Download Natural Selection

Natural Selection
The Process of Natural
Selection
In any population of individuals, there is genetic
variation.
• Genetic variation is the result of mutation and
recombination.
• In a given environment, some variations of
traits are lethal, some make no difference,
and some increase or decrease the chance
for survival.
If the environment changes, some individuals
might have one or more variations that
increase their chance for survival.
Organisms with the variation(s) are more
likely to survive, reproduce, and pass the
favorable variation on to their offspring.
Over time, the population changes as more
individuals within it have the selected trait.
This is called natural selection because it
occurs through natural processes.
The Factors Involved in the
Process of Natural Selection
1. Genetic variation from mutation and
recombination of genes (made up of DNA)
during meiosis.
2. Time
3. Selection, based on the environment, of
individuals with the trait that allows for
increased survival in the environment.
4. Those with the selected trait reproduce and
pass on the trait that allows for survival of
the offspring.
Darwinian Evolution
(Natural Selection)
Giraffes with longer necks tend to reach leaves
more easily. Longer-necked giraffes are more
likely to eat enough to survive and reproduce. The
offspring inherit their parents’ longer necks.
A Phrase to Summarize
Darwin’s theory of natural selection can
be summarized as:
Common ancestry and
descent with modification.
Is Evolution by Natural
Selection Random?
Overall, no.
Evolution requires
~Variation due to mutation and recombination
AND
~Selection of specific variations.
The effect of natural selection on survival of
organisms is NOT random.
However, some of the events that may lead to
mutation can be random:
– Mutations can occur in any part of a gene.
– Whether a mutation occurs at all is random.
An Example of Mutation and
Natural Selection
View the following segment on sickle
cell disease and malaria:
http://www.teachersdomain.org/resource/tdc02.sci.life.gen.mutationstory/
In your science notebook:
Explain why individuals who carry one copy of
the mutated allele for abnormal hemoglobin
are selected for in the natural selection of
some human populations.
Explain why genetic variation is important in
human populations that are exposed to new
diseases, which are a type of change in the
environment.
More on Mutation
Remember from genetics that humans have
two copies of every gene.
Sickle Cell Disease:
Two copies of the mutated disease gene
(homozygous state) of sickle cell disease can
be fatal.
However, people with one copy of the
mutated disease gene (heterozygous) have
an advantage in areas where there is malaria.
The heterozygous condition is selected for.