In biology, adaptation refers to the evolutionary process by which populations become better suited to their environment. At the deepest level, this concept describes inheritable traits that increase fitness within a given ecological niche, a process driven primarily by natural selection acting upon existing genetic variation. Adaptations can manifest over vast spans of geological time, leading to structural changes in organisms, or they can involve immediate physiological adjustments. Physiological adaptation refers to the short-term modifications an organism makes in response to immediate environmental stressors, such as changes in temperature or altitude, allowing for temporary homeostasis while the genetic, long-term evolutionary processes proceed.