Colour polymorphism in the salt-marsh isopod, Sphaeroma rugicauda; Evidence for stable equilibrium frequencies

If morph frequencies in a polymorphic population are maintained at stable equilibria, then if the frequencies are perturbed they should return to their equilibrium values. Artificial populations of the isopod were set up in cages on the salt-marsh with varying frequencies of the "pattern" morph. The cages were left undisturbed for five months over the breeding season. In the control cages the initial frequency of pattern (0.15) was the same as in the natural population. At the end of the experiment the frequency had not changed significantly, and was not significantly different from the frequency of pattern in the natural population (at the end of the experiment). High frequency cages started with pattern at a frequency of 0.5, showed significant decreases in frequency towards the control values, from which they were not significantly different by the end of the experiment. The average frequency in low frequency cages (started with pattern at 0.06) rose only slighly and was still significantly lower than the average control frequency by the end of the experiment. It is suggested that in these cages the restoring force may have been opposed by random processes, since pattern was lost entirely from some of the cages. The results indicate that the polymorphism is at a stable equilibrium, although the nature of the restoring forces were not identified.