Evolutionary trade-off causes multigenerational population cycles

Many wild populations undergo seasonality where they cycle between peaks of high and low numbers across several generations. 

Research from four members of the Department of Integrative Biology, Dr. Gustavo S. Betini, Dr. Andrew McAdam, Dr. Cortland Griswold, and Dr. Ryan Norris provides empirical and mathematical evidence that seasonality caused by density-dependence and evolutionary trade-offs may be driving these population cycles.

Dr. Betini, a post-doctoral fellow in the Department of Integrative Biology, first started his research on seasonality during his PhD research when he noticed something unusual in his populations of Drosophila melanogaster (fruit flies).

These populations, unlike regular fly populations kept in the lab, were submitted to seasonal changes in resource. During part of their life cycle, they were allowed to breed in regular fruit-fly food (the ‘breeding season’). But after their offspring emerged, they were given only a limited amount of water and sugar (the ‘non-breeding season’). This medium prevented females from laying eggs, but kept flies alive if the were at low density. After ~30 generations, Dr. Betini realized that these populations were undergoing multi-generational population cycles, unlike regular populations of fruit-flies kept in the lab.

The four researchers combined this observation with his interests in how organisms live in seasonal environments to hypothesize that adaptation to this seasonal environment could be driving these cycles.

Selection for traits that would help increase number of offspring during the breeding season might be harmful to the animal during the non-breeding season. Body size in fruit flies was an example of this evolutionary trade-off. Large flies have increased fecundity and therefore are more successful during the breeding season but as the population density increases during the non-breeding season and food was scarce, large flies were less successful. Dr. Betini had already observed this trade-off in his flies as part of his PhD work and hypothesized that this could be driving the cycles.

They tested this hypothesis using a laboratory experiment where they stopped selection on body size during the non-breeding period by providing high levels water-sugar medium to the flies, which kept them alive during the non-breeding season. They then haphazardly selected flies that would have died as a result of competition for food and removed them from the populations at the end of the non-breeding season.

As expected,  the stop-selection experiment showed no evidence of density-dependent selection, no change in body size after the non-breeding season (non trade-off on body size) and thus, no multigenerational population cycles.

To expand on their analysis, they also developed a mathematical model of the system. When selection on body size and density-dependence was removed from the model, it resulted in the elimination of multigenerational cycles similar to what they had observed in their stop-selection experiment

This confirmed their empirical experiments, that both evolutionary and ecological processes due to seasonal environments result in the persistence of multi-generational population cycles.

Their combination of empirical and mathematical evidence suggest that: seasonality, a life-history trade-off of body size, and density-dependence are three important ecology and evolutionary processes that can affect population cycles and that it may also explain why there are population fluctuations in many other animals.