Although some species do show little to no change through their duration,
others show either trends or simply variability through time.  This may be
related to environmental conditions: if conditions are very easy, variation
may not matter much; if conditions are somewhat variable, sticking to a
particularly effective pattern may be the best; if conditions are really
bad, change may be necessary (and competition low).  "If it ain't broke,
don't fix it" applies to evolution as well as machines.  If a particular
organism is well-suited to its environment, and mutations are significantly
detrimental, then evolution will cause it to not change.

Molluscan examples are common, given their good fossil record.
Macrocallista maculata shows almost no change in shape from at least the
mid-Pliocene if not the early Miocene to the present, whereas the ancestors
of Macrocallista nimbosa (M. reposta, M. greeni, etc.) show a gradual trend
through time and space from more rounded (older, more northern) to more
elongate shape.  (By the way, UV light can be used to show that the shell
color patterns have not changed).

Several species show high variability in the lower Pliocene in the
southeastern U.S.  This is when the sinistral Busycon and Conus, wrinkled
Glycymeris americana (forms quinquirugata and aberrans) and Mercenaria
(form "tridacnoides"), and various other variations appear.  A splitter can
come up with lots of species that, when compared with enough other
specimens, all intergrade.

David Campbell

"Old Seashells"

Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA

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919-962-0685
FAX 919-966-4519

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug