 Dear students, in this module, we shall discuss the concepts of adaptation, acclimatization and acclimation. Adaptation is an evolutionary process that occurs extremely slowly in a species. It involves thousands of generations. During this process, the physiology of the members of a species becomes well matched to the changing environment. This phenomena is very important in the survival of a species. The changes that are gathered through adaptation are generally not reversible. Related to the phenomena of adaptation, there is another phenomena which is called acclimatization. Acclimation is also an adaptive change, but it involves only one or a few members of a species. It happens due to the chronic exposure of a member of a species to naturally occurring changed environmental conditions. While acclimation is also an adaptive change that involves not natural environmental conditions, but induced or experimentally induced laboratory conditions. Acclimatization and acclimation both are acquired characters. They are not actually adapted and evolutionary characters. They are restricted to only one or few members of a species. These characters do not have a genetic basis, so they are not inheritable. An individual adopts any changes in his or her lifetime and does not inherit them. So they have no evolutionary significance. Such changes are usually reversed. Now we shall take an example of acclimatization. Let us consider an animal that voluntarily migrates from a valley to a high mountain area where there is low partial pressure of oxygen. What is the effect of this? When an animal goes to a mountainous altitude from a valley, its respiration rate, breathing rate increases. This rate increases after a few days and normalizes. This change now means that we say that an animal is acclimatized with new naturally occurring environmental conditions. In simple words, we would say that it has become a part of our new environment. We call it acclimatization in scientific terminology. For the example of acclimation, we consider that the same animal was brought into the laboratory and a scientist put the animal in a hypobaric chamber at low atmospheric pressure. The same types of effects will appear on its breathing rate. That is, it will increase initially and after a few days it will drop to normal. Let us take an example of adaptation. Adaptation is a naturally occurring phenomena that occurs over many thousand years and involving thousands of generations. Evolution is a phenomenon that has been adapted to this species for many thousands of years and all members of this species have this ability. This is adaptation.