Mutation - Change in the genetic code or sequence in the base
pair create a new allele that offer an evolutionary advantage.
Genetic drift - An allele becomes more frequent in a
population's gene pool by chance that give to an advantage lead to
evolution.
Environmental chance - Natural calamities like earthquake or
volcanic eruption will lead to increase in frequency of allele of
survivor
Non random mating - When individuals mate for specific traits
that are not same in other individuals will cause the increase of
that specific allele and that lead to evolution.
Immigration - The movement of members of a species into a
population adds allele to the gene pool and increase in frequency
of that allele lead to evolution.