Experimental evolution of competing bean beetle species reveals long-term reversals of short-term evolution, but no consistent character displacement.

Interspecific competition for shared resources should select for evolutionary divergence in resource use between competing species, termed character displacement. Many purported examples of character displacement exist, but few completely rule out alternative explanations. We reared genetically diverse populations of two species of bean beetles, and , in allopatry and sympatry on a mixture of adzuki beans and lentils, and assayed oviposition preference and other phenotypic traits after four, eight, and twelve generations of (co)evolution. specializes on adzuki beans; the generalist uses both beans. growing in allopatry emerged equally from both bean species. In sympatry, the two species competing strongly and coexisted via strong realized resource partitioning, with emerging almost exclusively from lentils and emerging almost exclusively from adzuki beans. However, oviposition preferences, larval survival traits, and larval development rates in both beetle species did not vary consistently between allopatric versus sympatric treatments. Rather, traits evolved in treatment-independent fashion, with several traits exhibiting reversals in their evolutionary trajectories. For example, initially evolved a slower egg-to-adult development rate on adzuki beans in both allopatry and sympatry, then subsequently evolved back toward the faster ancestral development rate. Lack of character displacement is consistent with a previous similar experiment in bean beetles and may reflect lack of evolutionary trade-offs in resource use. However, evolutionary reversals were unexpected and remain unexplained. Together with other empirical and theoretical work, our results illustrate the stringency of the conditions for character displacement.