Although the initial choice of left-handed versus right-handed seems to have been arbitrary, the predominance of one or the other does seem to have a definite evolutionary factor.  In those species (mostly terrestrial or freshwater) where variability is common, it seems more difficult for individuals of different handedness to mate than for individuals coiling the same direction.  Consistency, rather than which direction, is what's important.

The sinistral Busycon, along with sinistral cones now extinct, arose in the Early Pliocene in the southeastern U.S., along with various other examples of unusually high variability (wrinkled Mercenaria and Glycymeris, several highly variable species in the Tamiami, Pinecrest, Goose Creek, etc.).  For some reason (we're currently working on some possibilities), variability seems to be at a peak about then.

Dr. David Campbell
    "Old Seashells"

    NEW ADDRESS AS OF 8/21/00
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    Saint Mary's College of Maryland
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    "He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The Gold Bug