Feedback between environment and traits under selection in a seasonal environment: consequences for experimental evolution

Abstract

Batch cultures are frequently used in experimental evolution to study the dynamics of adaptation. Although they are generally considered to simply drive a growth rate increase, other fitness components can also be selected for. Indeed, recurrent batches form a seasonal environment where different phases repeat periodically and different traits can be under selection in the different seasons. Moreover, the system being closed, organisms may have a strong impact on the environment. Thus, the study of adaptation should take into account the environment and eco-evolutionary feedbacks. Using data from an experimental evolution on yeast Saccharomyces cerevisiae, we developed a mathematical model to understand which traits are under selection, and what is the impact of the environment for selection in a batch culture. We showed that two kinds of traits are under selection in seasonal environments: life-history traits, related to growth and mortality, but also transition traits, related to the ability to react to environmental changes. The impact of environmental conditions can be summarized by the length of the different seasons which weight selection on each trait: the longer a season is, the higher the selection on associated traits. Since phenotypes drive season length, eco-evolutionary feedbacks emerge. Our results show how evolution in successive batches can affect season lengths and strength of selection on different traits.

Keywords: fitness components; life-history traits evolution; mathematical modelling; seasonal environment.

Figure 1.

Example of predictions of the model for one strain evolved in a 1%–96 h environment (a) for the total density of cells, (b) for the logarithm of the density of living cells, (c) for the glucose concentration, (d) for the ethanol concentration. Grey curves are the 10 000 best simulations obtained from the ABC algorithm. Black points are experimental data.

Figure 2.

Competition between three strains and non-transitivity. On each panel, the frequency of the strain (blue for strain A, red for strain B and green for strain C) at the end of each batch is represented. The strains are propagated through 100 batches of 90 h. The text indicates the traits which influence fitness the most and are responsible for the invasion in the first batches (on the left) and in the later batches (on the right). Tolerance to ethanol and mortality are selected together, but we only indicate the traits responsible for the positive fitness. Parameter values are given in electronic supplementary material S3.