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A population is either a whole species or an isolated portion of a species that can not interbreed with other segments of the species.

Most mutations have no affect. Well, really, these mutations don't have an apparent affect. Remember that natural selection works through the adaptive pressures of the environment on members of a species. When, or if, the environment changes those organisms that can adapt will survive and prosper while the others will die. It is in these times that mutations that appeared to have no affect on an animal's fitness may (or may not) provide an evolutionary advantage.

Of the mutations that do have an affect, most of these are fatal. Usually the mutation results in a miscarriage, often in the first month or so of pregnancy. A very, very small percentage of mutations are actively and immediately beneficial.

Genetic diversity, that is, having individuals within a population that show variation in their genes, is important for a species to survive. When the environmental conditions change a genetically diverse species will be more likely to have individual members that are able to adapt to the changes and survive. While individuals may die, the species as a whole will continue.