Biologists are continually refining their ideas of the relationships of organisms and, as a result, groups are often redefined or undergo name changes as our knowledge increases. There is always a temptation to take evolutionary trees as representing progress. Evolution does not necessarily mean progress. Mollusks have been evolving for more than 550 million years, an immense amount of time for speciation and divergence. 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. Species Species are often patchy in distribution, and may not be found everywhere throughout their ranges, often because of lack of appropriate habitat in a given area.