Kevin M. Kelly
Like other concepts encountered in this volume, population has a variety of meanings (e.g., demographic, Malthusian, ecological, genetic, statistical, evolutionary, Darwinian). For example, statisticians define a population as the entire set of items or measures of interest. Demographic uses of the term focus on individuals circumscribed by geopolitical boundaries (Howell 1975:17-18; Kertzer and Fricke 1997; Levy and Lemeshow 1999). Similarly, population ecologists use the term to describe the aggregated members of a species within some defined area (e.g., Andrewartha 1971:10; Sutherland 1996:1-4). Population biologists (e.g., Hastings 1997:1) define a population as “a group of individuals of the same species that have a high probability of interacting with each other.” Population geneticists restrict the definition further, identifying a population as a group of interbreeding organisms of the same species (Futuyma 1997; Hanski 1999).
Underlying all of these manifestations is the fundamental notion of the population as a “defined collection.” Grouped as populations, individuals (and items) exhibit collective properties (e.g., density, size, age structure, a life history, a distribution in time and space, gene frequencies). Populations can be of any size and although the elements need not be identical, they must share at least one measurable attribute. The presence (or absence) of a single attribute is the simplest form of specification. The variables used to identify population membership may be interval (e.g., weight, birth order, antiquity), categorical (e.g., gender, location, language, presence/absence), ordinal (e.g., more/less, older/younger) or ratio (e.g, age, length) (see LeCompte and Schensul, 1999a: 115-119; Sokal and Rohlf 1981:10-