Escalation, Interdependence, and Source Populations

Evolutionary escalation of interacting species occurs when the chief agents of natural selection for each species in the interaction are enemies - competitors, predators and pathogens - rather than food species.  Abundant evidence indicates that the direction and extent of selection and of the resulting adaptations vary greatly from place to place and over time.  In the long run, however, much of the spatial variation is drowned out by the disproportionate influence of those populations that, by virtue of their large size and high fecundity, contribute the majority of individuals to subsequent generations.  The large, productive environments occupied by these dominant populations and the pattern of selection and adaptation that prevails in those environments thus influence the characteristics and evolution of each species.  When the location (and therefore the regime of selection) of dominant populations shifts as environmental conditions and geographical configurations change, the characteristics of species potentially undergo rapid adaptive evolution.  Many species in the wild today are threatened because the dominant populations that formerly exist can no longer seed other populations or act as source populations for the species as a whole.  The ability of species to adapt in general and to enemies (including introduced ones) in particular is therefore likely to be compromised.  Conservation efforts should therefore be targeted particularly to source populations and regions in which productivity is high and suitable habitats are extensive.