Harsh Environments and “fast” Human Life Histories: What Does the Theory Say?

A common belief among human life history researchers is that “harsher” environments i.e., those with higher mortality rates and resource stress select for “fast” life histories, i.e. earlier reproduction and faster senescence. I show that these “harsh environments, fast life histories” or HEFLH hypotheses are poorly supported by evolutionary theory. First, I use a simple model to show that effects of environmental harshness on life history evolution are incredibly diverse. In particular, small changes in basic but poorly understood variables e.g., whether and how population density affects vital rates can cause selection to favor very different life histories. Furthermore, I show that almost all life history theory used to justify HEFLH hypotheses is misapplied in the first place. The reason is that HEFLH hypotheses usually treat plastic responses to heterogeneous environmental conditions within a population, whereas the theory used to justify such hypotheses treat genetic responses to environmental changes across an entire population. Counter-intuitively, the predictions of the former do not generally apply to the latter: the optimal response to a harsh environment within a large heterogeneous environment is not necessarily the optimal strategy of a population uniformly inhabiting the same harsh environment. I discuss these theoretical results in light of the current state of empirical research.