7C, 7D, 7E

Natural Selection &
Unit 9
Natural Selection
• Gradual process by which biological
traits become either more or less
common in a population
Nature basically “selects” certain
individuals to be successful and
reproduce and over time the
population changes due to this
What has happened to the mice
population below?
The population has changed from 50% brown &
50% white to 80% white & 20% brown. WHY?
Adaptations Cause Fitness
• Adaptation – any inherited characteristic that
increases an organism’s chance of survival.
(Remember the source of any new trait is a
• Adaptations make an organism better suited
to the environment. So in the struggle for
survival, organisms with adaptations suited to
the environment will be more likely to survive
and reproduce.
• Fit offspring then survive and
reproduce and even more offspring
are “born” that have the adaptation.
• Over generations, the DNA in the
whole population changes, so that all
the population has the adaptation
that makes the population fit for its
• The ability of an organism to survive
and reproduce
If fitness is the
ability to survive
& reproduce
Which lion
is more fit?
This lion will hunt for more food, have
the energy to reproduce, and pass on
his genes to his offspring.
Traits that help an organism survive
Survive a predator
Survive disease
Compete for food
Compete for territory
Traits that help individuals reproduce
• Attracting a mate
• Compete for nesting sites
• Successfully raising young
“Survival of the Fittest”
• Faster, stronger, good hunter, better
• Best organism that survives and
Mesquite have the longest tap root of any desert species
• Any characteristic that increases fitness
Types of Selection
Types of Selection
Disruptive Selection
• Describes changes in population genetics in
which extreme values for a trait are favored
over medium values.
• The variance of the trait increases and the
population is divided into 2 distinct groups.
Types of Selection
Stabilizing Selection
• Genetic decreases as the population
stabilizes on a particular trait or
value. It varies medium traits.
“Average Joe wins”
• Opposite of disruptive selection.
Types of Selection
Directional Selection
• A single phenotype is favored, causing
the allele frequency to continuously
shift in 1 direction.
• Occurs under environmental changes
when populations migrate to new areas
with different environmental pressures.
Natural Selection Summary Points
• Inherited variation – there are genetic
variations (differences) between individuals of
a population
• The more genetic diversity, the more
variations of alleles that are suited for the
environment. The more offspring will survive
and reproduce.
• The population will then continue.
Natural Selection Summary Points
• Overproduction of offspring – there are more
offspring are produced than can survive
• Struggle for existence – there is competition
for survival among the members of the
Witness to Evolution
• Peppered Moth
– 2 types: dark vs. light
Peppered moth
Peppered moth
• Why did the population change?
– early 1800s = pre-industrial England
• low pollution
• lichen on trees = light colored bark
– late 1800s = industrial
• factories = soot coated trees
• killed lichen = dark colored bark
– mid 1900s = pollution controls
• clean air laws
• return of lichen = light colored
Peppered moth: Evolution in action
% dark
% light
clean air, light-colored bark
pollution, dark-colored bark
Clean Air Act, light-colored bark
industrial melanism
Natural Selection &
Unit 9
Genetic or Inherited Variation
• variation in alleles of genes that
occurs both within and among
What determines these different traits
or variations?
• Mutations in our DNA
• crossing over & independent
assortment in meiosis.
Let’s remember…
• Genotype is the individuals set of alleles
for a trait…
BB, Bb, or bb
• Phenotype is the appearance or
expression of the trait…
Homozygous black, heterozygous black, or
homozygous brown
Bb heterozygous black
BB homozygous black
bb homozygous brown
• 64% of the total mouse population
has black fur
• The black allele (B) occurs 40% of the
time while the brown allele (b)
occurs 60% of the time.
• The brown allele occurs more
frequently than the black. What
change in the habitat could change
the frequency of alleles?
Elements of Natural Selection
1. Inherited Variation
2. Producing More offspring than can
3. Limited (finite) supply of
Environmental Resources
1. Inherited Variation
• In an environment that favors black
fur, mice with brown fur will be
eaten, and the frequency of the
allele for brown fur (b) will decrease.
Black mice will have a better chance
of surviving and passing on their
alleles to their offspring.
The frequency for the while allele in
the mouse population below will
2. Producing More offspring than can
• Most populations produce more
offspring than can survive. Many
offspring are lost due to predation,
starvation, etc… The offspring that do
survive to adulthood will have
phenotypes that give them an
advantage within that environment over
those that didn’t survive & reproduce.
3. Limited (finite) Supply of Resources
• Individuals compete for food, water,
space, & shelter.
• When resources become scarce, such
as during a drought, competition
increases. Populations decline, and the
individual with advantageous traits will
survive and reproduce.
The ground finches
with larger beaks
can eat a wider
variety of seeds &
If a drought occurs, the finches with
smaller beaks will run out of food &
the population of finches with larger
beaks will increase.
Natural Selection &
Unit 9
• the evolutionary process by which
new biological species arise
• Every species has different DNA.
• Species can breed and produce
fertile offspring.
• How has this population changed
over time?
Body has become red with spots
• What has caused this change?
Mutations with redder body & blacker spots
have been beneficial & allowed these insects to
survive and reproduce. The mutations in the
nucleotide sequences have now changed the
DNA and a new species has evolved.
How do new species evolve?
Due to natural selection, individuals with
adaptations that give them an advantage
over other individuals in the same
environment are more likely to produce
and pass the adaptations to their offspring.
If the adaptation (mutation) continues to
be beneficial, over time, the trait is passed
on to generations, and a larger portion of
the population with have that adaptation.
Diversity in a Species
Let’s remember, individuals within a
species are genetically different.
If the snow & ice melt, will this polar
bear be able to grow thin brown fur?
But if a few mutated polar bears are
born with thin, brown fur…
Diversity among Species
• The process of natural selection and other
mechanisms of evolution have lead to the
millions of species of living organisms living on
Earth today and millions of different species
that lived in the past.
How does speciation occur?
Originally, there was 1 species of tortoise
on 1 of the Islands.
How does speciation occur?
• An event occurred, like a storm, that
stranded some tortoises on another island.
These individuals started a new population in
a different environment.
• Over time, mutations occurred, & the
genetic variation increased so the 2
populations couldn’t mate & produce fertile
offspring and were now 2 separate species.
How does speciation occur?
Now, each island has at least 1 different
species of tortoise.
Reproductive Isolation
• occurs when two populations cannot
interbreed and a new species is
formed (speciation occurs).
• A single species has evolved through natural
selection into new and different species
Things that CAN (but don’t always) cause
reproductive isolation:
1. Habitat isolation
2. Behavioral isolation
3. Geographic isolation
4. Temporal isolation
Habitat Isolation
• 2 populations breed in different
areas of a habitat
• Ex. snakes that breed in the water vs.
the land
Habitat Isolation
Water-dwelling Thamnophis
Terrestrial Thamnophis
Behavioral Isolation
• 2 populations can breed, but
behavior causes them not to.
• Ex. Eastern & Western meadowlarks
using 2 different songs to attract
Behavioral Isolation
Courtship ritual of different populations of
Blue-Footed Boobies
Geographic Isolation
• 2 populations are separated by a
geographic barrier (river, mountain,
• Ex. Colorado River separated
Geographic Isolation
Geographic Isolation
Geographic Isolation
Temporal Isolation
• 2 populations breed at different
• Ex. orchids that pollinates once a
Wood & Leopard Frog