by Dr. Gary Rosenberg
There are certainly more than 70,000 living species of mollusks
worldwide and probably more than 100,000, classified in thousands
of genera and hundreds of families. Mollusks have been evolving
for more than 550 million years, an immense amount of time for
speciation and divergence. Divergence can occur whenever a population
(a group of organisms of one species) is split into two or more
parts that have no chance to interbreed. This can happen as
a result of chance dispersal to isolated islands or through
geological changes, such as the uplift of the Isthmus of Panama,
which separated the Western Atlantic and Eastern Pacific.
Speciation can result from isolation.
Because of natural selection, separated populations of a species
will adapt to different conditions in different regions, perhaps
developing different mating behaviors or breeding seasons, or
accumulating enough genetic differences to render egg and sperm
incompatible. If they are reunited, they might not be able to
interbreed, or hybrid offspring might be at a disadvantage relative
to purebred ones. In organisms with two sexes, populations that
cannot interbreed are usually regarded as separate species.
In practice we usually do not know whether two supposed species
can interbreed successfully. Thus we usually must rely on morphological
(and genetic) criteria to define species.
The above material
has been adapted from Dr. Rosenberg's The Encyclopedia of Seashells,
published by Robert Halt, Ltd., London, 1992.