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Natural Selection &
I. Natural Selection &
Charles Darwin
 Due to earth’s long history; life is
thought to have evolved over
 Charles Darwin (1819-1882) an
English scientist considered the
founder of the evolutionary
 Darwin studied many forms of life
through the voyage of the Beagle
 Galapagos Islands was where
Darwin studied a majority of the
 Darwin's many observations led him to
the idea that species slowly change over
time (Gradualism)
 Darwin's comparison of the animals of
South America and the Galapagos
Islands caused him to conclude that
adaptation to the environment can
cause diversification, including origin of
new species
The Galapagos Islands:
 Volcanic islands off the South
American coast
 Island species varied from the
mainland species, and from island-toisland
 Each island had either long or short
necked tortoises depending on the
island's vegetation
The Galapagos Islands:
 Finches on the Galapagos Islands
resembled a mainland finch, but there
were more types
 Bill shapes are adaptations to
different means of gathering food.
 Galapagos finch species varied by
nesting site, beak size, and eating habits
Darwin's Theory of
 An adaptation is a trait that helps an
organism be more suited to its
 Darwin decided adaptations develop
over time
 Natural selection was proposed by
both Alfred Russell Wallace and
Darwin as a driving mechanism of
Natural Selection:
Natural Selection – a mechanism
for change in a population where
favorable conditions for a
particular environment which
help organisms, survive,
reproduce and pass on these
variations. This is also known as
“Survival of the Fittest”
There is a variation in Traits.
Green beetles tend to get
eaten more often therefore
lessen their chances to The Brown
Trait is an
allows more
brown beetles
to be produced
 Artificial Selection - a technique in
which a breeder selects the
particular traits
 Pigeon breeding experiments
helped him to further study his
theory of evolution
 This was an example of Artificial
 Mimicry – structural adaptation that
provides protection. Allows a harmless
species to resemble a harmful species.
This caterpillar
protects itself by
mimicking a snake
 Camouflage – structural adaptation that
enables an organism to blend in with it’s
into a variety of
 Structural adaptations can occur
over many generations
 Physiological adaptations can
occur very rapidly (organisms
adjusting to the changes of
 Eg: tanning of skin when exposed o
sun/ formation of callouses on
hands/ change color of coat
because of temperature
Evidence of Evolution
Fossils show changes which have occurred
over time:
1. Homologous Structure – modified structure
similar in structure and function. (PAP p.384/
2. Analagous Structure – similar in function
different in structure (Insect wings/Birds)
3. Vestigial Structure – structure which was once
used in an ancestral organism, but looses
function due to evolution. (mole eyes/ Whale
pelvic bones/ ostrich wings)
Bell Ringer 2:
1. Define Phenotype.
2. What provides evidence for evolution?
3. What is an adaptation?
II. Mechanisms of
 Phenotypes may determine the success
or failure of an organism
 If the phenotype is poorly adapted to it’s
environment the organism my not
survive, therefore not reproduce
 Populations evolve over time and not
individuals this is called “Population
Genetics Determines
 Gene Pool – All the alleles in a
populations genes. In other words every
form of a trait
 Allelic Frequency – the % of any specific
allele w/in a gene pool
Statsis (Genetic Equilibrium) – allele
frequency remains the same over
 Populations in genetic equilibrium
are not evolving
Factors that affects genes can affect
1. Mutations [negative, positive]
 Negative more than likely causes
 Positive may be beneficial
2. Genetic Drift – alteration of
allelic frequencies by chance
3. Gene Flow – transfer of alleles
from one population to another
population through immigration of
individuals . This disrupts allele
Trait Variations –
differences in Organisms
 Increases or decreases an organisms
chance of survival
 Inherited and controlled by alleles
 Allelic frequencies in a populations gene
pool will change due to natural selection
Bell Ringer 3:
7. How do alleles affect evolution?
8. How can geographic isolation change a
populations gene pool?
9. Why is rapid evolution more likely to
occur in small populations?
3 Types of Natural Selection
that Act on Variation:
1. Stabilizing – natural selection that
favors average individuals
3 Types of Natural Selection
that Act on Variation:
2. Directional selection – natural
selection that favors an extreme form of
a trait
3 Types of Natural Selection
that Act on Variation:
3. Disruptive selection – individuals w/
either extreme form of a trait
The Evolution of Species:
 Species – organisms that look alike can
interbreed to produce fertile offspring
 Speciation – evolution of a new species,
where members of similar populations no
longer interbreed to produce fertile
Physical Barriers Cause
Geographic Isolation – a physical barrier
divides a population
Reproductive Isolation – the inability of
organisms to mate and breed fertile
 Genetic material becomes so
different fertilization can not occur
 Behavior – mating seasons are
These Bowerbirds have different mating seasons and
rituals. These 2 factors contribute to Reproductive
Speciation Rates - the time it takes for
evolution to occur
Different Theories:
Gradualism – idea that species
originate through a gradual change of
Punctuated Equilibrium – idea that
species originate in rapid bursts followed
by long period of genetic equilibrium
 Sympatric Speciation: Is the process
through which new species evolve from a
single ancestral species while inhabiting
the same geographic region.
 Allopatric Speciation: occurs when the
same species become vicariant, or
isolated from each other. This can be the
result of geographical changes such as
mountain formation, island formation, or
large scale human activities (for example
agricultural and civil engineering
Patterns of Evolution
Adaptive Radiation – ancestral
species evolves into an array of species
to fit a particular environment.
Divergent Evolution – process of
adaptive radiation where individuals
become a totally new species
Convergent Evolution – distant
related organisms evolve and share
similar traits.
Both of the cactus are
located in different
parts of the world.
Their environment is
the same, so they
have adapted in some
of the same ways.