Answer:
mutation and sexual reproduction, natural selection
Step-by-step explanation:
Variation within individuals in a population is brought about through mutation and sexual reproduction, while pressures exerted by natural selection results in differential survival and reproduction, and adaptation to the environment.
Mutation is a change in gene structure which causes variation in population of individuals, examples are albinism, dwarfism.
Sexual reproduction also results in increased variation in a population, it involves the fusion of the male and female gamates (gene transfer from male and female parent) resulting in variation in the offspring.
Natural selection is the process whereby organisms struggle to survive in an environment by adaptation. Any organism which fails to adapt to the prevailing environment dies out.