As natural selection acts on the limiting components of biological systems, those features that are limiting are likely to show evolutionary change. This article has described several ways in which tick-borne pathogens are limited by host factors, but these alone do not appear to account for their evolutionary history as inferred from phylogenetic analysis. Climate imposes a further, more fine-grained limitation through its impact on the distribution, abundance, seasonal population dynamics and host relationships of ticks. The precise quantitative identification of these critical climate variables for each tick-borne disease system has only just begun. The quest raises a question of fundamental ecological significance: how have purely abiotic factors, climate, directed the responses of three distinct but interacting biotic agents -arthropod vectors, vertebrate hosts and micro-parasites - resulting in the evolution and dissemination of new strains of pathogens?

If climate has been a significant force in shaping the past evolutionary pathways of these pathogens, it is likely to be important in future events on both evolutionary and ecological timescales. Identifying the precise nature of these forces will provide a powerful tool for making reliable predictions, not necessarily about the appearance of new pathogens in the future, but about new appearances and spread of existing pathogens. Arguably, the latter may lead to the former - an expansion of the ecological envelope, exposing pathogens to new selective pressures. Because of their higher than average transmission potential, vector-borne, rather than directly transmitted, infections are likely to appear in places where environmental conditions suddenly become permissive through environmental change. But which pathogens are likely to emerge in new places and where will this be?

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