Natural Selection
Lab 14
Evolution - Defined

Evolution – a change in the genetic
composition of a population over
time.


A change in the frequency of certain
alleles.
On a larger scale, evolution can be
used to refer to the gradual
appearance of all biological
diversity.
A Mechanism for Evolution

Darwin presented a mechanism
for evolution – natural selection.



Organisms that are in some way more
successful at reproduction will pass on
more of their genes.
Over time the traits responsible for that
success will become widespread in the
population.
This theory holds up very well!!
Natural Selection

Natural selection occurs when
organisms with particular heritable
traits have more offspring that
survive & reproduce.
Natural Selection

Natural selection
can increase the
adaptation of an
organism to its
environment.
Natural Selection

When an environment changes, or
when individuals move to a new
environment, natural selection may
result in adaptation to the new
conditions.

Sometimes this results in a new
species.
Natural Selection


Individuals do not evolve;
populations evolve.
Evolution is measured as changes in
relative proportions of heritable
variations in a population over
several generations.
Natural Selection

Natural selection can only work on
heritable traits.

Acquired traits are not heritable and
are not subject to natural selection.
Natural Selection

Environmental factors are variable.

A trait that is beneficial in one place or
time may be detrimental in another
place or time.
Natural Selection

When natural selection is occurring,
some individuals are having better
reproductive success than others.

Alleles are being passed to the next
generation in frequencies that are
different from the current generation.
Genetic Drift

The smaller the sample, the greater
the chance of deviation from
expected results.


These random deviations from
expected frequencies are called
genetic drift.
Allele frequencies are more likely to
deviate from the expected in small
populations.
Genetic Drift


Which allele was lost is due to random
chance.
Over time, drift tends to reduce genetic
variation through random loss of alleles.
Natural Selection & Adaptation

Natural selection is a way of altering
the gene pool that results in
adaptation.


Genetic drift alters the gene pool but
does not result in adaptation.
Selection depends on variation.