NON-MUTUALISTIC YUCCA MOTHS AND THEIR EVOLUTIONARY CONSEQUENCES

Interspecific mutualisms are regarded as having evolved from antagonis tic or commensalistic interactions, with most mutualisms remaining fac ultative but some having coevolved into obligate reciprocal dependency (1-4). Underlying mutualism is an intrinsic conflict between the parti es, in that each is under selection for increased exploitation of the other(3-7). Theoretical models suggest that this conflict is a source of evolutionary instability, and that evolution of 'cheating' by one p arty may lead to reciprocal extinction(4,6,7). Here we present phyloge netic evidence for reversal of an obligate mutualism: within the yucca moth complex, distinct cheater species derived from obligate pollinat ors inflict a heavy cost on their yucca hosts by laying their eggs but not pollinating the yucca. Phylogenetic data show the cheaters to hav e existed for a long time. Coexisting pollinators and cheaters are not sister taxa, supporting predictions that evolution of cheating within a single pollinator is evolutionarily unstable. Several lines of evid ence support a hypothesis that host shifts preceded the reversal of ob ligate mutualism. Host or partner shifts is a mechanism that can provi de a route of evolutionary escape among obligate mutualists in general .