Certain types of organisms produce dormant (i.e. idle) stages that can remain in the environment for a very long time. Spores of many microorganisms or the seeds of some plants are typical examples of these dormant stages. It is known that the seeds of many plant species accumulate in the soil for long periods of time and sprout only when the particular location offers convenient conditions, for example after the forest in that location has been destroyed by fire. Just as migrants can transfer genes in space from one local population to others, even very distant ones, dormant stages can transfer genes from one generation to others, also very distant ones, in time. In a similar way as the heterogeneity of the environment and the ensuing heterogeneity of selection pressures can lead to differences in the gene pools of two distant populations, the gene pool of a local population can also gradually change as a result of changing local conditions. Migrants in space and migrants in time can thus introduce alleles into the gene pool of the local population, which are no longer present there or which occur with low frequency. In this way, gene flow in time facilitated by dormant stages can enhance the genetic polymorphism of populations or hinder their optimal adaptation to local conditions (see below).