In the biosciences, the population is the totality of all individuals , usually of the same species , that occur in a certain area . As a result of the social interactions between the members of this population unit, it may a. to a constant mixing of their genetic material through gene flow . Populations are therefore also a starting point for evolutionary changes in the species concerned.
Which interactions are of interest in individual cases is not always congruent within the biological disciplines. In population genetics , for example, particular emphasis is placed on the fact that individuals are connected to one another due to their development processes. In population ecology , special emphasis is placed on the fact that the individuals are in a uniform area at the same time. In demography and epidemiology , in addition to the characteristics of individuals and their variation, genetic and areal geographic factors, social interactions in particular also play an essential role in the definition of populations.
In addition to these specifically biological definitions, biology often speaks of a “population” in a purely statistical sense as a (arbitrarily defined) population from which certain properties and their distribution are determined using random samples .
When considering genetic and evolutionary issues, a population is usually defined as a group of individuals of the same species who are able to mate with one another during sexual reproduction and, at least in principle, can have common offspring. It is not only a question of the genetic and physiological ability to mate, but also an opportunity to do so. Individuals of a species that live in spatially completely separate habitats and can only produce offspring with experimental manipulation by humans, for example in the laboratory or in captivity, are therefore (mostly) counted as belonging to the same species, but never to the same population. A species therefore usually comprises many populations, with very small populations, such as local endemic populations , possibly only one. If individuals live next to one another in the same habitat, who would be physiologically capable of mating with one another, but never do this in practice (called: reproductive isolation ), they do not belong to the same population. This is an essential difference to ecologically defined populations. Species that reproduce through asexual reproduction also do not - in this sense - form a population.
As a rule, a population defined in this way has no sharp boundary. Spatially separated habitats often exchange migrating individuals with one another, since individuals or, in the case of fixed species, at least reproductive units such as diaspores , are mobile. A distinction is made here between local populations with almost unlimited reproduction among each other and neighboring populations that are less frequently involved in reproduction but regularly enough over longer periods of time. This is then called a metapopulation with multiple subpopulations.
In the model assumption, for the sake of simplicity, it is often assumed that there are no pairing barriers between individuals of the same population. Thus the pairing between them is random in the statistical sense. This is called panmixie . In the case of panmixia, all individuals in a population are part of the same gene pool . Every difference between subgroups of them, which may arise by chance ( genetic drift ) or due to directed environmental factors ( selection ), will be leveled again and again by gene flow . Such a population forms an evolving unit. This means: their characteristics and the frequency of the alleles can change ( anagenesis ). However, it cannot split itself into two populations, which could eventually result in separate species (no cladogenesis ). Statistically, the difference between two groups of individuals is often examined using some form of the F-test . If their allele frequencies differ only below a certain threshold value, gene flow between them can be assumed. Indeed, different test procedures and different thresholds are in use for whether two groups are considered two populations or two sub-populations of a single population. This also depends on the question: an evolutionary biologist who examines the divergence of traits in a possible process of speciation will use different threshold values than a nature conservation biologist who wants to prevent the extinction of small local populations of an endangered species due to genetic factors.
A Mendel population is made up of organisms with sexual reproduction.
- Werner Kunz: What is a species? Tried and tested in practice, but vaguely defined. In: Biology in Our Time. 32, No. 1, 2002, , pp. 10-19.
- Formulated based on: Klaus Immelmann (Ed.): Grzimeks Tierleben . Special volume behavioral research. Kindler Verlag, Zurich 1974, p. 634
- Robin Waples, Oscar Gaggiotti (2006): What is a population? An empirical evaluation of some genetic methods for identifying the number of gene pools and their degree of connectivity. In: Molecular Ecology. 15: 1419-1439, doi: 10.1111 / j.1365-294X.2006.02890.x .
- Nancy Krieger (2012): Who and What Is a “Population”? Historical Debates, Current Controversies, and Implications for Understanding “Population Health” and Rectifying Health Inequities. In: Milbank Quarterly. 90 (4): 634-681.