Why can an organism that is fit in one environment become less fit after conditions change?

An organism that is considered "fit" in one environment may become less fit when conditions change because natural selection favors specific traits that are advantageous only within a particular set of physical and biological circumstances.12

Herbert Spencer, who coined the term "survival of the fittest," recognized that evolution is a continuous process of development. Because environments are "forever changing"—both physically, through shifts in climate or configuration, and biologically, through the evolution of predators, parasites, and food sources—the traits that once provided a reproductive advantage may no longer be beneficial.341

When an environment shifts, the previously favored genotype may no longer maximize an organism's ability to survive and reproduce. Consequently, natural selection will begin to favor different traits that are better suited to the new conditions, causing the relative fitness of the original population to decline. This dynamic ensures that adaptation is not a static state but an ongoing response to a constantly evolving world.12