by Dr. Gary Rosenberg

Origins of Diversity

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.