One challenge in population biology is how to delimit the assemblage of individuals. Population can be anything from the individuals in a resource to all individuals within a species. The population can also be defined from its geographical borders. Practical limitations for wood-decomposing fungi have often been set by the size of the woodland or forest in which the fungus occurs. Although it might be possible to define the geographic borders they may not represent true borders for actual gene flow, since airborne basidiospores can travel over long distances, for example H. annosum spores have been collected on islands more than 300 km from any wood resources (Rishbeth, 1959). From a population genetics point of view, population limits can be defined as when gene flow is below a certain limit. Then a natural approach could be to set the borders at obstacles to gene flow/colonization. This could be either high mountain ranges or vast areas with non-colonizable environment, although what actually constitutes a strong barrier to gene flow is not clear for most fungal species. The frequency of mating then becomes a function of geographic distance, and it is almost impossible to give a fixed limit. Population genetics provides a tool by which we try to understand the range of the actual breeding population.
Also relevant here is the metapopulation approach (Hanski, 1999). In essence a metapopulation is a regional population of local populations each with a certain probability of going extinct. Uncolonized patches can then be recolonized from other populations within the metapopulation system. Hence, the long-term persistence of the species can only occur at the regional or metapopulation level.
The ultimate outer boundary for a population is set by the species limit. The boundary then depends on the species concept, be it morphological, biological or phylogenetic (Harrington and Rizzo, 1999; Taylor et al., 2000; Kohn, 2005; Giraud et al., 2006). While some fungal species defined by morphology show global geographic ranges, when fungal species are defined phylogenetically they typically harbour several to many endemic species (Taylor et al., 2006). It has been further argued that, as a consequence of differences in rates of genetic and morphological change, genetic isolation occurs before a recognizable morphological change. The final step in speciation—reproductive isolation, also follows genetic isolation and may precede morphological change (Taylor et al., 2006). In determining the breeding population both the biological and the phylogenetic species concepts provide basis for experimental work. In Schizophyllum commune, sequencing a number of genes followed by phylogenetic analysis showed that regional populations exist in this cosmopolitan species (James et al., 1999). The biological species concept has been widely applied to wood decomposers (e.g. Korhonen, 1978a, 1978b; Chase and Ullrich, 1990; Hallenberg and Larsson, 1992). On many occasions, partial interfertility has been detected, which can be illustrated by the H. annosum species complex: European members of the S and F intersterility groups are intersterile, but both groups are interfertile with the North American S group (Capretti et al., 1990; Stenlid and Karlsson, 1991). Intersterility is controlled by at least five genes, and it is necessary for at least one of the loci to have a common plus allele to achieve interfertility between two homokaryotic strains (Chase and Ullrich, 1990). The genes have recently been mapped, which should allow for further characterization of the interacting molecules (Lind et al., 2005). The European S, P and F intersterility groups, with preferences for spruce, pine and fir, respectively, have been given scientific names, H. parviporum, H. annosum and H. abietinum, respectively (Niemela and Korhonen, 1998).
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