Download Natural Selection

Bellringer
 Are evolution and natural selection related to one
another? Explain. Who is Charles Darwin?
Darwin and Natural
Selection
Learning Targets
 Describe the factors that Darwin considered when
developing his Theory of Evolution.
 Explain the process of natural selection and how it
leads to descent with modification.
 Describe the significance of genetic variation within
a population and identify sources of genetic
variation.
 Describe how evolution occurs through changes in
gene pools, including genetic drift, gene flow, and
sexual selection.
Evolution is a Theory – Just like Einstein's
Theory of General Relativity, which helps to
explain gravity!
• Evolution is a well
supported explanation
of phenomena that
have occurred in the
natural world
• A theory in science is a
well tested hypothesis,
not just a guess

Charles Darwin (1809-1882)
Sailed around the world 1831-1836

What did Darwin’s
travels reveal
 The diversity of living
species was far greater
than anyone had
previously known!!
 These observations led
him to develop the
theory of evolution!!

Diversity on the Galapagos
 Each island had
its own type of
tortoises and
birds that were
clearly different
from other
islands.
Galapagos
Tortoise
Influences on Darwin’s
Theories






Theories of geologic change set the stage
for Darwin’s theory
Fossils: traces of organisms that
existed in the past.
Theory of catastrophism states that
natural disasters like floods and
volcanic eruptions have shaped
landforms and caused species to go
extinct.
Theory of gradualism states that
changes in landforms resulted from
slow changes over a long period of
time.
Theory of uniformitarianism states
that the geologic processes that shape
Earth are uniform through time
(Charles Lyell’s theory).
Influences on Darwin’s
Theories
Lyell argued that the earth is many
millions of years



layers of rock take time to form
processes such as volcanoes and earthquakes
shaped the earth and still occur today

Influences on Darwin’s
Theories
Jean-Baptiste Lamarck:


Theory of acquired
characteristics

Organisms acquired
traits by using their
bodies in new ways

These new characteristics
were passed to offspring

Lamark was wrong!

Influences on Darwin’s
Theories
 Thomas Malthus
 If the human population
continued to grow
unchecked, sooner or
later there would be
insufficient living space
and food for everyone
Influences on Darwin’s
Theories
 Artifical Selection: Process in which humans change
a species by breeding it for certain traits. Examples:
Dog breeds, livestock, crops, vegetables, ect.

Darwin’s The Origin of Species (1859)
 Other naturalists, like
Alfred Wallace, were
developing the same theory
as Darwin’s
 Even though Darwin was
afraid of the Church’s
reaction to his book he
wanted to get credit for his
work.
 Summary
of Darwin’s Theory of
Evolution by Natural Selection
1. Organisms differ; variation is inherited
2. Organisms produce more offspring than survive
3. Organisms compete for resources
4. Organisms with advantages survive to pass those
advantages to their children (“survival of the
fittest”)
5. Species alive today are descended with
modifications from common ancestors
Summary of Darwin’s Theory
POINT 1 of 5
Organisms differ; variation is
inherited
OBSERVED
THEORY
OBSERVED
?
HYPOTHESIS
Discuss …
Summary of Darwin’s Theory
POINT 2 of 5
Organisms produce more
offspring than survive
OBSERVED
THEORY
OBSERVED
?
HYPOTHESIS
Discuss …
Summary of Darwin’s Theory
POINT 3 of 5
Organisms compete for resources
OBSERVED
THEORY
OBSERVED
?
HYPOTHESIS
Discuss …
Summary of Darwin’s Theory
POINT 4 of 5
Organisms with advantages survive to
pass those advantages to their children
OBSERVED
THEORY
OBSERVED
?
HYPOTHESIS
Discuss …
Summary of Darwin’s Theory
POINT 5 of 5
Species alive today are descended with
modifications from common ancestors.
THEORY,
OBSERVED
THEORY
much evidence
?
exists
HYPOTHESIS

What is evolution?
 changes in living
organisms over
time
 explains how
modern
organisms have
descended from
ancient organisms

Fitness and Adaptation
 Organisms compete for limited resources and
some organisms are more “fit” than others
 Fitness – the ability of an individual to survive
and reproduce in its specific environment
 Adaptation – any inherited characteristic that
increases an organism’s chance of survival,
including reproduction

What determines survival?
 Traits that help individuals survive
 survive predators
 survive disease
 compete for food
 compete for territory
 Traits that help individuals reproduce
 attracting a mate
 compete for nesting sites
 successfully raise young

Natural Selection
1. There is variation (difference in the physical
traits of an individual from those of other
individuals in the group) in traits.
For example, some beetles are green and some
are brown.
2. There is differential reproduction.
Since the environment can’t support unlimited
population growth, not all individuals get to
reproduce to their full potential. In this
example, green beetles tend to get eaten by
birds and survive to reproduce less often than
brown beetles do.

Natural Selection
3. There is heritability.
The surviving brown beetles have brown baby
beetles because this trait has a genetic basis
(trait is passed down from one generation to
the next).
4. End result: Natural Selection
The more advantageous trait, brown
coloration, which allows the beetle to have
more offspring, becomes more common in the
population. If this process continues,
eventually, all individuals in the population
will be brown.

Natural Selection
 Over time, natural selection results in changes in the
inherited characteristics of a population.
 These changes increase a species’ fitness in its
environment.
 Species: Group of organisms so similar to one another
that they can reproduce and have fertile offspring.
Sources of Genetic
Variation
 Genetic Variation is stored in a populations Gene
Pool, which is the combined alleles of all of the
individuals in a population.
 Each allele exists at a certain rate in a population.
Allele Frequency, is a measure of how common a
certain allele is in the population.

Sources of Genetic Variation
 Gene flow – movement of genes from one population to another
(migration)
 Mutation – any change in a sequence of DNAMain Source
 Some mutations can affect an organism’s fitness while others
have no effect on fitness.
 Recombination –(occurs during meiosis) Main Source
 Independent assortment
 Crossing over
 Sexual reproduction
Evolution as Genetic Change
 Natural selection determines which alleles are
passed from one generation to the next.
 As a result, it can change the relative frequencies of
alleles in a population over time.
 Evolution is any change in the relative frequencies
of alleles in a population’s gene pool.
 Evolution acts on populations, not on individuals.
*Remember:
Populations include
individuals of the
same species living in
an area.
Genetic Drift
 Natural selection is not the only source of evolutionary
change
 Genetic drift – random change in allele frequency
 In small populations, individuals that carry a particular
allele may leave more descendants than other
individuals, just by chance.
 Over time, a series of chance occurrences of this type can
cause an allele to become common in a population.
Bottleneck Effect
 Mechanism of genetic drift, occurs after an event
drastically reduces the size of a population. This
causes the population to have very little variation.
The reason for this is that certain alleles have
become fixed in the population and others have
been lost.
Founder Effect
 Occurs when a small group of individuals colonizes a
new habitat.
 Members of the colony may carry alleles in different
relative frequencies than the larger population from
which they came.
 If so, the population that they found will be genetically
different from the parent population.
Sexual Selection
 Certain traits increase mating success, so these traits
will be passed down from generation to generation.