Modeling reproduction/longevity tradeoffs and phenotypic plasticity in fluctuating environments
A year ago, I was passing through beautiful Brisbane (in the news recently because of disastrous flooding) on my way back from the Applied Evolution Summit on Heron Island. This week, I'll discuss one figure from a paper I wrote for that meeting. An online-early version of "Past evolutionary tradeoffs represent opportunities for crop genetic improvement and increased human lifespan" is up at Evolutionary Applications, which will publish a special issue of papers from the meeting.
The basic idea is that, if there is even a small trade-off between reproduction and longevity, natural selection will sometimes sacrifice reproduction in favor of longevity. The graph shows predicted numbers for three genotypes, based on a computer model (Hormesis8.py). Fertile adults are assumed to have a 25% chance of dying in a given year, while those delaying reproduction have a 20% chance of dying. That slightly lower chance of dying isn't enough to give genotypes that always delay reproduction (dots) an advantage over those that never delay reproduction (solid line).
But what if individuals delay reproduction only when some environmental cue predicts a population decline? That's what the dashed line represents. In good years, their population grows as fast as the never-delay genotype. In bad years, 50% of the dashed-line genotype are assumed to detect the population-decline cue and delay reproduction, so that they die off a bit more slowly. In an environment where conditions fluctuate (assumed to affect the survival of juveniles), this form of phenotypic plasticity beats both of the fixed strategies.
So what's the "population-decline cue"? The model just assumes there is such a cue and that half of the dashed-line individuals detect it. In a PLoS One paper in 2009, we proposed "eating famine foods" as an example of such a cue. We hypothesized that our ancestors ate less-desired foods mainly when more-desired foods weren't available, that is, during a famine. Famines often lead to population declines. Delaying reproduction until after the population has declined means that each offspring makes a greater relative contribution to the gene pool.
Plants high in insect-repelling toxins might be an example of such "famine foods", even if some modern humans have developed a taste for kale, coffee, or hot peppers. These plant toxins might have small negative effects on our health. But, if our bodies respond to the information carried by those toxins -- famine! population decline likely! delay reproduction! -- then those negative effects may be outweighed by the health benefits of setting our hormone levels etc. to values optimized for longevity rather than reproduction.