A change in which variable would not necessarily change an organism's fitness?

A change in mutation rate does not necessarily affect an organism's fitness in a straightforward manner. While mutations are essential for generating genetic variation and can lead to beneficial adaptations over time, the mutation rate itself merely indicates how frequently these variations occur. An increase in mutation rate may result in more genetic changes, but these changes can be neutral, beneficial, or deleterious. Therefore, unless the mutations produced confer a direct advantage or disadvantage to the organism in its specific environment, the mutation rate alone will not determine fitness.

In contrast, population size, environmental conditions, and genetic diversity are more directly tied to fitness. Population size influences genetic drift and can affect allele frequencies, potentially impacting fitness over time. Environmental conditions can have an immediate and profound effect on what traits are advantageous for survival and reproduction, thus directly influencing fitness. Genetic diversity provides a wider range of traits for natural selection to act upon, which can enhance the ability of a population to adapt to changing conditions and ultimately affects overall fitness.