a change in DNA, the
hereditary material of life. An organism's DNA affects how it looks, how
it behaves, and its physiology all aspects of its life. So a change in
an organism's DNA can cause changes in all aspects of its life.
Mutations are random
Mutations can be beneficial, neutral, or harmful for the organism, but mutations
do not "try" to supply what the organism "needs." In this
respect, mutations are random whether a particular mutation happens or not
is unrelated to how useful that mutation would be.
Not all mutations matter to evolution
all cells in our body contain DNA, there are lots of places for mutations to occur;
however, not all mutations matter for evolution. Somatic mutations occur in non-reproductive cells
and won't be passed onto offspring.
For example, the golden color on half of this Red Delicious apple was caused by a
somatic mutation. The seeds of this apple do not carry the mutation.