CHAPTER 16 EVOLUTION OF POPULATIONS

SECTION 1 GENES AND VARIATION

KEY CONCEPT QUESTIONS What are the main sources of

inheritable variation in a population?

How is evolution defined in genetic terms?

What determines the number of phenotypes for a given trait?

1859 Darwin published his theory of evolution

1866 Mendel worked with peas to explain inheritance

These two ideas did not come together until the 1930’s

Today, genetics, molecular biology, and evolutionary theory work together to explain how inheritable variation appears and how natural selection operates on that variation

What is a species? Biological species concept

defined by Ernst Mayr population whose members can interbreed &

produce viable, fertile offspring reproductively compatible

POPULATION a collection of individuals of the same

species in a defined area GENE POOL

the combined genetic information of all the members of a particular population

common group of genescontains two or more alleles—or forms

of a certain gene—for each inheritable trait

Changes in populations Evolution of populations is really

measuring changes in allele frequency all the genes & alleles in a population =

gene pool Factors that alter allele frequencies

in a population natural selection genetic drift

founder effect bottleneck effect

gene flow

Populations evolve Natural selection acts on individuals

differential survival “survival of the fittest”

differential reproductive success who bears more offspring

Populations evolve genetic makeup of

population changes over time

favorable traits (greater fitness) become more common Bent Grass on

toxic mine site

Individuals DON’T evolve!!!

RELATIVE FREQUENCY the number of times that allele

occurs in a gene pool compared with the number of times other alleles occur

expressed in percents

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

 Relative Frequencies of Alleles

What are the main sources of genetic variation in a population?The two main sources of

genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

Mutation & Variation Mutation creates variation

new mutations are constantly appearing

Mutation changes DNA sequence changes amino acid sequence? changes protein?

change structure? change function?

changes in protein may change phenotype & therefore change fitness

Sex & Variation Sex spreads variation

one ancestor can have many descendants

sex causes recombination offspring have new combinations

of traits = new phenotypes

Sexual reproduction recombines alleles into new arrangements in every offspring

Gene Shuffling independent assortment during

meiosiscrossing-over during meiosisWhen alleles are recombined during

sexual reproduction, they can produce dramatically different phenotypes. Thus, sexual reproduction is a major source of variation within many populations.

Variation impacts natural selection Natural selection requires a source of

variation within the population there have to be differences some individuals must be more fit than

others

SINGLE-GENE TRAIT trait controlled by a single gene that

has two allelescan have two phenotypes only

widow’s peak hairline

Fre

qu

ency

of

Ph

eno

typ

e(%

)100

80

60

40

20

0 Widow’s peak No widow’s peak

Phenotype

 Distribution of Phenotypes for Single-Gene Trait in a Population

POLYGENIC TRAITSTraits controlled by two or more

geneseach gene has two or more allelesone polygenic trait can have many

possible genotypes and even more possible phenotypesheight in humans

Fre

qu

enc

y o

f P

hen

oty

pe

Phenotype (height)

Generic Bell Curve for Polygenic Trait in a population

Most people fall in the middle of the bell curve

KEY CONCEPT QUESTIONS What are the main sources of inheritable

variation in a population? Mutations and sexual reproduction

How is evolution defined in genetic terms? genetic makeup of population changes

over time favorable traits (greater fitness) become

more common What determines the number of phenotypes

for a given trait? The number of genes that control the trait

SECTION 2 EVOLUTION AS GENETIC CHANGE

KEY CONCEPT QUESTIONS How does natural selection affect

single-gene and polygenic traits?

What is genetic drift?

What 5 conditions are needed to maintain genetic equilibrium?

Natural selection affects which individuals having different phenotypes survive and reproduce and which do not

In this way, natural selection determines which alleles are passed from one generation to the next.

Any factor that causes alleles to be added to or removed from a population will change the relative frequencies of alleles.

Whenever an individual dies without reproducing, its genes are removed from the population.

But if an individual produces many offspring, the proportion of that individual’s genes in the gene pool will increase.

In genetic terms, evolution is any change in the relative frequencies of alleles in a population’s gene pool.

Thus, evolution acts on populations, not on individuals.

Take a look:

Brown is the normal color

Besides a mutation for red color, what other mutation occurred in the lizard population?

A mutation for black color How does color affect the fitness of the

lizards?Both red and brown lizards are less fit than

black lizards

What do you predict the lizard population will look like by generation 50? Explain.The lizard population will have

more black lizards, fewer brown lizards, and no red lizards by generation 50. The environment determines the favorable color.

As you learned earlier: the action of multiple alleles on traits such

as height produces a range of phenotypes that often fit a bell curve

The fitness of individuals close to one another on the curve will not be very different.

But fitness can vary a great deal from one end of such a curve to the other.

And where fitness varies, natural selection can act.

Witness to Evolution Peppered Moth

dark vs. light variants

Peppered moth

Peppered moth

Year % dark % light1848 5 951895 98 21995 19 81

Peppered moth

Why did the population change? early 1800s = pre-industrial England

low pollution lichen growing on trees = light colored bark

late 1800s = industrial England factories = soot coated trees killed lichen = dark colored bark

mid 1900s = pollution controls clean air laws return of lichen = light colored bark

industrial melanism

Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection.

DIRECTIONAL SELECTION When individuals at one end of the

curve have higher fitness than individuals in the middle or at the other end

Ex) The supply of small seeds runs low in a particular environment. Take a look at the graph below and explain what is happening.

- the birds with larger beaks are more likely to survive and reproduce because their beaks are adapted to the available food. There is a shift in the beak size of a population.

STABILIZING SELECTIONWhen individuals near the center

of the curve have higher fitness than individuals at either end of the curve

EX) Figure shows that human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average

DISRUPTIVE SELECTION when individuals at the upper and

lower ends of the curve have higher fitness than individuals near the middle

EX) average-sized seeds become less common, and larger and smaller seeds become more common. As a result, the bird population splits into two subgroups specializing in eating different-sized seeds.

Effects of Selection Driving changes in a population

GENETIC DRIFT random change in allele frequencies

that occurs in small populations In small populations, individuals that

carry a particular allele may leave more descendants than other individuals do, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a populationa

Genetic drift Effect of chance events

founder effect small group splinters

off & starts a new colony

bottleneck some factor (disaster)

reduces population to small number & then population recovers & expands again

Bottleneck effect When large population is drastically reduced

by a disaster famine, natural disaster, loss of habitat… loss of variation by chance

alleles lost from gene pool narrows the gene pool

Cheetahs All cheetahs share a small

number of alleles less than 1% diversity as if all cheetahs are

identical twins 2 bottlenecks

10,000 years ago Ice Age

last 100 years poaching & loss of

habitat

Conservation issues Bottlenecking is an

important concept in conservation biology of endangered species loss of alleles from gene

pool reduces variation reduces ability to

adapt at risk populations

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

FOUNDER EFFECT situation in which allele

frequencies change as a result of the migration of a small subgroup of a populationDarwin’s FinchesFruit flies on Hawaii

Founder effect When a new population is started by only a

few individuals some rare alleles may be at high

frequency; others may be missing skew the gene pool of

new population human populations that

started from small group of colonists

example: white people colonizing New World

HARDY-WEINBERG PRINCIPLE allele frequencies in a population will

remain constant unless one or more factors cause those frequencies to change

GENETIC EQUILLIBRIUM in which allele frequencies remain

constant If the allele frequencies do not change,

the population will not evolve

Five conditions are required to maintain

genetic equilibrium (alleles don’t

change) from generation to generation:

1. There must be random mating

2. The population must be very large

3. There can be no movement into or out of the population

4. No mutations

5. No natural selection.

How do allele frequencies change?

Human evolution today Gene flow in human

populations is increasing today transferring alleles

between populations

Are we moving towards a blended world?Are we moving towards a blended world?

KEY CONCEPT QUESTIONS How does natural selection affect

single-gene and polygenic traits?affect the distributions of phenotypes

in any of three ways: directional selection, stabilizing selection, or disruptive selection.

What is genetic drift?random change in allele frequencies

that occurs in small populations

Focus Questions What 5 conditions are needed to

maintain genetic equilibrium? Random mating Large population No movement into or out No mutations No natural selection

SECTION 3 THE PROCESS OF SPECIATION

KEY CONCEPT QUESTIONS What factors are involved in the

formation of a new species?

Describe the process if speciation in the Galapagos Finches.

Speciation New species are created by a series of

evolutionary processes populations become isolated

reproductively isolated geographically isolated

isolated populations evolve independently

Isolation allopatric

physical separation sympatric

still live in same area

Allopatric speciation Allopatric = “other country”

geographic separation migration physical barrier

Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white–tailed antelope squirrel

Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white–tailed antelope squirrel

GEOGRAPHIC ISOLATION two populations are separated by

geographic barriers such as rivers, mountains, or bodies of water Squirrels at the Grand Canyon Darwin’s Finches

Sympatric speciation Sympatric = “same country”

some type of isolation even though populations live in same area

what causes this isolation? behavioral differences

non-random mating physiological differences

chromosomal changes polyploidy

mostly in plants: oats, cotton, potatoes, tobacco, wheat

TEMPORAL ISOLATION two or more species reproduce at

different times Orchids and pollination

REPRODUCTIVE ISOLATION when members of two populations

cannot interbreed and produce fertile offspring

BEHAVIORAL ISOLATION occurs when two populations are capable

of interbreeding but have differences in courtship rituals or other types of behavior

Mating songs of meadowlarks

Darwin studied birds on the Galapagos Islands.

He thought they were blackbirds, warblers, and other kinds of birds!

The species he examined differed greatly in the sizes and shapes of their beaks and in their feeding habits, as shown on p. 406.

Some species fed on small seeds, while others ate large seeds with thick shells. One species used cactus spines to pry insects from dead wood. One species, not shown here, even pecked at the tails of large sea birds and drank their blood!

Once Darwin discovered that these birds were all finches, he hypothesized that they had descended from a common ancestor. Over time, he proposed, natural selection shaped the beaks of different bird populations as they adapted to eat different foods

Darwin’s hypothesis relied on two testable assumptions.

First, in order for beak size and shape to evolve, there must be enough inheritable variation in those traits to provide raw material for natural selection.

Second, differences in beak size and shape must produce differences in fitness that cause natural selection to occur

Two scientists, Peter and Rosemary Grant, tested Darwin’s hypothesisconcluded there is great variation of

inheritable traits among Galapagos Finches

individual birds with different sized beaks had different chances of survival

When food for the finches was scarce, individuals with the largest beaks were more likely to survive

Beak size also plays a role in mating behavior, because big-beaked birds tend to mate with other big-beaked birds

they found natural selection takes place frequently and sometimes very rapidly

Speciation of Darwin’s Finches

1. Founders arrive

2. Separation of populations

3. Changes in the gene pool

4. Reproductive isolation

5. Ecological competition

6. Continued evolution

KEY CONCEPT QUESTIONS What factors are involved in the

formation of a new species? Allopatric isolation

physical separation Sympatric isolation

still live in same area

KEY CONCEPT QUESTIONS Describe the process of speciation

in the Galapagos Finches.• Founders arrive• Separation of populations• Changes in the gene pool• Reproductive isolation• Ecological competition• Continued evolution