Environmental changes: Environmental changes
leads to evolution, because organism will survive only if it can
accomodate itself to the environment. For example, camouflage of
chemelion is an evolutionary change to protect itself from
predators.
Mating preferences: Mating with strong and
better breeds will lead to better offspring.
Population Immigration: If mating parteners
with better quality breeds immigrates in a particular region, the
offspring will have new qualities and better capabilities.
Genetic Mutation/Sexual reproduction: In
complex organisms, due to sexual reproduction, sharing of
chromosomes from both parents creates variation in the population,
and offspring are not exactly similar to the parent.