Animal physiology encompasses the study of how various structures within organisms function, distinguishing it from anatomy, which focuses on the physical structures themselves. For instance, the human cardiovascular system, comprising veins, arteries, capillaries, and the heart, serves the critical function of pumping blood to deliver oxygen to tissues throughout the body. This interplay between structure and function is fundamental to understanding how organisms adapt to their environments.

Adaptations are traits that evolve over generations, enhancing an organism's survival and reproductive success. A classic example is the color change in moths during the industrial revolution, where darker moths became more prevalent due to their camouflage in polluted environments, reducing predation risks. This contrasts with acclimatization, which refers to short-term physiological adjustments to environmental changes, such as increased oxygen-carrying capacity in red blood cells when training at high altitudes. However, these acclimatizations are temporary and diminish once the individual returns to lower altitudes.

Evolution does not lead to perfect adaptations due to fitness trade-offs, which highlight the limitations of an organism's ability to adapt based on finite energy resources. Just as in role-playing games where characters have limited stat points, organisms must allocate their energy efficiently to maximize survival and reproductive success. This often results in compromises, such as the design of the human spine, which is not ideally suited for upright walking due to its evolutionary history.

Moreover, the energy investment in adaptations can affect reproductive strategies. For instance, an organism may prioritize survival over immediate reproduction, ensuring it can reproduce successfully in the future. The immune system exemplifies this, as mounting a robust immune response requires significant energy, which may detract from reproductive efforts.

In terms of genetics, if a mutant allele enhances an individual's survival and reproductive efficiency, its frequency will increase within the population, aligning with principles of Hardy-Weinberg population genetics. This concept illustrates how advantageous traits can proliferate through generations.

Real-world examples, such as the cheetah and giraffe, further illustrate fitness trade-offs. Cheetahs have evolved longer legs for speed, but excessively long legs risk bone brittleness. Similarly, giraffes possess long necks to access high vegetation, but this trait demands significant energy to maintain. Both species exemplify the necessity of finding a balance between advantageous traits and the energy costs associated with them.