Final answer:
Genetic drift is a process that increases genetic variation without changing phenotypes. It occurs due to chance events that affect which individuals reproduce. Mutations also increase genetic diversity and contribute to genetic variation but do not always affect phenotypes.
Step-by-step explanation:
The process that will not change phenotypes but will increase genetic variation, allowing a population to carry more alleles that may help it respond to environmental change, is known as genetic drift. This phenomenon is the effect of chance where certain individuals in a population may have more offspring than others—not due to having advantageous traits, but merely due to random events. Genetic drift can introduce new variations to a population's gene pool, and over time, these changes may increase the genetic diversity within the population.
Mutations also contribute to the increase in genetic variation by introducing new alleles into the population. Unlike genetic drift, mutations occur at the molecular level and can introduce entirely new genetic information into the gene pool. However, not all mutations will affect the phenotype.
Sexual reproduction, another mechanism for increasing genetic variation, results in offspring with a mix of alleles, which contributes to diverse phenotypes and genotypes within a population. However, if the alleles produced do not affect the observable characteristics, they contribute to genetic variance without altering the current phenotypic distribution.