7/23/2009
1
Natural Selection
Both genetic variation and environmental
factors are causes of evolution and
diversity of organisms.
Natural selection explains how a
population changes in response to its
environment.
•Those changes are called
adaptations.
•Adaptations are inherited, therefore they
bid
m
ust
b
e carr
i
e
d
on genes.
•Since Darwin developed his theory before
Gregor Mendel’s experiments, he knew
nothing about genes.
Mutations
What causes genetic variation?
Since Darwin’s time, there has been a
growing body of knowledge about
heredity.
•That knowledge explains many of Darwin’s
observations and supports the theory of
evolution.
•For example, Darwin observed that
individuals in a population show variation in
their traits.
•Today, scientists know that variations in the
population of a species are caused by
random mutations in genes.
Random mutations in genes produce
variations of traits in a population.
•A single mutation, for example, caused this
cat's ears to curl backwards slightly.
Mutations lead to alleles.
•Recall that alleles are different forms of a
gene.
•A gene mutation leads to different alleles of
that gene which in turn leads to variations
that
gene
which
in
turn
,
leads
to
variations
of a trait.
•Mutated alleles may cause favorable and
unfavorable adaptations.
•Favorable alleles are selected.
