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Genetic drift is an important concept in conservation biology, where small, spatially isolated populations of endangered plant and animal species are often studied. Often, the deciding factor is the estimation of the minimum viable size of the population. The minimum viable population size is an estimate of the number of individuals required for a high probability of survival of a population over a period of time. A commonly used definition is a probability of more than 95% persistence over 100 years. An important prerequisite for preventing the extinction of a population is the preservation of sufficient genetic diversity of the population to allow adaptation to changing environmental conditions. The genetic drift must then be minimized and at most correspond to the mutation rate within the population. In 1980, Ian Franklin proposed that there is a balance between the loss of genetic diversity by drift and the gain of genetic diversity by mutation, if 1/2Ne = 0.001, then ne = 500 individuals. This rule of thumb for minimum viable population size is widely used by conservation biologists, although recent meta-analyses have suggested minimum viable population sizes of at least a few thousand individuals. Note that the size of the census population should generally be much larger than the actual population size, as not all individuals in a population contribute to the genetic diversity of the offspring. Many factors can increase the difference between the actual population size and the size of the census population: an unequal number of males and females, a variance between individuals in the probability of producing offspring, a high variance in the number of offspring between individuals, non-random mating, and fluctuations in the number of breeding individuals from one generation to the next. Populations where Ne is equal to the size of the census are considered ideal. An emerging rule of thumb is that if a population begins to fall below several thousand individuals, it has a high probability of disappearing.

The Red List`s evaluation criteria are based on the “50/500 rule”. This indicates that to avoid inbreeding depression (the loss of “fitness” due to genetic problems), an effective population size of at least 50 individuals in a population is required. Recently, a relatively new discipline called phylogeography has been applied to study the principles and processes that determine the geographical distribution of genealogical lineages within and among closely related existing species. Phylogeographic studies focus on understanding the contribution of historical ecological processes to contemporary ecological processes to the design of today`s species distributions. Phylogeographic conclusions are based on DNA sequences taken from the same locus from many individuals collected throughout the geographic range of a species. Statistical analysis is based on coalescence theory, which uses a sample of individuals in a population to trace all the alleles of a gene shared by all members of the population to a single ancestral copy. Sophisticated model-based approaches are used to answer specific questions for the derivation of population history. Such studies can provide significant new information on the processes responsible for the formation of spatial patterns of genetic variation within and between populations and their distribution. So, sensational analogies about the apocalypse aside, do people follow the same rule? We`re not entirely sure, but the evidence suggests that most species from very different groups follow pretty much the same trend. What population size qualifies a small population? Although the most accurate answer depends on the life history of some species, the “50/500” rule of thumb in conservation biology (Franklin, 1980) provides a useful starting point when detailed life history information is lacking.

This rule states that isolated areas with an effective population size (Ne) of more than 500 are relatively safe from the dangers of loss of genetic diversity. These risks gradually increase as the Ne increases from 500 to 50. At Ne 50 is needed for the short>term viability of the population and a Ne>500 for long>term viability. The “50/500” rule was based on the objective of preserving genetic diversity (Franklin, 1980). Shaffer (1981) described the concept known as a minimum viable population. Estimates of an MVP include three parameters: a Ne, a probability of extinction (or persistence), and a specific period of time. For example, “An MVP for a particular species in a given habitat is the smallest isolated population with a 99% probability of being preserved for 1000 years despite the foreseeable effects of demographic, ecological and genetic stochasticity and natural disasters” (Shaffer 1981, p. 132).

Taking into account the dangers of demographic and environmental stochasticity, the minimum Ne should be in the order of thousands rather than hundreds (Traill et al., 2010). The 50/500 rule has been used as a conservation guiding principle for the assessment of the minimum effective population size (N(e)). There is a lot of confusion in the recent literature about how the value of 500 should be applied to assess extinction risk and set priorities in conservation biology. Here, we argue that confusion arises when the genetic basis of a short-term N(e) of 50 is used to prevent inbreeding depression in order to justify a long-term N(e) of 500 to maintain evolutionary potential. This confusion can lead to misleading conclusions about how genetic arguments alone are sufficient to set minimum thresholds for viable populations (PVPs) to assess the risk of extinction of endangered species, particularly those that point out that MVPs would need to multiply by the thousands to maintain evolutionary potential. The ideal population size used in population genetics theory, which would have the same rate of increase in inbreeding or decrease in genetic diversity as the population actually studied. Metapopulations are populations of subpopulations located in a defined area in which a spread from one local population (subpopulation) to at least other habitat sites is possible. There is a significant renewal of the local population, local extinction and resettlement by dispersal. The concept of metapopulation is at the heart of much of ecology and conservation theory, and the dynamics of metapopulations of single and multi-species species is examined by Hanski and Gilpin (1996). The genetically efficient size of a metapopulation is influenced by the carrying capacity of habitat sites, extinction and resettlement rates, the number and source of founders, the number of local populations, and the rate of gene flow between spots.

It is difficult to determine the actual size of the metapopulation on the basis of genetic data, as it is strongly influenced by the dynamics of eradication and recolonization. As with genetically efficient individual population size, the effective metapopulation size in many species is 10 to 100 times smaller than the census size. Metapopulation dynamics, with frequent local extinction and colonization of habitat patches by a few founders, can reduce Ne to a small fraction of N, resulting in a loss of genetic variability associated with a demographic bottleneck. Detailed metapopulation studies are reviewed by Hanski and Gilpin (1997) and include the glanville fritillary butterfly (Melitaea cinxia) and the red bladder campion (Silene dioica), both in Sweden; the checkerboard butterfly (Euphydryas editha) limited to serpentine outcrops in California; and the Pikas (Ochotona principles), a small mammal confined on isolated embankments in alpine areas. Although metapopulation theory is well developed and the relevance of metapopulation theory for the management of small semi-isolated populations of threatened species is clear, the empirical study of ecological and genetic predictions has only just begun. Evolutionary theory predicts that peripheral populations in a species` range are likely to have lower genetic diversity and genetic differentiation due to greater distance and smaller effective population size compared to more central populations (Eckert et al. 2008; Wulff, 1950). This suggests that populations bordering ranges should be less able to adapt to rapid changes in environmental conditions.

In fact, Pearson et al. (2009) found that populations at the southern end of the range of Fucus serratus, a tidal brown algae, were more sensitive to dehydration and increased response to heat shock than their geographically central congeners. This has been interpreted as an increase in cellular stress during heat shock and a decrease in resistance to environmental stressors.