Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through homeostasis (literally, вЂњsteady stateвЂќ)-the ability of an organism to maintain constant internal conditions.

These conditions may, however, change from one moment to the next. In order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. Polar bears ( Ursus maritimus) and other mammals living in ice-covered regions maintain their body temperature by generating heat and reducing heat loss through thick fur and a dense layer of fat under their skin. As an environment changes, natural selection causes the characteristics of the individuals in a population to track those changes. All adaptations enhance the reproductive potential of the individuals exhibiting them, including their ability to survive to reproduce. Examples of adaptations are diverse and unique, from heat-resistant Archaea that live in boiling hot springs to the tongue length of a nectar-feeding moth that matches the size of the flower from which it feeds. Biologists refer to this fit as adaptation, and it is a consequence of evolution by natural selection, which operates in every lineage of reproducing organisms. AdaptationДЄll living organisms exhibit a вЂњfitвЂќ to their environment. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape. When reproduction occurs, genes containing DNA are passed along to an organismвЂ™s offspring. Multicellular organisms often produce specialized reproductive germline cells that will form new individuals. Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Watch this video to see how plants respond to a stimulus-from opening to light, to wrapping a tendril around a branch, to capturing prey.

Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response. Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light ( phototaxis). For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (Figure 2).

After a few minutes, the plant returns to normal. The leaves of this sensitive plant ( Mimosa pudica) will instantly droop and fold when touched.

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