218 The Evolution of Coinmimities 



of its close relatives (Fig. 93) (Michener, 1944; Linsley, 1958). 

 The parasitic bees of the genera Psithtjrus and Sphecodes, for ex- 

 ample, are morphologically so similar to the bees they parasitize 

 that the ancestral parasitic species in each genus undoubtedly 

 started out simply as a "lazy" or cuckoo bee laying its eggs on the 

 provisions of a sister species. This might readily be traced to a 

 situation in which two previously allopatric species having iden- 

 tical nesting habits came to occupy the same range so that the 

 nest-finding instincts of one would frequently lead it to the nests 

 of the other. 



The discovery of a host for the parasitic conifer Podocarpus iistus 

 of New Caledonia illustrates another case of one species becoming 

 a predator on a closely related form. This unique gymnosperm, a 

 woody shrub with some chlorophyl in its leaves, was found grow- 

 ing out of a specimen of Dacrydium taxoides, a closely related tree 

 member of the same family Podocarpaceae (de Laubenfels, 1959). 



The First Communities 



From the foregoing observations one may conclude that as soon as 

 geomorphic changes resulted in a mixing of species, these mixtures 

 evolved into the first biological communities which had the same 

 fundamental characteristics as those occurring today. Competition, 

 chance inherent differences in the species, and innate ecological 

 differences within the range of the mixtures would interact and 

 constitute the mechanisms of this evolution. Without much doubt, 

 the first relationships which evolved were coexistence and exploita- 

 tion or the predator-prey relationship. It seems probable that even 

 in mixtures of only a few species both of these patterns of inter- 

 species relationships were evolving simultaneously. 



From these community relationships emerged an ecological ag- 

 gregation progressing through time. Each aggregation is a mixture 

 of species held together by ecological forces, in part obligatory as 

 in exploitation and in part simply coincidental because of the similar 

 ecological tolerances of the species. However, each individual phy- 

 logenetic line in this ecological aggregation has either a potential 

 or a constant influence on the success of other companion phy- 

 logenetic lines. 



EVOLUTION OF COMPLEX COMMUNITIES 



After communities became established they gradually became more 

 complex until now they are vastly richer in species and far more 



