Evolution usually occurs so slowly that it's imperceptible to the naked eye. Simple adaptations can take thousands of years to take hold, far too long for any of us to notice. But just because it's slow doesn't mean it's not happening. Humans, for example, aren't growing wings or gills (as far as any of us can tell), but we are evolving "faster than ever before," mostly in subtle ways such as food tolerance, disease resistance, and brain size.
One fascinating project that was started decades ago has allowed scientists to track and measure evolutionary changes in real time by speed-running evolutionary adaption in a heavily controlled lab setting.
In 1954, Japanese ecologist Syuiti Mori launched what would become one of the longest-running active experiments in the world. He enclosed a colony of fruit flies in glass bottles, covered them with cloth, and placed them in complete darkness.
They would remain there for the next 72 years and counting.
Dark flies and wild fruit flies look almost identical, but have significant genetic differences. Photo by Luke Peterson on Unsplash
Fruit flies are often used in this type of research because they reproduce incredibly quickly. Eggs typically hatch into maggots within a day or two. Within two weeks, they mature into sexually active adults that begin mating and laying eggs. New generations can cycle rapidly, which makes fruit flies the perfect candidate for tracking genetic changes over time.
Perhaps more surprisingly, fruit flies actually share quite a bit of DNA with humans. Researcher Stephanie Mohr tells Harvard Health that fruit fly genes are about 60% similar to human genes, with even greater overlap in disease-related markers. That makes them a surprisingly effective model for studying human genetics at a simplified level.
Mori's flies bred in complete darkness, rapidly producing generations that had never seen the light of day. Today, the experiment continues under successive generations of leading researchers, during which the colony has produced more than 1,500 generations.
Researchers access, feed, and study the flies using a weak red light that the flies cannot detect. For decades, they've been able to perfectly control the conditions from generation to generation. The Genetics Society of America writes: "The stock of flies has now spent more than 1,500 generations without light. In human terms, that would be like sequestering generations of our ancestors in the dark for 30,000 years."
Adaptation is slow, but that is more than enough time to measure changes, if any occur.
The project has been passed down from researcher to researcher for 70 years. Photo by CDC on Unsplash
The research team quickly discovered that after several generations, the new variety they had created, fittingly called "Dark-fly," performed better in darkness than wild flies.
The two subspecies look nearly identical, but the Dark-fly has longer head bristles used for sensory perception, a stronger sense of smell, and a tendency to lay more eggs in dark conditions.
Researchers even created mixed colonies of wild flies and dark flies, allowing them to interbreed in darkness, and found that Dark-fly DNA was more likely to be passed on to future generations.
In 2012, the full genome of the Dark-fly was sequenced by Naoyuki Fuse, who had taken over the project a few years earlier.
According to Bioedonline.org, "Fuse and his colleagues sequenced the dark-fly genome, identifying 220,000 single-letter differences in its DNA and several thousand larger DNA insertions or deletions, compared with a normal fruit-fly strain."
The team was able to pinpoint which genes changed over time and how, offering rare insight into how evolution works.
Scientists are quick to note that living in darkness does not directly cause changes in the flies' DNA. Instead, natural selection favors mutations and traits that help the flies thrive, making them more likely to be passed on to future generations. Some differences may simply be due to random chance, or genetic drift.
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Adaptations in extreme environments have been observed before. Some insects, including bedbugs, are becoming resistant to pesticides. The Mexican tetra, also known as the blind cave fish, has lost functional eyesight after generations of living in pitch-black caves. And a number of animals have adapted to rapidly warming climates.
But recording and measuring these changes in a controlled laboratory environment is extremely rare. Outside the Dark-fly experiment, other long-running studies have tracked colonies of E. coli and yeast under various conditions. But there's something incredibly exciting about observing a living, breathing species as it rapidly adapts to thrive in unnatural conditions.
One day, studies like this one could help scientists rewire human brains and genetics to protect against disease, among other amazing applications.
Worried mother and children during the Great Depression era. Photo by Dorthea Lange via Library of Congress 
A mother reflects with her children during the Great Depression. Photo by Dorthea Lange via Library of Congress 
Families on the move suffered enormous hardships during The Great Depression.Photo by Dorthea Lange via Library of Congress
Dark flies and wild fruit flies look almost identical, but have significant genetic differences. Photo by 
The project has been passed down from researcher to researcher for 70 years. Photo by