Evolution under a microscope

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Charles Darwin saw evidence of evolution in all the animals and plants he studied. He boxed, bottled, and pressed thousands of specimens: mockingbirds and iguanas from the Galápagos, corals from the Indian Ocean, lichens and seaweeds from Tierra del Fuego, and even the skull of an extinct giant sloth from Argentina. He turned the gardens and greenhouses at Down House into personal laboratories for breeding pigeons, fertilizing orchids, and dissecting barnacles.

But one form of life was conspicuously absent from Darwin’s work: microorganisms.

Darwin might be impressed at how these invisible creatures have come to reinforce and shape his theory. Over the past four decades, scientists have used microbes to reproduce evolution under laboratory conditions and test hypotheses about how natural selection works, in evolution experiments that have led to surprising new discoveries.

Natural selection is driven by a few basic processes: births, deaths, mutation, selection, and competition. Individuals better adapted to their environment tend to survive and reproduce more successfully than their competitors, passing their advantageous traits down to their offspring. Over generations, beneficial traits increase in frequency in the population until traits that were once found in just a few are shared by many.

In the large organisms Darwin studied, natural selection happens over many thousands of years. This is because these species’ reproductive cycles are long and those with the most adaptive genes do not have many more children than those with the least. More time between generations means more time is needed for genes to spread through a population. However, in species with shorter generation times, it can progress much more quickly.

For example, elephants have a generation time of 20 to 30 years, while the fruit fly grows from egg to adult in a mere ten days. Tens of generations of fruit flies can pass in a month or two, and one elephant takes up as much space in a lab as millions of fruit flies. Geneticists have a long and successful history using flies to study natural selection, but large-scale evolutionary changes, such as the origin of new functions and new species, typically require thousands of generations. Even with fruit flies, experiments attempting to capture ...