Mutualism In Arctic Subspecies

Subspecies variation has been repeatedly noted as present in many arctic species (Table 6.2). A common explanation advanced for ecotypic variation is based on the need for plants to optimize their use of resources in response to competition. There is, however, an alternative interpretation for the frequency of ecotypic variation, which does not depend on competition and which has a particular aptness for the High Arctic situation where competition is minimal, and that is the increase in long-term fitness that comes from mutualism.

Selection acting on individuals gives rise to population variations, which as a result of competition are associated with specialized habitats. This increases the habitat range occupied by the species and can be considered as increasing immediate fitness. In the Arctic these ecotypic variants are frequently associated with warm or cold, wet or dry, early or late sites. Thus they can also be considered as increasing long-term fitness by providing a reservoir of genetic variability which has pre-adapted species to climate change. This argument for the mutualistic advantages of ecotypic variation for species survival in fluctuating environments becomes apparent in those regions where the major constraints on survival are physical and not biotic. Competition does exist in the Arctic in favoured sites, but over large expanses of polar desert and semi-desert as well as tundra, there is much open ground where competition is minimal. In such situations a series of polymorphic metapopulations (see Table 1.1) can act as a genetic reservoir.

For the ancient (autochthonous) arctic species which are largely diploid and of low ploidy level the advantages of gene flow can be readily expressed due to the lack of masking genes.

At high latitudes Saxifraga oppositifolia exists with distinct ecotypes adapted to differences in growing season length. In areas with late snow-lie and cold, wet soils, increased metabolic rates and rapid shoot production compensate for ultra-short growing seasons (see Section 3.2.3) but do not conserve carbohydrate or water for adverse periods. An opposing strategy is evident in ecotypes living in sites with an earlier resumption of growth, where soils are warmer and drier and the growing season longer. Here metabolic rates are lower and result in a greater ability to conserve both carbohydrate and water. The existence of opposing strategies for survival in warm and cold habitats suggests that even in the minimal thermal conditions of the High Arctic a high degree of population diversity gives the species as a whole a wider ecological amplitude. This degree of diversity not only increases the range of sites in which the species can survive but confers an ability to adapt to climate change by altering ecotype frequencies to accommodate climatic fluctuations. Such a facility may have contributed to the survival of these polymorphic populations of this and other species in the High Arctic during the Last Glacial Maximum. Table 6.3 lists a number of incompatible characters, both morphological and physiological, which have the capacity to adapt Saxifraga oppositifolia ecotypes to different environmental conditions.

In a review of regional and local vascular plant diversity in the Arctic (Murray, 1997), attention was drawn to the observation that in the Arctic it is only common species that exhibit a broad ecological amplitude, which may be due to phenotypic plasticity of the individuals, or else is the result of the species in question being an aggregation of a series of ecotypes. It is this latter possibility that is advanced here as accounting for the outstanding capacity for survival in the Arctic and sub-Arctic over a very long time of Saxifraga oppositifolia and other common and variable arctic species. The phenomenon, particularly noticeable at high latitudes, by which a species maintains a range of interfertile ecotypes, instead of evolving breeding barriers as a response to competition, has been described as suspended speciation (Murray, 1997) and can be considered as one of the consequences of mutualism.

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