The importance of the timescale of the fitness metric for estimates of selection on phenotypic traits during a period of demographic change

Abstract

Although fitness is central to the evolutionary process, metrics vary by timescale. Different timescales may give rise to different estimates of selection, especially during demographic transitions caused by rapid environmental and socioeconomic change. In this study, we used a dataset of a human population in Finland from 1775 to 1950 to compare two fitness metrics and their estimates of selection pressures, before and during a demographic transition. Both metrics, lifetime reproductive success and an annual metric of individual performance, declined while selection on the ages at first and last reproduction remained nearly constant, favouring individuals with wider reproductive windows. The ability to partition the annual metric into contributions from reproduction and survival revealed the short-term effects of a famine and the reversal of selection pressure via the survival component of annual fitness. Although the metrics generally agreed, the annual metric detected the effects of environmental variation and demographic change occurring within a generation.

What have humans done for evolutionary biology? Contributions from genes to populations

“…Compared to researchers with no-exposure, those with direct-exposure were more likely to agree that humans are still evolving (OR ¼ Thus, in summary, most respondents agreed that humans are still evolving, with the majority agreeing on a role for natural selection in that process, but this varied among levels of relevant education and experience. Importantly, this finding is in line with an increasing number of studies showing that, despite large decreases in mortality and reproduction over the last centuries, and in contrast to some beliefs [4,5], contemporary human populations are subject to selection [6]. In other words, genetic variation still influences survival to adulthood, reproduction, and fitness, fuelling continued evolution by natural selection in current human populations [7].…”

Rainfall variability and fine‐scale life history tradeoffs help drive niche partitioning in a desert annual plant community

“…Indeed, investigating how demographic rates and fitness changes at multiple temporal scales will likely be key to understanding how many populations and communities will respond to changes in both average conditions and in short‐term variability due to climate change (Scranton et al . ; Shriver ).…”

“…However, the relevant scale of variation likely differs from species to species depending on their biology, for example while daily variation in soil moisture is relevant for annual plants, it is likely that deeper rooted organism such as trees experience moisture variation on much longer timescales. Indeed, investigating how demographic rates and fitness changes at multiple temporal scales will likely be key to understanding how many populations and communities will respond to changes in both average conditions and in shortterm variability due to climate change (Scranton et al 2016;Shriver 2016). A growing literature on winter annual plants in the Sonoran desert has greatly expanded our understanding of how annual plants cope with variable environments.…”

The Reproductive Ecology of Industrial Societies, Part I

“…These caveats aside, it is clear that empirical assessments are needed to determine whether natural selection is operating in contemporary populations or whether the strength of, or response to, such selection has changed over time (see also Scranton et al 2016); measuring fitness-related traits such as fertility and lifetime reproductive success is an obvious place to start such an investigation. Thus, the idea that natural selection has been of limited importance in more recent times seems to have little empirical support given the highly suggestive nature of the work cited above.…”