Chapter 23: The Evolution of Populations

Chapter 23: The Evolution of

Populations

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).

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.

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.

Population Genetics The study of genetic variation

in populations. How do populations change,

genetically, over time? Represents the reconciliation

of Mendelism and Darwinism.

Population A localized group of individuals

of the same species. Must produce viable offspring

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

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.

Microevolution Changes in the relative

frequencies of alleles in the gene pool.

Micro = small Microevolution is how we

study evolution at the genetics level

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.

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)

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

allele q = %/frequency of recessive

allele

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)

Genotypes p2 = Homozygous Dominant

frequency2pq = Heterozygous frequencyq2 = Homozygous Recessive frequency

Example Calculation Let’s look at a population

where: A = red flowers a = white flowers

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)

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

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

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.

Example What is the frequency of the

PKU allele? PKU is expressed only if the

individual is homozygous recessive (aa).

PKU Frequency PKU is found at the rate of

1/10,000 births. PKU = aa = q2

q2 = .0001 q = .01 (frequency of

recessive alleles)

Dominant Allele p + q = 1 p = 1- q p = 1- .01 p = .99

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

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%

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

Hardy-Weinberg Assumptions

1. Large Population2. Isolation3. No Net Mutations4. Random Mating5. No Natural Selection

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?

Microevolution Caused by violations of the

5 H-W assumptions.

Causes of Microevolution

1. Genetic Drift2. Gene Flow3. Mutations4. Nonrandom Mating5. Natural Selection

Genetic Drift Changes in the gene pool of a

small population by chance. Types:

1. Bottleneck Effect 2. Founder's Effect

By Chance

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

Importance Reduction of population size

may reduce gene pool for evolution to work with.

Ex: Cheetahs

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

Importance Very common in islands and

other groups that don't interbreed.

Gene Flow Movement of genes in/out of

a population. Ex:

Immigration Emigration

Result: change in gene frequency

Mutations Inherited changes in a gene.

Result May change gene frequencies

(small population). Source of new alleles for

selection. Often lost by genetic drift.

Nonrandom Mating Failure to choose mates at

random from the population.

Causes Inbreeding within the same

“neighborhood”. Assortative mating

(like with like).

Result Increases the number of

homozygous loci. Does not in itself alter the

overall gene frequencies in the population.

Natural Selection Differential success in

survival and reproduction. Result - Shifts in gene

frequencies.

Comment As the environment changes,

so does natural selection and gene frequencies.

Result If the environment is

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

Genetic Basis of Variation

1. Discrete Characters – Mendelian traits with clear phenotypes.

2. Quantitative Characters – Multigene traits with overlapping phenotypes.

Polymorphism The existence of several

contrasting forms of the species in a population.

Usually inherited as Discrete Characteristics.

Examples

Garter SnakesGaillardia

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

Quantitative Characters Allow continuous variation in

the population. Result –

Geographical Variation Clines: a change along a

geographical axis

Yarrow and Altitude

Sources of Genetic Variation

Mutations. Meiosis - recombination

though sexual reproduction. Crossing-over Random fertilization

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

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?

Relative Fitness Contribution of one genotype

to the next generation (when compared to other genotypes)

Rate of Selection Differs between dominant and

recessive alleles. Selection pressure by the

environment/nature.

Modes of Natural Selection

1. Stabilizing2. Directional3. Diversifying4. Sexual

Stabilizing Selection toward the average

and against the extremes. Ex: birth weight in humans

Directional Selection Selection toward one extreme. Ex: running speeds in race

animals Ex. Galapagos Finch beak size

and food source

Diversifying(Disruptive) Selection toward both

extremes and against the norm.

Ex: bill size in birds

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.

Sexual Mate selection May not be adaptive to the

environment, but increases reproduction success of the individual.

Result Sexual dimorphism. Secondary sexual features

for attracting mates.

Comments Females may drive sexual

selection and dimorphism since they often "choose" the mate.

Question Does evolution result in

perfect organisms? No!?

Compromises Chance occurrences

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.