Fruits

Evolution can happen at shorter timescales, a fruit fly study shows

Have we grossly underestimated the pace of evolution? That was a question that we sought to answer with our 2014 study on dynamic adaptation in fruit flies. The premise was that if adaptive tracking did occur in populations experiencing rapid environmental change, it would alter our understanding of the importance of evolutionary change for ecological outcomes.

Thus began a large-scale research initiative spread over eight years by evolutionary biologists across three universities — Stanford University, the University of Pennsylvania and Ahmedabad University.

The results of the study — direct observation of adaptive tracking on ecological timescales — in Drosophila was published in the journal Science.

Evolution process

The study has vastly changed our understanding of evolution and questioned the old notion of evolution being a slow process occurring over a longer period of time. We now have evidence that the change is actually happening faster than we ever thought and that not all evolution happens due to habitat destruction, invasion or pollution. It establishes that evolutionary changes happen quickly and researchers can track these in real time — within a single seasonal turn in a year. It’s that rapid.

Experiment on fruit flies

Taking fruit flies ( Drosophila melanogaster) as the sample, we began a large-scale controlled field experiment in Pennsylvania for four months from July-November 2014. What we did was directly measure phenotypic and genomic evolution in response to natural seasonal change across 10 independent replicate field populations (mesocosms) , each comprising up to 1,000,000 fruit flies. This study was vastly different from the ones done earlier to track adaptive changes in shorter time spans.

The population size and replication in our study were the distinguishing factors. We conducted a large-scale research to ensure accuracy of results. Replication was also of utmost importance to establish our premise. Which is why not only was our sample size large but we also had 10 replicate cages (mesocosms), each of which gave comparable results.

Predatory conditions

In these mesocosm cages, the fruit flies experienced the same seasonal changes — heat, rain, cold, they had the same food, and were exposed to the same predatory conditions. Changes were then observed in six phenotypes related to reproductive output or stress tolerance. We sampled individuals from the original population, and 2,500 eggs from each cage at the first four time points of observation. The genomes were sequenced to measure genomic change over time.

Population dynamics were largely consistent among the replicates — population size increased sharply during summer, peaked in early fall and then declined steadily as minimum daily temperatures reduced in late fall. Egg production showed a similar pattern while overall egg to adult growth was low.

historical perspective

Comparable outcomes were seen in fruit flies kept in the replicate cages too. Eminent evolutionary biologists Professor Ary A. Hoffmann from Melbourne University, Australia, and Professor Thomas Flatt from University of Friborg, Switzerland, wrote a perspective piece in the same issue of Science about this study. They appreciated the large study and gave a historical perspective around this long-standing problem in evolutionary biology.

A study as large as this has manifold outcomes. First, in understanding the field life of fruit flies a little more which will have a large impact on our basic understanding of evolution. We hope to see parallel reports in other taxa over the years (that is, observable evolutionary changes in ecological time scales). Also, biologists could better predict the future of species experiencing rapid changes in climatic variables. More work needs to be done on these lines.

Interesting question

Finally, this study raises several interesting questions around species conservation efforts. Do populations with large standing genetic variation turn out to be more successful in handling changing climate and others not?

(Subhash Rajpurohit is Associate Professor, Biological and Life Sciences, School of Arts and Sciences, Ahmedabad University.)

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