Larger context for LACK in Corpus USbrown_UKbncw/UK_bncw.txt DBL-CLICK WDS FOR DEF
The shell-shape patterns reviewed here (and more fully by Crothers, 1983b) coupled with the chromosome variation, can be "explained" by envisaging the N. lapillus populations of the British Isles as comprising two main groups (Fig. 33).
A southwestern faction, bearing the genes for short squat " exposed shore" shells and n = 18 chromosome number, confronting and gradually displacing a northeastern faction which lacks those particular genes and is monomorphic n = 13.
The pattern of "teeth" variation, originally interpreted as purely phenotypic variation dependent upon water temperature, is not far removed (Fig. 20).
Variation in colour, however, does not correlate with the other patterns.
