Most living things require free oxygen from the air or from water, but yeasts, many bacteria, and some other simple forms obtain the oxygen required for oxidation from molecules of substances that contain the element. Various animals and plants are adapted for securing their food and for surviving the extremes of temperature and of water supply in desert, tropical, and polar regions. For most organisms the optimum temperature is between about 20°C; (68°F;) and 40°C; (104°F;). Some algae and protozoans live in hot springs, and some bacteria can survive freezing or survive on chemicals, without light, in the ocean depths. Cacti can survive heat and drought. Certain fish and other aquatic animals live in deep water and are so specialized to withstand the great pressure that they burst if lifted to sea level.

Animals show anatomical adaptations—e.g., the body of the fish is suited to life in the water; the body of the bird is adapted for flight; and the land mammals show a wide variation in the structure of limbs and body that enables some to run swiftly, some to climb, some to swing from tree to tree, some to glide through the air, and others to jump. The whale, an aquatic mammal, can adjust to great pressure changes at different levels in the water. The beaks of birds vary in shape and size according to what they feed on—e.g., on seeds, on insects, on aquatic animals, or on small mammals. The feet and legs of birds also show modifications that fit them for perching, for wading, or for paddling through the water. Adaptive coloration is observed in many animals (see protective coloration). Among communal insects, such as ants and honeybees, the individuals are highly adapted to perform their functions in the community.

It is believed by many scientists that life originated in the sea and that through gradual evolutionary changes some forms became adapted to life on land. Variations may arise as a result of mutation, or of recombinations of the genes in the germ cells. Such variations are inherited (see genetics). Those that aid the organism to meet the conditions of a changing environment or help it in its competition with other living things enable it to survive and reproduce, the changes thus being passed on from one generation to another and in this way perhaps producing a new species.

See ecology; evolution; selection.

In physical anthropology, the genetic adaptation of human beings to different environmental conditions, such as extreme cold, humid heat, desert habitat, and high altitudes. Extreme cold favours short, round bodies with short arms and legs, flat faces with fat pads over the sinuses, narrow noses, and a heavy layer of body fat. These adaptations provide minimum surface area in relation to body mass for minimum heat loss and protect the lungs and base of the brain against cold air in the nasal passages. In conditions of humid heat, where body heat must be dissipated, selection favours tall and thin bodies with maximum surface area for heat radiation. A wide nose prevents warming of the air in the nasal passages, and dark skin protects against harmful solar radiation. The desert-adapted person must compensate for water loss through sweating. A thin but not tall body minimizes both water needs and water loss; skin pigmentation is moderate, since extreme pigmentation is good protection from the sun but allows absorption of heat from direct sunlight, which must be lost by sweating. Adaptation to night cold, often part of a desert environment, provides increased metabolic activity to warm the body during sleep. High altitudes demand, in addition to cold adaptation, adaptation for low air pressure and the consequent low oxygen, usually by an increase in lung tissue. Seealso acclimatization; race.

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In biology, the process by which an animal or plant becomes fitted to its environment. It is the result of natural selection acting on inherited variation. Even simple organisms must be adapted in many ways, including structure, physiology, and genetics; movement or dispersal; means of defense and attack; and reproduction and development. To be useful, adaptations must often occur simultaneously in different parts of the body.

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