³Lower² animals, particularly clonal invertebrates, show well developed mechanisms of kin discrimination and tend use these mechanisms to exclude unrelated invididuals from habitat patches. Highly polymorphic (hypervariable) genetic systems allow allorecognition. This interesting paper deals with a hydroid that lives on gastropod shells occupied by hermit crabs (how specialized can you get?). The hydroids colonize the shells and reproduce clonally. If a shell is colonized by multiple hydroids then they have the option of fusing or fighting. The choice is driven by the degree of genetic similarity between the pair of hydroids (allorecognition): Hart MW, Grosberg RK 1999 Kin interactions in a colonial hydrozoan (Hydractinia symbiolongicarpus): Population structure on a mobile landscape. Evolution 53:793-805. Hart and Grosberg give a detailed discussion of their view on the factors that maintain the hypervariability in the allorecognition system in this species. Another good paper on lower animals is: Cohen, C.S., Y. Saito and I. L. Weissman. 1998. Evolution of allorecognition in botyrellid ascidians inferred from a molecular phylogeny. Evolution 52:746-756. [Ascidians = sea squirts] ³Phylogenetic reconstruction of the evolution of allorecognition behavior using molecular systematics in this study shows that allorecognition at the whole colony level for purposes of space competition is unlikely to have been the original evolutionary driving force for a highly polymorphic recognition system in which botyrellid ascidians, and perhaps, other colonial protochordates as well.² They go on to suggest that allorecognition evolved for internal discriminatory functions and may later have been ³coopted² for external space competitive purposes.