Douglas, A. E. “Host benefit and the evolution of specialization in symbiosis.” Heredity 81, no. 6 (1998): 599-603.
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Various animals and plants benefit from symbiotic micro-organisms, but the effectiveness of the microbial symbionts (i.e. the amount of benefit that the animal/plant host derives from the symbiosis) varies among symbiont genotypes in natural populations. Recent studies on symbioses with horizontal transmission suggest that three factors may contribute to this variation: (a) selection pressure on the microbial symbionts to exploit the host, resulting in reduced host benefit (b) variation with environmental circumstances in the amount of benefit derived by a host from different symbiont genotypes and (c) unpredictable or low abundance of the microbial partner available to infect hosts from the free-living environment. The latter two factors would counter the selection pressure on hosts to specialize (i.e. to form symbioses exclusively with highly effective symbiont genotypes), despite variation in symbiont effectiveness. Vertical transmission is identified as a key route to host specialization on effective symbionts.