosis - recombination

though sexual reproduction. Crossing-over Random fertilization

Page 54: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Comment

Population geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time.

Ex – Sickle Cell and Malaria, Tay-Sachs and Tuberculosis

Page 55: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural
Page 56: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Fitness - Darwinian

The relative contribution an individual makes to the gene pool of the next generation. How likely is it that an organism

will survive and reproduce in a given environment?

Page 57: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Relative Fitness

Contribution of one genotype to the next generation (when compared to other genotypes)

Page 58: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Rate of Selection

Differs between dominant and recessive alleles.

Selection pressure by the environment/nature.

Page 59: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Modes of Natural Selection

1. Stabilizing

2. Directional

3. Diversifying

4. Sexual

Page 60: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Stabilizing

Selection toward the average and against the extremes.

Ex: birth weight in humans

Page 61: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Directional Selection

Selection toward one extreme. Ex: running speeds in race

animals Ex. Galapagos Finch beak size

and food source

Page 62: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural
Page 63: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Diversifying(Disruptive)

Selection toward both extremes and against the norm.

Ex: bill size in birds

Page 64: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Comment

Diversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed.

Page 65: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural
Page 66: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Sexual Mate selection

May not be adaptive to the environment, but increases reproduction success of the individual.

Page 67: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Result Sexual dimorphism. Secondary sexual features

for attracting mates.

Page 68: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Comments

Females may drive sexual selection and dimorphism since they often "choose" the mate.

Page 69: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Question

Does evolution result in perfect organisms?

No!? Compromises Chance occurrences

Page 70: Chapter 23: The Evolution of Populations. Essential Knowledge u 1.a.1 – Natural selection is a major mechanism of evolution (23.2). u 1.a.2 – Natural

Summary

Recognize the modern synthesis Theory of Evolution. Identify and use the Hardy-Weinberg Theorem for

population genetics. Identify the Hardy-Weinberg assumptions and how

they affect evolution of populations. Recognize causes and examples of microevolution. Identify modes of natural selection. Recognize why evolution does not produce "perfect"

organisms.


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