From Holling to Darwin to Gould

CS_HollingThis year marks the 200th anniversary of Darwin’s birth and the 150th of the publication of The Origin of Species. Natural selection has had a fundamental impact on the way we view the evolution of pathogens and their geographies of spread. In Farming Human Pathogens we attempt to better wed Darwin’s contribution to more recent work on the dynamics of ecosystems.

Views of Evolution

Darwin was a gradualist who believed that selection on small variations conferred small improvements in fitness. He largely discounted the influence of climate and believed that the relationships between species of an ecosystem drove natural selection. However, in his time catastrophists were prominent, especially among the geologists, who believed that catastrophic events winnowed out species. As Stephen Jay Gould pointed out, Darwin’s advances came only by way of ignoring the data catastrophists had until then accumulated.

In 1972, Niles Eldredge and Gould published their theory of punctuated equilibrium which combined gradualism and catastrophism: their analysis of the fossil record showed a given species persisting with little change and then suddenly disappearing. The successor species were markedly different and appeared around the time of the disappearance of the first species. By chance, catastrophic events eliminated long-standing species in favor of previously marginalized taxa. The theory raises other questions. Does such a process occur the same way everywhere? Are some ecosystems better able to resist catastrophes and their effects on resident organisms?

Views of Ecosystems

In 1973, C.S. Holling (pictured) published his theory of ecological resilience. From this view, an ecosystem is a mix of loose and tight ties between component species. Where loose ties predominate, ecological impacts—including catastrophes—are damped. The system is resilient. On the other hand, where tight ties predominate, the impact is amplified. It broadcasts across the entire ecosystem and is felt by many if not all resident species. Such impacts can also shift a once resilient ecosystem toward tight ties, as the impacts reduce populations of certain species or remove them entirely and, thus, diminish the functional redundancies on which loose ties depend. Impacts eventually lead to a brittle, fragile ecosystem which drastically and suddenly changes without warning. Holling called this change a “domain shift”.

Implications for Pathogens

In Farming Human Pathogens, we combine the Hollingian ecosystem with the Eldredge/Gould model of evolution. A domain shift acts as a great change in selection pressure and helps drive the great change in species composition, the punctuation. Coupling ecological resilience with punctuated evolutionary equilibria has important implications for human infectious disease and its spread.

The ecosystem of pathogens is the individual host and groups of hosts. Pathogens have evolved two main strategies of survival and spread to deal with the growth of immunity in exposed populations. One strategy is to wait for a new generation of susceptibles to appear and to mutate only rarely, as do the typical childhood diseases (e.g., measles, mumps and rubella) and such infections as small pox. Vaccines work well for immunizing against pathogens with this reproductive strategy.

The other pathogen strategy is to generate susceptibility continually via rapid mutation. Examples of pathogens with this strategy are influenza, Trypanosoma protozoa, HIV, TB, and malaria. For this group effective vaccines are transient at best (flu) or almost impossible to produce (HIV and TB). These diseases thereby represent frequent and serious public health threats.

The physicochemical environment and the socioeconomic system comprise a large part of the modern human host’s ecosystem. Culture and historic trajectories are also important factors in determining the effect of the environment (physical and socioeconomic) on host epidemiology: on individual immune systems, herd immunity, risk behavior, and use of and access to modern health care. Historically oppressed groups typically support fewer redundant loose ties that define resilient ecosystems. Catastrophic impacts, including infectious diseases of our second group, are thereby more easily broadcast through the exposed population. A domain shift in the public health of a population can have a fundamental effect on the evolution of persistent pathogens. From Holling to Darwin to Gould.

In short, the geography of these mutating diseases depends on the intersection of the past and present—for both pathogen and host populations. The occurrence, epidemiology, and evolved virulence of mutating pathogens depend on the state of human ecosystems and their resilience. And a population’s ecological resilience depends on the effects of the population’s political and cultural histories.

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