HONORS LIVING ENVIRONMENT
TOPIC 22: EVOLUTION AND BIODIVERSITY
MS. ETRI
PART 2: DARWIN’S THEORY OF NATURAL SELECTION
ADAPTATION: heritable characteristics that enhance organisms’ ability to survive and reproduce in specific
environments.
EX. Desert foxes have large ears, which radiate heat; Arctic foxes have small ears, which conserve
body heat.
NATURAL SELECTION explains how adaptations arise.

The process whereby organisms better adapted to their
environment tend to survive and reproduce more
offspring.

It is the result of environmental factors that vary from
place to place and over time.

A trait that is favorable in one situation may be useless—
or even detrimental—in different circumstances.
o
TWO IMPORTANT IDEAS:

1. Natural selection is more an editing
process than a creative mechanism.

A pesticide does not create new
alleles that allow insects to
survive. Rather, the presence of
the pesticide leads to natural
selection for insects in the
population that have those alleles.

2. Natural selection is contingent on time
and place.

It favors those heritable traits in a
varying population that fit the
current, local environment. If the
environment changes, different traits may be favored.
COMPONENTS OF DARWIN’S THEORY OF NATURAL SELECTION
1. NATURAL VARIATION EXISTS IN THE ORIGINAL POPULATION

Individuals in a population vary in their traits, many of which are heritable.
2. OVERPRODUCTION & COMPETITION
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 A population can produce far more offspring than can survive. With more individuals
than the environment can support, competition is inevitable.

Organisms struggle for food, water, shelter, and space.
3. SURVIVAL OF THE FITTEST—“MAY THE BEST MAN WIN”

Individuals with inherited traits that are better suited to the local environment are
more likely to survive and reproduce (TRANSMISSION OF FAVORABLE TRAITS)
than individuals less well-suited.
o
Favorable traits are therefore passed onto their offspring.
4. ACCUMULATION OF FAVORABLE TRAITS AND DESCENT WITH MODIFICATION

The accumulation of favorable traits will lead to a species that is highly adapted to its
environment.

Evolution occurs as the unequal reproductive success of individuals ultimately
leading to adaptations to their environment.
o
Over time, natural selection can increase the match between organisms and
their environment.
NOTE: If an environment changes, or if individuals move to a new environment, natural selection may result in
adaptation to these new conditions, sometimes giving rise to new species in the process.
HOW DOES THE
GIRAFFE
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EXAMPLE ILLUSTRATE DARWIN’S THEORY OF NATURAL SELECTION?
1. NATURAL VARIATION EXISTS IN THE ORIGINAL POPULATION

Early giraffes most likely had necks of varying lengths.
2. OVERPRODUCTION AND COMPETITION

If the shorter necked giraffes were not able to reach the food on the trees, they would
most likely die out; therefore, the longer necked giraffes were stronger and outcompeted the shorter necked giraffes for resources.
3. SURVIVAL OF THE FITTEST

Which trait was favored in this example? Longer necks.

This means that the longer-necked giraffes were more successful and survived to
reproduce.

Therefore, the long-necked trait was passed down from generation to generation
(transmission of favorable traits).
4. ACCUMULATION OF FAVORABLE TRAIT

Because the long-necked giraffes continued to pass down the inheritable trait to future
generations, the favorable trait accumulated within the population of giraffes.

This led to a species that is highly adapted to its environment.
FINCHES AND NATURAL SELECTION

Beak sizes in a population of ground finch species
have changed in response to altered environmental
conditions in the Galapagos Islands.

Ground finch species eat mostly small seeds.

In dry years, when all the small seeds are in short
supply, birds must eat more large seeds.

Birds with larger, stronger beaks have a feeding
advantage and greater reproductive success.
o
Scientists (the Grants) measured an
increase in the average beak depth for the
population, following the dry years.

During wet years, smaller beaks are more efficient
for eating the now abundant small seeds; scientists found a decrease in the average beak depth.

Depending on the environment and the food source in various regions of the Galapagos, only finches
with beaks suited for a particular environment survived to reproduce in that area.
WEAKNESSES OF DARWIN’S THEORY
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 Darwin did not know the source of variation that was so central to his theory. He did not know how a
species develops different traits.

He could not explain how inheritable traits were passed from one generation to the next.
o
We now know that our traits are determined by the genes you have. Genes are passed from
one generation to the next.
IMPORTANT ASPECTS OF EVOLUTION

Although natural selection occurs through interactions between individual organisms and their
environment, individuals DO NOT evolve.
o
Rather, it is the population—the group of organisms—that evolves over time as adaptive traits
become more common in the group and other traits change or disappear.

Natural selection can amplify or diminish only heritable traits.
o
An organism may become modified through its own interactions with the environment during its
lifetime, and those acquired characteristics may help the organism survive.
o
But unless coded for in the genes of an organism’s gametes, such characteristics cannot be
passed on to offspring.

RECALL: Evolution is not goal-oriented.
ADDITIONAL EXAMPLES OF NATURAL SELECTION AND RECENT EVOLUTION
1. BACTERIAL RESISTANCE TO ANTIBIOTICS

ANTIBIOTICS: drugs that kill infectious microorganisms.
o
Penicillin, the first antibiotic to be developed, has been widely prescribed since the
1940s. A revolution in human health followed its introduction, rendering many previously
fatal diseases easily curable.

In the same way that pesticides select for resistant insects, antibiotics select for resistant
bacteria.
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o A gene that codes for an enzyme that breaks down an antibiotic or a mutation that alters
the site where an antibiotic binds, can make a bacterium and its offspring resistant to
that antibiotic.

HOW DO WE CONTRIBUTE TO ANTIBIOTIC RESISTANCE?
o
Doctors may overprescribe antibiotics—for example, to patients with viral infections,
which do not respond to antibiotic treatment.
o
Patients may misuse prescribed antibiotics by prematurely stopping the medication
because they fell better. THIS IS NOT A GOOD THING!

This allows mutant bacteria that may be killed more slowly by the drug to survive
and multiply.

Subsequent mutations in such bacteria may lead to full-blown antibiotic
resistance.
2. INSECT RESISTANCE TO INSECTICIDES

Pesticides control insects and prevent from eating crops or transmitting diseases.

Whenever a new type of pesticide is used to control pests, the story is similar:
o
A relatively small amount of poison initially kills most of the insects, but subsequent
applications are less and less effective.
o
The few survivors of the first pesticide wave are individuals that are genetically resistant,
carrying an allele (alternative form of a gene) that somehow enables them to survive the
chemical attack.
o
So the poison kills most members of the population, leaving the resistant survivors to
reproduce and pass on the alleles for pesticide resistance to their offspring.
o

The proportion of pesticide-resistant individuals thus increases in each generation.
EXAMPLE: DDT AND MOSQUITOES
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o The World Health Organization’s campaign against malaria is a real-world example of
the evolution of pesticide resistance.
o
Some mosquitoes in the populations that were sprayed with DDT carried an allele that
codes for an enzyme that detoxifies the pesticide.

When the presence of DDT changed the environment, the individuals carrying
that allele survived to leave offspring, while nonresistant individuals did not.

Thus, the process of natural selection defeated the efforts of WHO to control the
spread of malaria by using DDT to kill mosquitoes.
3. THE ENGLISH PEPPERED MOTH

The peppered moth can benefit from blending into its environment; its coloration should match
with the trees on which it perches.

What would happen if the trees began changing, and
the peppered moths were no longer able to blend in?
o
The individuals could move (to try and find
trees that match its color).
o
The species could have altered behavior, or
even change over time to adapt to the new
surroundings.

Species has two adult forms:
o
1. typica—a pale, lighter color that is
peppered with black speckles.
o

2. carbonaria—a much darker color that is peppered with light speckles.
COLLECTORS IN ENGLAND NOTICED
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o Most peppered moths collected in early 1800s were light gray peppered with bits of
black; many years later (1845), most of the moths collected were almost black.
o
WHAT CAUSED THIS CHANGE?

During the 1800’s, Europe and America experienced the Industrial Revolution
which led to the building of factories that heavily used coal as their new fuel
source.

Coal burning releases large amounts of smoke and smog into the surrounding
environment.

This left a layer of black soot on the once lighter-colored trees.

The pollution also killed the light speckled colored lichens that grew on
the tree trunks.

The tree bark was now exposed and dark without the lichens. How did
this affect the peppered moth?
RESULTS:

As the trees darkened with soot, the light-colored moths were
easier to see.
o
They were eaten by birds more and more, while the dark
colored moths blended in better on the darker trees.
o
This made the dark colored moths to have a higher
survival rate.

They lived longer and passed their dark colored
genes onto their offspring.

Natural selection favored the dark colored moths.

Recently, as a result of environmental pollution laws, the moth
population is slowly changing back to light-colored moths.
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