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Natural Selection
• In science, theories are
statements or models that
have been tested and
confirmed many times.
In science, the term "Theory" does
not express doubt.
• They explain a wide variety of data
and observations
• They can be used to make
predictions
• They are not absolute, can be
changed as new evidence is found
Natural Selection
• Lamarke's Theory of
Acquired Characteristics
• Some thought that you would
gain or lose features if you
overused or didn't use them,
• PROVEN TO BE WRONG!
Example:
A giraffe acquired its long neck because its
ancestor stretched higher and higher into the trees
to reach leaves, and that the animal’s increasingly
lengthened neck was passed on to its offspring.
Photo courtesy of ucumari,
creative commons, flickr
Summary of Darwin’s Theory
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
5. Species alive today are descended with
modifications from common ancestors
• Darwin’s main ideas can be
summarized in three points.
• Natural selection is differential success in
reproduction (unequal ability of individuals to
survive and reproduce).
• Natural selection occurs through an interaction
between the environment and the variability
inherent among the individual organisms
making up a population.
• The product of natural selection is the
adaptation of populations of organisms to their
environment.
1. Variation exists among individuals in a
species.
2. Individuals will compete for resources (food,
mates, and space)
3. Competition would lead to the death of some
individuals while others would survive
4. Individuals that had advantageous variations
are more likely to survive and reproduce.
This process came to be
known as Natural Selection
The favorable variations are
called Adaptations
Photo courtesy of
digitalART2, flickr
creative commons
Natural
Selection
 The traits that help
an organism
survive in a
particular
environment are
“selected” in
natural selection
 Natural selection acts on the phenotype, or the
observable characteristics of an organism, such that
individuals with favorable phenotypes are more likely
to survive and reproduce than those with less
favorable phenotypes.
 The phenotype's genetic basis, the genotype
associated with the favorable phenotype, will
increase in frequency over the following generations.
Over time, this process may result in adaptations that
specialize organisms for particular ecological niches
and may eventually result in the emergence of new
species.
• While natural selection involves
interactions between individual
organisms and their environment, it is
populations, not individuals that
evolve.
• Populations are defined as a group of
interbreeding individuals of a single
species that share a common
geographic area.
Natural Selection and Species
Fitness
 Overtime, natural selection results in changes in
the inherited characteristics of a population.
 These changes increase a species fitness (survival
rate)
• Say in a species of blob….there exists
blobs of all shapes and sizes (variation)
Blobs eat the little purple organisms that live
underground and on the surface.
During a particularly hot year, food became less
abundant (competition), blobs that had the ability to
dig into the soil to get food had a better chance of
survival.
Many blobs died that year…….
The ones that survived mated and passed their
genes to the next generation. (reproduction)
The next generation had move blobs with the pointed
noses. That is NATURAL SELECTION.
1. Variation
2. Competition
3. Survival
4. Reproduction
Genetic Drift
 Genetic drift or allelic drift is the change in the
frequency of a gene variant (allele) in a population due
to random sampling.[1] The alleles in the offspring are a
sample of those in the parents, and chance has a role
in determining whether a given individual survives and
reproduces. A population's allele frequency is the
fraction of the copies of one gene that share a
particular form.[2] Genetic drift may cause gene variants
to disappear completely and thereby reduce genetic
variation.