Summary
The frequencies of banding morphs in Cepaea nemoralis, and in its sibling species C. hortensis, have been examined in relation to the relative distributions of the two species.
The frequency of unhanded C. nemoralis decreases significantly on moving from allopatry to sympatry. This occurs in two well-separated populations. In C. hortensis, the frequency of unbandeds also decreases in sympatry. Again the effect is shown in two different populations and is significant in both.
It is argued that this particular type of parallel variation in morph-frequency in two species, which would not result from interspecific hybridisation even if this were to occur, constitutes good evidence for natural selection (directional rather than balancing). It is stressed that the data present a strong case for each species being the selective agent acting on morph-frequency in the other; but also that the data are not consistent with the theory of character displacement.
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Arthur, W. Morph-frequency and co-existence in Cepaea. Heredity 41, 335–346 (1978). https://doi.org/10.1038/hdy.1978.104
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DOI: https://doi.org/10.1038/hdy.1978.104
