A fitness trade-off between seasons causes multigenerational cycles in phenotype and population size.

Gustavo S Betini,Andrew G Mcadam,D Ryan Norris,Cortland K Griswold

doi:10.7554/elife.18770

Abstract

Although seasonality is widespread and can cause fluctuations in the intensity and direction of natural selection, we have little information about the consequences of seasonal fitness trade-offs for population dynamics. Here we exposed populations of Drosophila melanogaster to repeated seasonal changes in resources across 58 generations and used experimental and mathematical approaches to investigate how viability selection on body size in the non-breeding season could affect demography. We show that opposing seasonal episodes of natural selection on body size interacted with both direct and delayed density dependence to cause populations to undergo predictable multigenerational density cycles. Our results provide evidence that seasonality can set the conditions for life-history trade-offs and density dependence, which can, in turn, interact to cause multigenerational population cycles.

Highlights

In many organisms, reproduction is confined to seasonal fluctuation in periods of high resource, in which both fecundity and viability selection can occur, and periods of low resources, when reproduction stops and natural selection occurs only through viability selection
In order to address potential environmental changes in the lab, we conducted a third experiment using the same protocol as in the ’long-term control’, but under the same initial conditions and at the same time as the ’stop selection’ treatment. This ’short-term control’ experiment had 13 replicate populations tracked over 31 generations (Figure 1). In addition to these experiments, we developed a mathematical model to investigate the contributions of both viability selection and delayed density dependence to population dynamics
The autocorrelation functions (ACF) function was characterized by stationary periodic dynamics, which resulted in an oscillatory decay to zero (Figure 2a inset)

Summary

Introduction

Reproduction is confined to seasonal fluctuation in periods of high resource, in which both fecundity (reproduction) and viability (survival) selection can occur, and periods of low resources, when reproduction stops and natural selection occurs only through viability selection. When body size is positively related to fecundity, but small individuals survive better in the non-breeding season (Stockhoff, 1991; Munch et al, 2003; Monaghan, 2008; Betini et al, 2014), these opposing patterns of selection could cause population cycles if selection is density-dependent. We tested the role of viability selection during the non-breeding season by tracking 13 additional populations over 31 generations using a similar protocol to the ‘long-term control’, but in which we experimentally prevented viability selection in the ‘non-breeding’ season by providing high levels of food during this season (the ‘stop-selection’ treatment; Figure 1) This protocol maintained direct density effects on fecundity and Figure 1. We first investigated if lag effects were present in all three experiments, and used the mathematical model to understand whether they played a role in the dynamics of their populations

Results

Long-term control

Discussion

Funding Funder