American Society of Naturalists

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“How does the evolution of universal ecological traits affect population size? Lessons from simple models”

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Peter A. Abrams (June 2019)

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Theory shows that adaptive evolution should frequently decrease population size, even in simple systems

When does evolution decrease population size?

Does evolutionary change via natural selection usually increase population size? Do maladaptive processes (e.g., deleterious mutations) generally decrease population size? Many biologists would answer both questions in the affirmative, while acknowledging some exceptions. Nevertheless, many simple and widely used ecological models predict that adaptation via natural selection should often reduce population size (and that maladaptive processes should have the opposite effect). Both processes can be termed ‘adaptive decline’. Adaptive change in any of the three basic ecological properties that characterize all biological species—rate of taking up resources, rate of converting them to offspring, and probability of survival—is expected to lead to an ‘overly high’ rate of resource uptake; i.e., a rate greater than that which maximizes its population size. Overly high uptake rates may cause lower population size due to three mechanisms: (1) higher death rates or lower conversion efficiencies due to tradeoffs with uptake ability; (2) decreased productivity of the resource due to its lower population size; and/or (3) decreased productivity due to increased defense by the resource. Models predict that species experiencing a new environment are almost equally likely to increase or decrease their equilibrium population size as a result of their subsequent adaptive evolution. Even new genotypes that only affect all of the basic ecological properties in a fitness-increasing manner can decrease population size via decreased resource productivity or increased resource defense. Unfortunately, changes in population size during a period of adaptive genetic change are seldom measured, and we know little about the occurrence of overexploitation. If adaptive decline is in fact rare, it implies that our most widely used models of predator-prey or consumer-resource interactions are missing some element that is key to understanding how population sizes change as a consequence of evolution.

The author, Peter Abrams, became interested in this topic while working on its ecological analogue, i.e., when environmental change causes an immediate harm to individuals of a species, but nevertheless increases its ultimate population size.


This article argues that adaptive evolutionary change in a consumer species should frequently decrease (and maladaptive change should increase) population size, producing ‘adaptive decline’. This conclusion is based on analysis of multiple consumer-resource models that examine evolutionary change in consumer traits affecting the universal ecological parameters of attack rate, conversion efficiency, and mortality. Two scenarios are investigated. Under one, evolutionary equilibrium is initially maintained by opposing effects on the attack rate and other growth-rate parameters; the environment or trait is perturbed and the trait then evolves to a new (or back to a previous) equilibrium. Here evolution exhibits adaptive decline in up to one-half of all cases. The other scenario assumes a genetic perturbation having purely fitness-increasing effects. Here, adaptive decline in the consumer requires that the resource be self-reproducing and overexploited, and requires a sufficient increase in the attack rate. However, if the resource exhibits adaptive defense via behavior or evolution, adaptive decline may characterize consumer traits affecting all parameters. Favorable environmental change producing parameter shifts similar to those produced by adaptive evolution has similar counter-intuitive effects on consumer population size. Many different food web models have already been shown to exhibit such counterintuitive changes in some species.