Most organisms experience competition for resources, probably most of the time. As the structure and requirements of closely related species are generally liable to be more similar than in distantly linked species, Darwin suggested that the potential for competition was greater in the former. Since that time, studies have concentrated on interactions of either conspecifics or congeneric species. Shared critical resources, which organisms compete for, are generally mates, food and space (for access to the former). Whilst mates are valued only within species, in that the definition of a species requires it so, both food and space have the potential to be shared by very different organisms. It is now clear that vertebrates may compete with remotely related species: e.g. with squid for krill and with insects for nectar or seeds. Diamond suggested that (i) mutual aggression, (ii) displacement and (iii) evolutionary change in morphology would be increasingly asymmetric with competitor dissimilarity. Thus, with increasing taxonomic distance between two competitors (A and B), increasing aggression is exhibited between them and, increasingly, one consistently displaces the other. Here, Darwin' suggestion and Diamond' first two theories are tested across a taxonomic spectrum for the first time to the best of the author' knowledge. The proportion of spatial competitors in two different marine invertebrate groups demonstrating mutual aggression and displacement increases with taxon divergence (Nei' genetic identity). Congenerics were twice as likely to fight as conspecifics, and confamilial competitors were three times as likely to fight as conspecifics. This relationship seems robust to taxonomic and environmental variability. Competitors do not need to be as distant as birds and bees for complete asymmetry, a different family seems sufficient.