Artificial and Natural
Selection
Angela Gula & Audrelyn Watkins
Artificial Selection


Human intervention in animal or plant
reproduction to ensure that certain desirable
traits are represented in successive generations.
Examples?
The Theory of Natural Selection



Originally presented in The Origin of Species in
1859.
The theory of natural selection was not
accepted by the scientific community until 60
years later.
Today, natural selection is considered one of the
most important causes of evolution.
What leads to natural selection?
1.
2.
3.
4.
Variation of individuals in the population
Inheritance of variation
Overproduction
Non-random Survival and Reproduction
Variation


Take a look around—individuals within
populations vary.
Similar variations occur in plant populations
Some plants bloom sooner than other
 The amount of caffeine produced in the seeds of a
coffee bean varies


Each different type of individual in a population
is termed a variant.
Inheritance of Variation




When The Origin of Species was written, Darwin did not
understand the process of inheritance.
Darwin hypothesized that offspring tend to have the
same characteristics as their parents.
Until Gregor Mendel’s work on inheritance was
accepted, there was controversy over this component
of natural selection.
Now understood, natural selection operates on genetic
variation that can be passed from one generation to the
next.
Overproduction


The capacity for reproduction outstrips the
resources of the environment.
Therefore, many individuals do not survive to
reproductive maturity.
Non-random Survival and
Reproduction




The subset of individuals that survive long
enough to reproduce is not arbitrary.
Some variants in a population have a better
likelihood of survival and reproduction.
This is known as fitness.
Traits that increase an individuals fitness in that
particular environment are adaptations.
So….What does it mean?

If the four underlying observations Darwin
made are true, what can we infer?
Types of Natural Selection



Directional Selection—when an extreme phenotype is
favored and the distribution shifts in that direction.
(peppered moth, bacteria resistance, pesticide
resistance)
Stabilizing Selection—occurs when an intermediate
phenotype is favored. (birth weight in humans)
Diversifying Selection—two or more phenotypes are
favored over any intermediate phenotype. (Sickle-cell
anemia)
Graphic Representation of Selection
Types
A. ________________
B. ________________
C. ________________
Addressing Misconceptions
Three dreaded words…Try

Individuals
or
populations
can try to
evolve, but
it won’t
work!
www.evolution.berkley.edu
Three dreaded words…Need

Natural
selection
cannot
provide
what an
organism
needs.
www.evolution.berkley.edu
Three dreaded words…Want

Individuals
or
populations
cannot
control
what they
want.
The lifetime problem…


Natural selection acts on the inherited traits.
Individuals cannot change within their lifetime
in order to survive environmental changes.
Good, Better, Best…


Natural selection does cause populations to
become better fit to the environment, BUT…
The result of that process is not necessarily a
“better” organism.
The Predetermined Goal


Natural selection does not occur with a goal in
mind; instead, it is situational.
Consider:

Environmental conditions—drought, temperature
change, food availability, etc.