Portal de Periódicos da CAPES

Stability of a Steep Cline in Morph Frequencies of the Snail Cepaea nemoralis (L.)

1969; Wiley; Volume: 38; Issue: 3 Linguagem: Inglês

10.2307/3039

1365-2656

H. Wolda,

Isotope Analysis in Ecology

There seems to be a general consensus in the literature that polymorphism in Cepaea nemoralis (L.) is maintained by heterozygote advantage, or frequency-dependent selection (Cain & Sheppard 1954; Cain & Currey 1963a; Wolda 1969; Clarke 1962). However, there are still very few facts showing that polymorphism is maintained by either of these mechanisms or, for that matter, that it is maintained at all. The situation is very similar to that in population dynamics where it is often stated that density is governed although critics of this view argue that there are hardly any facts to support this conclusion. Deterministic theories and concepts, such as equilibrium, are of little value for the understanding of natural populations. We must use stochastic theories and concepts instead (Reddingius, 1968). We can formulate two alternative hypotheses concerning the maintenance of polymorphism analogous to Reddingius' stochastic formulation for population dynamics. (1) In a polymorphic population, living in a stationary environment (=a non-constant environment which shows no systematic trend) the morph frequencies are governed, i.e. they are kept within a narrow range of values, deviations from which are counteracted by some governing mechanism such as heterozygote advantage or frequency-dependent selection. (2) In a polymorphic population, living in a stationary environment there are no frequency-governing mechanisms. Morph frequencies usually remain within a narrow range of values because in a natural, i.e. fluctuating, environment, sign and intensity of natural selection vary in time and space. Under either hypothesis there is a positive chance that a morph frequency will reach either 0 or 1, but the probability of such an event will be smaller under the governed frequency hypothesis and also if the population is larger and if the frequency concerned is closer to 05. If the environment is not stationary but changes systematically, then frequencies will be expected to change, unless the various morphs are ecologically identical, i.e. if no selection can occur, a condition which does not seem to be very realistic. Data on stability of morph frequencies in time are directly relevant for the testing of the above hypotheses and such data for Cepaea are rare. The data that are available concern populations that are sampled twice with a fairly long interval in between (Cain & Currey, 1963a; Clarke, Diver & Murray, 1968; Currey & Cain, 1968; Clarke & Murray, 1962; McConnell, 1935; Goodhart, 1956, 1958; Schilder, 1957, 1958). Sometimes significant differences have been found between the two samples, sometimes not. Such data give useful information, but for testing the hypotheses, one must compare the actual amount of fluctuation in morph frequencies with that expected under either hypothesis and this is not known. If differences in morph frequencies are found between the two samples this could mean that either the environment is gradually changing or it