What are three modalities of increasing variation in a population?
Not exactly sure what is meant by "modalities of increasing variation" but maybe this will help
Mutation - random changes in DNA caused by various sources.
Founder effect - when a small group of organisms (maybe even just one pregnant female) enter a new, unused environment. If they survive, they will populate the area and evolve to fill available food niches (ways of life). This situation can lead to:
Adaptive radiation - when one species evolves to fill many niches of a relatively empty environment. (A good example of this is marsupials in Australia. All of species of pouched animals in Australia are probably related to the first marsupial group to colonize the continent.)
Genetic drift - the accumulation of neutral mutations in a species. Since the mutations are "neutral" they are not acted upon by natural selection (having the mutation does not effect your survival rate). In this way populations can change their genetic makeup even without natural selection being a factor. [Note: just because the mutation is neutral at this point does not mean that the environment will not change in some way to select for or against the mutation in the future.] The allele for blue eyes probably began as a case of genetic drift.
Genetic bottlenecks - when a cataclysm (large extinction event) removes competition in an area (or perhaps the whole world) the surviving species can then repopulate and fill available niches. The death of the dinosaurs may have been the opportunity needed by mammals in order to become a dominant group.