What term describes a modification that enhances an organism's fitness for its environment?

The term that describes a modification enhancing an organism's fitness for its environment is adaptation. Adaptations are characteristics or traits that have evolved over time to enable an organism to better survive and reproduce in its specific environment. These can be physical traits, such as camouflage or specialized feeding structures, or behavioral traits that improve survival and reproductive success.

Adaptations result from the process of natural selection, where individuals with traits better suited to their environment are more likely to survive and reproduce. Over generations, these advantageous traits become more common in the population.

In contrast, mutation refers to a change in the DNA sequence that may lead to new traits but does not inherently imply an improvement in fitness. Evolution is the overarching process by which populations change over time, resulting in adaptations among other outcomes. Variation refers to the differences among individuals in a population, which is a prerequisite for natural selection to act upon, but it does not specifically denote a beneficial enhancement to fitness.