THE EFFECT OF COEXISTENCE ON COMPETITIVE OUTCOME IN TRIBOLIUM CASTANEUM AND TRIBOLIUM CONFUSUM.

We describe an experiment exploring the effects of coexistence and population differentiation on the competitive outcome of two species of Tribolium flour beetles, T. castaneum and T. confusum. The only manipulation was whether the populations used in the competitive phase of the study were raised initially in mixed-species communities, single-species populations, or in the standard culture conditions used to maintain stocks in the laboratory. Any treatment effects observed were due to natural selection acting within populations and genetic drift. In the competitive phase, we examined 10 mixed-species communities and 10 pairs of single-species populations. We replicated each community 15 times to provide an assessment of the distribution of competitive outcome. Statistical analysis demonstrates the lineages within the treatments became highly differentiated for all measures of competitive outcome: the outcome of competition (which species won), time to extinction of one of the competing species, and census history. The fraction of the variance that is among lineages has been referred to as the group or community heritability. All of these measures of competitive outcome had high community level heritabilities indicating that competitive ability would evolve rapidly as a result of group or community level selection. In contrast, competitive outcome was not affected by whether the two species had coexisted prior to the competitive phase. This indicates that the outcome of competition was not systematically changed by processes acting within the two species communities.