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°C) and 40°C (104°C). 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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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