In animals and humans, genes determine the general size and shape of the organism, but there may also be variation in physical characteristics due to environmental influences. All organisms function most efficiently under certain environmental conditions; however, an individual organism may react to some change in the environment with responses that change the form or internal structure of the body.
An organism’s environment includes nonliving factors such as temperature, water, and nutrients. The environment also includes biological factors: all the other organisms that are part of any individual’s environment. Environmental stimuli can have important effects on individual organisms, groups, and whole populations.
The influence of environmental factors is particularly effective during the formative stages of life, both before and after birth. The experiences of early life determine the physical and mental attributes of the young and thereby determine the characteristics of the adult. Early influences can have favorable or unfavorable effects on the young organism’s development. Environmental factors exert such a strong influence on development that some physical or mental characteristics are difficult or impossible to change later in life.
Research on many types of animals and observations of human life have revealed that early environmental influences affect an individual’s initial rate of growth and resistance to various types of stress, as well as the ultimate body size of the adult. Environmental factors also affect learning ability and behavioral patterns in the young.