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NATURAL SELECTION
The unequal survival and reproduction of
organisms due to environmental forces,
resulting in the preservation of favorable
adaptations.
The Two Steps of Natural Selection are:
1. The production of variation in a population
2. Limited aspects of survival and reproduction
DARWIN’S NATURAL SELECTION:
3 REQUIREMENTS
1. There is variation in traits in the
population:
For example; some beetles are
green & some are brown.
2. There is limited reproduction:
- Not all individuals get to reproduce to
their full potential.
3. Traits are able to be inherited
- Traits have a genetic basis.
The more advantageous traits allow
more offspring.
If you have variation, limited reproduction, and heredity, you will have evolution by
natural selection.
THE 4 MAIN PRINCIPLES
OF NATURAL SELECTION ARE:
1.
Variation: Differences in physical traits of an individual from the group to
which they belong.
2.
Overproduction: Raises the chances of survival and results in competition
for resources.
3.
Adaptation: Inherited trait that is selected for over time because it allows
organisms to better survive in their environment.
4.
Descent modification: Overtime natural selection will result in species
with adaptations that are well suited for survival and reproduction in an
environment.
NATURAL SELECTION
Step One: The Production of Variation. (Chance)
•Mutations
•Meiosis:recombination due to crossing-over
•Random mate selection & fertilization
What different wild species do we see variation in?
Step Two: Limited aspects of survival and reproduction
•Superior success of certain phenotypes
•Non-random mate choice
ARTIFICIAL SELECTION

Nature provides variation, humans select
variations that are useful.
Ex: a farmer breeds only his best livestock.
ARTIFICIAL SELECTION
Agriculture
Corn looks very different from its ancestor
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NOBODY!!
There is no agent involved in
natural selection.
Natural selection is a process of
elimination.
INDIVIDUALS THAT HAVE TRAITS
THAT ARE BEST ADAPTED FOR THE
CURRENT ENVIRONMENT ARE THE ONES THAT SURVIVE TO
BREED AND PASS ON THEIR GENES TO THE NEXT
GENERATION.
Organisms not possessing the beneficial traits either
die or don’t have as many offspring or adapt to a new
way of life.
EVOLUTIONARY FITNESS
Evolutionary Fitness: The probability
that the line of descent from an
individual with a specific trait will not
die out
If a gene is ‘fit’, then it
is likely to benefit an
organism in their
environment. If the
environment changes,
then it may no longer
be ‘fit’
ENVIRONMENTAL CHANGE

When faced with a change in environmental
condition, a population of a species can get
MAD:
 MIGRATE to a more favorable location
 ALREADY be adapted
 DIE - Extinction
MUTATIONS, MY FRIENDS!
Changes in the structure of the
DNA
 Adds genetic diversity to the
population – Speciation.
 May or may not be adaptive

 Depends
on the environment!
MECHANISMS OF NATURAL SELECTION
Gene Flow: the movement of genes into or
out of a population.
MECHANISMS OF NATURAL SELECTION
SELECTION PRESSURE ON A POPULATION
• Competition for food
• Competition for a mate
• Changes in the environment
• Predators
• Parasites