Variation and polymorphism in Viola

Abstract

In this paper, variation and polymorphism in flowering plants will be illustrated by reference to the genus Viola and in particular to V. riviniana . This is a tetraploid species, probably allopolyploid, with a wide European distribution; it is found on soils of a wide range of base status and texture, and in both woodland and grassland communities from the lowlands to subalpine regions. The species is somewhat gregarious, and large populations are not uncommon. The aim is to indicate the complexity of the populations and of the factors which control them, rather than to single out one aspect for special treatment. Certain size characters which are plastic and probably polygenically controlled will be discussed first. Variation in these characters is not sharply discontinuous; it is correlated with habitat and can be regarded as adaptive. Phenotypic plasticity is common in flowering plants and it is clearly important in habitat adaptation. Thus a range of habitats can be occupied by a single biotype, as, for example, by certain microspecies of Taraxacum and Alchemilla which are obligate apomicts. Even in outbreeding species, populations from different habitats which differ in phenotype can sometimes be shown to differ little, if at all, in genotype. More commonly, however, such phenotypic differences are found to be partly under genetic control; and this is the case in Viola riviniana . Here, dwarf small-leaved forms are found in exposed habitats, such as grassland, and large-leaved and more loosely branched forms in sheltered habitats, such as woodland. Intermediates occur in intermediate habitats, and the series can be regarded as an ecocline. In cultivation under uniform conditions, the differences are diminished but do not disappear; and breeding experiments show that they have become genetically fixed. It would appear that genotypic response is buffered by the direct response of the phenotype to the environment; extreme genotypic specialization thus does not occur, and the process may be able to reverse itself fairly quickly should conditions change. In western Europe most habitats are at best semi-natural, and many have been extensively modified by man. The result is a patchwork pattern of plant communities which are rarely in a state of stable equilibrium; and changes from one type to another, e.g. from grassland to woodland, and vice versa, are frequent. It is likely that plants such as V. riviniana may be able to adapt themselves successfully to the rapidly changing conditions.