Marginal distribution (biology)
The geographical limits to the distribution of a species are determined by biotic or abiotic factors. Core populations are those occurring within the centre of the range, and marginal populations are found at the boundary of the range.
The inability of a species to expand its range beyond a certain geographic area is because of some limiting factor or factors to which the species cannot successfully adapt. In some cases, geographical range limits are entirely predictable, such as the physical barrier of an ocean for a terrestrial species. In other cases the specific reasons why species do not pass these boundaries are unknown, however, ecology is the main determinant of the distribution of a species. The fitness of a species falls at the edges of its distributional range, with population growth and fitness falling to zero beyond where a species can survive.
For many species of invertebrate animals, the exact geographic range limits have never been precisely ascertained, because not enough scientific field work has been carried out in many parts of the world to map distribution more precisely, therefore finding a range extension for species, especially marine species, is not an uncommon occurrence.
Marginal distributions can have conservation implications.
Terminology
The science of understanding the distributions of organisms is known as chorology, a branch of biogeography. The core population of a species are those individuals occurring within the centre of the range. Although one cannot ever truly know the ideal niche of a particular species, it can be approximated from the core of the distribution, this is known as the "realized ecological niche". Marginal or peripheral populations are those found at the boundary of the range. When the distribution of a species is changing, the leading edge populations are at the expanding geographic edge of the distribution range whilst rear edge populations are undergoing retreat.The central‐marginal hypothesis, also sometimes called the "central-peripheral population hypothesis", posits that there is less genetic diversity and greater inter‐population genetic differentiation at the range margins, as compared to the range cores. This is based on the assumption that the habitat is most ideal at the centre of a distribution and ecological conditions decline towards the margin. Because the population size at the margin is likely to be smaller, genetic drift can have a larger effect and reduce the genetic variation of marginal populations. Reduced gene flow between central and peripheral populations also limits the genetic diversity at the margins. High selection pressure, due to a less than ideal habitat at the margin, furthermore reduces genetic diversity. Although exceptions to this hypothesis are common, in general this rule appears to hold empirically true. The spatial distribution often differs, with the population being more dense in the centre as opposed to the margins, this can often have a simple probability distribution pattern. The gene flow between central and peripheral populations may prevent range expansion when it does not allow the gene pool at margin to differentiate. Conditions at the centre of the range differ from those at the periphery, therefore adapted alleles at the centre may not benefit marginal populations experiencing different conditions. The asymmetrical gene flow hypothesis posits that there is more gene flow from central to peripheral populations. Empirical data supporting this theory is less robust.
When circumstances, usually climatic, restrict the distribution to a small area, this is known as a refugium. In Europe, for example, the geographical spokes sticking out of the continent in the south - the Iberian Peninsula, Italy and the Balkans served as refugia for warmth-adapted species during the Ice Ages.