Chapter 17: Evolution

of Populations

1. When Darwin developed his theory of evolution, he did not understand:

• how heredity worked.

This left him unable to explain two things:

a. source of variation

b. how inheritable traits pass from one generation to the next

Evolution of Populations

In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began to combine the ideas of many branches of biology to develop a modern theory of evolution. When studying evolution today, biologists often focus on a particular population. This evolution of populations is called microevolution.

Evolution of Populations

Vocabulary:

Population: group of individuals of the same species living in the same area that breed with each other.

gene poolcombined genetic

info. for all members

of a population

Allele: one form of a gene

2. relative frequency of an allele: # times an allele occurs in the gene pool compared to other alleles (percent)

Example

Relative Frequency:

70% Allele B

30% Allele b

3. Sources of Variation:

a. mutations: any change in DNA sequence

♦ Can occur because of:

♦mistakes in replication

♦ environmental chemicals

♦ May or may not affect an organism’s phenotype

3. Sources of Variation

b. Gene Shuffling: recombination of genes that occurs during production of gametes♦ Cause most inheritable differences between relatives♦ Occurs during meiosis♦ As a result, sexual reproduction is a major source of

variation in organisms.♦ Despite gene shuffling, the frequency of alleles does

not change in a population. Explain why this is true.Similar to a deck of cards – no matter how many

times you shuffle, same cards (alleles) are always there.

4. Gene Traits:

A) Single gene trait: controlled by single gene with two alleles

♦ Examples: widow’s peak, hitchhiker’s thumb, tongue rolling

(4. Gene Traits:)

B) Polygenic trait: controlled by 2 or more genes, each with 2 or more alleles

♦ Examples: height, hair color, skin color, eye color

Most human traits are polygenic.

Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain.

type: single gene

why? Only two phenotypes possible

Example: tongue roller or non-tongue roller

type: polygenic

why? Multiple (many) phenotypes possible

Example: height range 4feet to 9 feet all

5. Natural selection acts on phenotypes, not genotypes.

Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better brown or white?

Brown, more hidden.

5. If brown is dominant can a predator tell the difference between:

Mouse with highest fitness will have the most alleles passed on to the next generation.

White mouse will have low fitness

BB Bb

?

5. Which mouse will have the lowest fitness?

White, bb (recessive)

Will the fitness of BB and Bb differ? Why?

No, Both BB and Bb have the same fitness advantage of being brown

BB Bb

?

Three ways in which natural selection affects polygenic traits.

Natural Selection

•Directional Selection

•Stabilizing Selection

•Disruptive Selection

Directional Selection: individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait

♦ Individuals with highest fitness: those at one end of the curve

♦ Example: Galapagos finches – beak size

Food becomes scarce.

KeyLow mortality, high fitness

High mortality, low fitness

Directional Selection

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness

Directional Selection

Stabilizing Selection: individuals at the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curve

Key

Per

cen

tag

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f P

op

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Birth Weight

Selection against both

extremes keep curve narrow and

in same place.

Low mortality, high fitnessHigh mortality, low fitness

Stabilizing Selection

Individuals Individuals with highest with highest fitness: fitness: near near the center of the center of the curve the curve (average (average phenotype)phenotype)

Example: Example: human birth human birth weightweight

Disruptive Selection: individuals at both ends of the curve survive better than the middle of the curve.

♦ Individuals with highest fitness: both ends of curve

♦ Example: birds where seeds are either large or small

Disruptive Selection

Largest and smallest seeds become more common.

Nu

mb

er o

f B

ird

sin

Po

pu

lati

on

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Nu

mb

er o

f B

ird

sin

Po

pu

lati

onKey

Low mortality, high fitness

High mortality, low fitness

Key

Per

cen

tag

e o

f P

op

ula

tio

n

Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Stabilizing Selection

Disruptive Selection

Disruptive Selection

Largest and smallest seeds become more common.

Nu

mb

er o

f B

ird

sin

Po

pu

lati

on

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Nu

mb

er o

f B

ird

sin

Po

pu

lati

onKey

Low mortality, high fitness

High mortality, low fitness

However:No examples ever observed in animals

A couple examples that may demonstrate speciation exist in plants and some insects.

Genetic Drift

random change in allele frequency that occurs in small populations

Two phenomena that result in small populations and cause genetic drift

1. Founder Effect2. Bottleneck Effect

Genetic Drift

The results of genetic crosses can usually be predicted using the laws of probability.

In small populations, however, these predictions are not always accurate.

Genetic Drift

Founder effect Allele frequencies change due to

migration of a small subgroup of a population

Founder Effect

Two groups from a large, diverse population could produce new populations that differ from the original group.

Major change in allele frequencies when population decreases dramatically due to catastrophe

♦ Example: northern elephant seals

decreased to 20 individuals in 1800’s, now 30,000

no genetic variation in 24 genes

2. Bottleneck effect

Bottleneck Effect: Northern Elephant Seal Population♦ Hunted to near extintion♦ Population decreased to 20

individuals in 1800’s, those 20 repopulated so today’s population is ~30,000

♦ No genetic variation in

24 genes

Bottleneck Effect

Original population

Bottleneck Effect

Original population

Catastrophe

Bottleneck Effect

Original population

Catastrophe

Surviving population

Evolution Versus Genetic Equilibrium

What conditions are required to maintain genetic equilibrium?

According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium:

(1) The population must be very large(2) there can be no mutations(3) there must be random mating(4) there can be no movement into or out of the

population(5) no natural selection can occur

Genetic equilibrium = no evolutionA population is in genetic equilibrium if allele

frequencies in the population remain the same. If allele frequencies don’t change,

the population will not evolve.

The Hardy-Weinberg principle describes theconditions under which evolution does not occur.

The Hardy-Weinberg principle states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.

Hardy-Weinberg principle 1. Large Population

Genetic drift can cause changes in allele frequencies in small populations. Genetic drift has less effect on large populations.  Large population size helps maintain genetic equilibrium

2. No Mutations

If mutations occur, new alleles may be introduced into the gene pool, and allele frequencies will change.

Hardy-Weinberg principle 3. Random Mating

All members of the population must have an equal opportunity to produce offspring. Individuals must mate with other members of the population at random.  In natural populations, however, mating is not random.

•Female peacocks, for example, choose mates on the basis of physical characteristics such as brightly patterned tail feathers. •Such non-random mating means that alleles for those traits are under selection pressure.

Hardy-Weinberg principle 4. No Movement Into or Out of the Population

Individuals who join a population may introduce new alleles into the gene pool. (Immigration) Individuals who leave may remove alleles from the gene pool. (emigration) Thus, for no alleles to flow into or out of the gene pool, there must be no movement of individuals into or out of a population.

Hardy-Weinberg principle 5. No Natural Selection

All genotypes in the population must have equal probabilities of surviving and reproducing. No phenotype can have a selective advantage over another.

Sexual Reproduction and Allele Frequency

Meiosis and fertilization do not change the relative frequency of alleles in a population.

The shuffling of genes during sexual reproduction produces many different gene combinations but does not alter the relative frequencies of alleles in a population.

The Process of Speciation

The formation of new biological species, usually by the division of a single species into two or more genetically distinct one.

Three Isolating Mechanisms: Isolate species forming subspecies and perhaps causing speciation.

1. Geographic Isolation2. Behavioral Isolation3. Temporal Isolation

Example: Eastern and Western Meadowlark

Male birds sing a matting song that females like, East and West have different songs. Females only respond to their subspecies song.

1. Geographic Isolation

Two populations separated by a geographic barrier; river, lake, canyon, mountain range.

Example: 10,000 years ago the Colorado River separated two squirrel populations.

Kaibab Squirrel Abert Squirrel

This resulted in a subspecies, but did not result in speciation because the two can still mate if brought together.

Kaibab Squirrel Abert Squirrel

Example: Eastern and Western Meadowlark Eastern and Western Meadowlark

populations overlap in the middle of the US

2. Behavioral IsolationTwo populations are capable of

interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ.

3. Temporal IsolationPopulations reproduce at different times

January

1 2 3 4 5 6

7 8 9 10 11 12 13

Example: Northern Leopard Frog & North American Bullfrog Mates in: Mates in: April July

Conclusion:Geographic, Behavioral andTemporal Isolation are allbelieved to lead to

speciation.