Recent empirical studies have shown that genetic factors can influence the invasion success of alien species. Like species diversity, higher genetic diversity can increase plant performance or invasion success via selection effects or complementarity effects. The latter have also been referred to as nonadditive effects because in this case individual genotypes show a different performance in mixtures than in monocultures. Based on a manipulation experiment in the field, a recent study found that such non-additive effects of genotype mixing become stronger over time in an invasive plant, probably due to higher resource uptake or facilitation between genotypes. Other researchers criticized this interpretation because in their view complementarity or non-additive effects should lead to transgressive over-performance which could not occur based on selection effects alone, where simply differences of performance between genotypes would enhance the invasion ability of multiple-genotype patches. In a commentary paper we applied Monte Carlo permutation and Loreau and Hector’s additive partitioning to two datasets and found that although transgressive over-performance cannot occur due to only selection effects and thus is the most stringent test for the existence of complementarity effects, complementarity effects also commonly occur without transgressive over-performance.