Many small, numerically abundant animals of low trophic position exhibit
polymorphisms for colour and pattern. This variation is assumed to be adaptive and
maintained by frequency-dependent selection, the fitness of phenotypes being
negatively correlated with their frequency in the population.
Noctua pronuba is an abundant moth species of the western Palaearctic and is now
established in north eastern America. When the moth is at rest it is apparently
cryptically coloured with the visible surfaces polymorphic in colour and pattern. The
limitation of this variation to the exposed surfaces of the moth suggests that the
variation may be maintained by visual selection by predators.
The forewing polymorphism in N.pronuba can be split into three distinct phenotypes:
rufous, ochre and silver, with the variation probably controlled by a single locus, with
three alleles in a dominance hierarchy. The expression of these alleles is
influenced by sex with females lighter in colour than males. The aim of this work
was to establish whether natural selection maintains variation in forewing colour and
pattern to give a balanced polymorphism, using the null hypothesis that the variation
was neutral to selection and non-adaptive. This has been approached by
experiment and the analysis of temporal and geographic variation in forewing
Background resting experiments failed to show different phenotypes adopting
different backgrounds on which to rest but experimental conditions did affect the
behaviour of individuals. Selection acting in opposite directions on males and
females has been suggested as a mechanism maintaining the forewing
polymorphism. Little evidence has been found to substantiate this claim with
phenotype frequencies in light trap samples similar in males and females. No
systematic changes in phenotype frequencies were observed in an analysis of
temporal variation over a twenty-five year period. Only the ochre allele varied
significantly but the variation is minimal suggesting that the polymorphism is
Geographically there was remarkably little variation in phenotype frequencies with
only samples from Finland, Scotland and N.lreland having significantly differentiated
A study of polymorphic allozymes suggested that large amounts of gene flow occur
in the species. The consequence of this gene flow will be to unite geographically
separate populations into one panmictic unit. High levels of gene flow, in
conjunction with the local abundance of the species, mean that the effective
population size will be large.
Previous authors have considered that as crypsis is an adaptive trait, variation in the
colour and pattern of a cryptic species must also be adaptive, and maintained by
selection. This is not necessarily true, and there may be a number of colour
patterns that are equally cryptic in the same habitat. It is hypothesised that the
large population size and magnitude of gene flow in N.pronuba gives the forewing
polymorphism inherent stability both temporally and geographically, without the
need to invoke balancing selection.
Faculty of Health and Life SciencesDepartment of Biological and Medical Sciences
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