 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 aseptic states. 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. According to the phenomena of adaptation, there is another phenomena which is called acclimatization. Acclimatization 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 life time and does not inherit them. So they have no evolutionary significance. Such changes are generally 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 acclimatization? When an animal migrates from a valley to a high altitude mountain area, its respiration rate, breathing rate increases. This rate increases and becomes normalised after a few days and becomes normalised. This means that the animal is acclimatised with new, naturally occurring environmental conditions. In simple words, the animal is acclimatised by the scientific terminology. For the example of acclimation, we consider that the same animal was brought into the laboratory. 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. But since this is happening in the laboratory, we will say that this animal has acclimated. Let us take an example of adaptation. Adaptation is a naturally occurring phenomena that occurs over many thousand years and involving thousands of generations. This is an example of bar-headed goose. A bar-headed goose flies at a height of as much as a mount average. This is an example of evolution that occurs over many thousands of years and adapts to this species. This is an example of all the members of this species. This is adaptation.