Individual

Morphology, physiology, and behavior are attributes of individual organisms that can be observed. These attributes are known as the phenotype. Two factors that influence the phenotype are: (1) the specific genetic information of the individual (its genotype), and (2) environmental influences on the expression of the individual's genetic potential. The term phenotypic plasticity refers to the variable growth, physiology, and behavior that an individual organism displays, depending on environmental conditions experienced during its lifetime.

Because organisms vary in character (phenotype), they also differ in their abilities to cope with environmental stresses and opportunities. Under certain conditions, an individual with a particular phenotype (and genotype) may be relatively successful, compared with other individuals. In evolutionary biology, the "success" of an individual is measured by how many offspring it has produced, and whether those progeny go on to reproduce. This is similar to fitness, or the genetic contribution of an individual to all the progeny of its population. A central element of evolutionary theory is that individuals seek to maximize their fitness, and thereby to optimize their genetic influence on future generations.

Biologists believe natural selection is the most important means by which evolution occurs. Natural selection can only proceed if: (a) there is genetically based variation among individuals within a population, and (b) some individuals are better adapted to coping with the prevailing environmental conditions. Better-fit organisms tend to be more successful having offspring, and they have a greater influence on the evolution of subsequent generations. Individuals themselves do not evolve, however, they are capable of phenotypic plasticity.