Zooplankton show a unique pattern of evolution with successive waves of invasion into the water column from the benthos. We have found that 18 of 21 planktonic groups whose ancestry can be traced originated in the benthos. New recruits have survived and radiated if preadapted to remain in the plankton, but no major clades have evolved there (with the possible exception of some protists). The innovative steps into the planktic realm do not coincide with major global events such as mass extinctions. Recruitment into the plankton can occur either at the larval stage or in adulthood. No groups have returned to a benthic mode of life from a planktic one, except possibly some of the cnidarians. This unusual pattern of evolution, a one-way track into a particular environment, demonstrates the profound effect of the ecosystem on large-scale patterns and processes of evolution.

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