ABSTRACT

S.M. Rudman et al, Science 375, eabj7484 (2022), DOI: 10.1126/science.abj7484

It is a common belief that evolutionary changes take place at a much slower pace than ecological change. However, this is not completely true, and there is now mounting evidence to the contrary albeit under certain circumstances. In this line of thought, an experimental study of Drosophila melanogaster, or fruit fly, has shown that the pace of adaptation can match that of environmental and seasonal changes. The work, published in Science, studies adaptive tracking, which is defined as continuous adaptation in response to rapid environmental change. Adaptive tracking is known to be the critical mechanism by which living beings continue to thrive in a changing environment, but little is known about the pace, extent and magnitude of adaptive tracking in response to a changing environment.

Preferred model

Rudman et al conducted experiments on 10 independent replicate populations of Drosophila melanogaster, each being a group of up to 1,00,000 individuals. The flies were kept in boxes 2m X 2m X 2m in size and placed near a dwarf peach tree, outdoors, in Philadelphia.

The fruit fly is the preferred animal model for many experimental studies of evolution because it is relatively easy to breed and maintain, and multiplies rapidly, allowing many generations to be studied in a short time. The researchers measured the evolution of heritable and observable physical characteristics over time. These could be stress tolerance such as survival under cold, hunger and dessication or related to fitness or reproductive traits such as developmental rate and egg-laying, respectively.

The flies were exposed to a changing season from July-December in 2014, and monthly measurements were made. The authors write that they focussed on generating highly accurate measurements, taking these measurements on a time-scale matching that of environmental change and collecting measurements from 10 independent populations. Taking measurements from parallel lineages of fruit fly and finding similar changes in the populations which did not themselves interact ensured that the changes were indeed due to selection and not due to random inherited factors.

There was an interesting observation, as pointed out in a Perspective piece about the work in Science, written by Ary H. Hoffmann and Thomas Flatt, that the rate of the phenotypic (observable, physical characteristics of an organism) evolution varied according to the trait. While chill coma recovery, a marker of resistance to cold, increased as winter progressed, dessication resistance increased, plateaued and then decreased. But overall, the rates of evolution of these traits was rapid and matched the requirements of adaptive tracking.

While the paper establishes that fly populations can rapidly adapt to seasonal changes, the authors of the Perspective article remark that this may perhaps be anticipated theoretically because fruit fly populations are large, with huge genetic variation, which makes the effects of rare mutations on the evolution minimal. This means even a weak selection could be sufficient to drive a fast evolution. However, they point out that while theories suggest that adaptive tracking may be hindered by factors such as the reduction in fitness due to the lag in adaptation after an environmental change, this is not observed in the real drosophila populations.

Evolutionary rates underestimated

The researchers conclude that this experiment demonstrates how you can see adaptive tracking in response to environmental change in real time by observing multiple parallelly evolving populations. The action of fluctuating selection implies that evolutionary rates may have been underestimated and that fluctuating selection may play a role in maintaining biodiversity. They write: “Determining whether adaptive tracing is a general feature of natural populations, and elucidating the mechanisms by which it occurs, can be transformative for understanding the generation and maintenance of biodiversity.”