[Seminar Blog] Fall Flies and Spring Flies - The Rapid Pace of Adaptation in Fruit Flies

written by: Ben Gregory

Compare the ears of these two species of rabbits: the Eastern cottontail rabbit (left), native to the temperate forests of the eastern United States; an6d the black-tailed jackrabbit (right), which lives in the deserts of the Western US and Mexico. The jackrabbit has much larger ears relative to the size of its body. Evolutionary biologists think that this species’ large ears may be an adaptation to deal with high temperatures. Rabbits living in the desert face a much higher risk of overheating than rabbits living in the much cooler forests on the east coast. Larger ears allow for more surface area for the rabbits to dissipate heat into the air far from their high-temperature core. Natural selection on rabbits in these very hot environments favored rabbits with larger ears because rabbits with smaller ears were more likely to experience the negative consequences of overheating. Over many generations, this continued pressure led to rabbits with larger and larger ears, while rabbits on the east coast did not experience this pressure, and so did not gain this trait.

These rabbits demonstrate that animals have adaptations that help them to survive the specific stressors in their environment. But what if those stressors change over the course of the year? Snowshoe hares (Lepus americanus) which live primarily in Canada and Alaska, deal with seasonal change by molting into a snow-camouflage white coat in the winter, and then molting back into a brown coat when spring and summer come. This type of adaptation is an example of phenotypic plasticity, and allows individuals to adapt to changing environmental conditions within their lifetime. Unlike the jackrabbit example above however, changes in traits mediated by phenotypic plasticity are not encoded in the underlying genetic code. There simply aren’t enough generations of rabbits during the change from winter to summer for these adaptations to occur within the genome.
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Insects, on the other hand, breed quickly, often go through multiple generations per year, and we have a wealth of genetic information to probe underlying genetic changes. Fruit flies (Drosophila sp.), for example, live for only a few weeks and go through many generations over the course of the year. Most of these generations occur in the summer, when it’s warm and they can breed quickly. They wait out the winter in a hibernation-like state called diapause. But do these fast generation times result in genetic differences at different times of year? That’s the question that Dr. Alan Bergland and his team sought to answer. 

The team collected flies from a fruit orchard in Pennsylvania multiple times throughout the year in 2009, 2010, and 2011. Then, they divided these samples into spring and fall samples and performed pooled, whole genome sequencing. From this sequencing effort, they were able to calculate the frequencies of a large number of genetic variants (SNPs, Single Nucleotide Polymorphisms). They compared these across time and identified SNPs that fluctuate in the populations from season to season.

Insects, on the other hand, breed quickly, often go through multiple generations per year, and we have a wealth of genetic information to probe underlying genetic changes. Fruit flies (Drosophila sp.), for example, live for only a few weeks and go through many generations over the course of the year. Most of these generations occur in the summer, when it’s warm and they can breed quickly. They wait out the winter in a hibernation-like state called diapause. But do these fast generation times result in genetic differences at different times of year? That’s the question that Dr. Alan Bergland and his team sought to answer. 
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The team collected flies from a fruit orchard in Pennsylvania multiple times throughout the year in 2009, 2010, and 2011. Then, they divided these samples into spring and fall samples and performed pooled, whole genome sequencing. From this sequencing effort, they were able to calculate the frequencies of a large number of genetic variants (SNPs, Single Nucleotide Polymorphisms). They compared these across time and identified SNPs that fluctuate in the populations from season to season.

Bergland et al., 2014

This result is an indication that, yes, there are seasonal adaptations in wild populations of Drosophila. A population collected in the spring, right after the flies are coming out of diapause in the cold winter, will have different genetics than a population collected in the fall, after the populations have been through many generations in the heat of the summer, and this trend seems to repeat year after year. Dr. Bergland and his team next asked whether these seasonally fluctuating variants are related to survival at different temperatures.

By identifying low temperature frost events, and comparing the allele frequencies of those SNPs that were identified to fluctuate seasonally before and after these events, the researchers showed that extreme temperature events do seem to have a meaningful effect on the genetic composition of the populations. Populations after the frost events look more like “spring” populations, while populations before the frost event look more like “fall” populations. This provides some initial evidence that temperature is, in fact, at least one of the factors driving this rapid adaptation throughout the season. However, more research is needed to draw stronger conclusions on the influence of temperature, and to identify whether these seasonally-fluctuating SNPs code for traits that help with tolerance of extreme temperatures. 
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Ultimately, these results provide evidence that adaptive natural selection can occur rapidly over the course of the year in a seasonally changing environment in populations of animals with short generation times. This means that, rather than populations settling on an “optimal” genotype that allows them to resist the changing temperatures, the changing temperatures maintain genetic diversity that fluctuates over the course of the year, a different but still stable setup. These results will help inform our understanding of how animals–insects in particular–will adapt to changing climates and shifting seasonal trends as the Earth warms due to climate change. Longer-term adaptations require genetic diversity in the population, and seasonally-fluctuating SNPs represent one mechanism that maintains that diversity. Rather than taking place over hundreds of thousands or millions of years, insects keep showing us that evolution can happen on shorter timescales that we previously imagined.