Investigating the genetics of populations

Species, such as the Atlantic salmon, are collections of individuals which are born, develop and die. To survive over time they must reproduce and recruit new individuals. In a few species, including some fish (e.g. guppies: Poecilia spp.), recruitment results from asexual reproduction, i.e. clonal reproduction or parthenogenesis (a type of reproduction where the egg develops into a new individual without fertilisation). However, in most species, recruitment occurs from sexual reproduction. As discussed in Chapter 3, this is a genetic process with important implications for the genetic character of the individual. However, it also has important implications for the genetic dynamics of the species as a whole and, thereby, for the overall survival and character of a species.
The distribution of sexual reproduction in time and space determines which genetic types are produced and where. In most species, simply though not exclusively by virtue of geographic isolation, there is a subdivision into groups of individuals within which reproduction is more or less random but among which interbreeding is more or less constrained. As they are defined by their interbreeding, and interbreeding is necessary for reproduction, these groups represent the basic biological units underlying species survival. Such a biological unit is referred to by geneticists as a deme, a genetic population, or simply a population.
The term ‘population’ is used throughout the biological sciences, including fisheries biology, but there is no consensus about its use. It will be used by some to denote individuals of a species within national borders, e.g. ‘Norwegian salmon’. By others it will be used to denote individuals of a species within a well-defined area such as a bay, a lake or a river. Here we use it in the genetic sense to denote a group of randomly mating individuals that is partly or totally reproductively isolated from other such groups within the same species. A population geneticist would strongly argue in favour of the latter definition as being the best use for the term, as its definition is not arbitrary but defined by the fundamental biological processes of reproduction and recruitment.