ABSTRACT
Adaptation is sometimes expressed in morphological characters. Some succulent desert plants, like cacti, store water in their tissues in the brief rainy season. Others lose their leaves or replace them with thorns, reducing transpiration and water loss. Many examples of close adaptation of parasites to their hosts are provided by gall-forming insects. The host-specific aphids feeding on Pistacia illustrate the phenomenon. Only the fundatrix nymph, emerging from an overwintering egg, is able to induce a gall. Gall shape is insect species-specific, and is induced on a specific host tree. L. B. Slobodkin described a model for the evolution of adaptation. The model assumes that every organism is at equilibrium with the surrounding environment: a change in the environment disturbs the balance and is potentially deleterious. In every individual there exists a hierarchy of possible responses to change, attempting to restore the balance and prevent damage, with a minimal loss of energy.
