Dumpster diving has never been my cup of tea.
But when it comes to science, it turns out that you can convince me to tackle all sorts of less-than-glamorous tasks, including creeping around the back alleys of elegant small towns with a net and some plastic tubing.
Luke — a roommate and hero of past, current and future adventures — and I were hunting Drosophilia, tiny flies that love humans and rotting compost so much that you can usually find them hovering around every rubbish pile in California.
Which is why our friend Heather — a researcher of genetic differences in Drosophilia populations up and down the Pacific Coast — equipped Luke and me with nets when we told her we’d be in Humboldt County last weekend. (Heather samples the Hawaiian flies herself.)
Of those us who’ve met Drosophilia in a scientific context, most probably remember it from general biology class, where it’s used as a simple genetic model to demonstrate dominant and recessive traits. Researchers still use flies to untangle tricky genetic pathways and unlock the mysteries of development.
But Heather’s questions are both more basic and more complex. She wants to know how evolution works. And Drosophilia, with their large, widespread populations and fast generation times, are the ideal study subject.
In theory, flies the world over are responding to a world of different selection pressures — things like pesticides, or harsh winters, or tough competition for fruit — that drive adaptation to fit local conditions and, writ large, evolutionary diversification of populations.
Take pesticide resistance, for example. Because fly populations are big, when you start spraying a new chemical, at least one fly probably has a mutation that allows it to survive. Since fly generations are so short (a couple weeks from egg to egg), we can witness evolution in action, as pesticide-resistant genotypes climb to dominance within a few years. Today, we find flies all over the world carrying the same pesticide-resistance mutations on different genetic backgrounds (that is, on different sets of ancestral genes), indicating that resistance evolved multiple times in different locations. Eventually, the accumulation of new mutations — both at random, and under selection — may lead to speciation, in which populations become so different that they either can’t or won’t interbreed, and scientists identify them as different species.
Of course, not all signals are as strong as the pesticide resistance one. And even in Drosophilia, they can get quite complicated, in part because we humans interfere every time we ship a truckload of fruit (with, doubtless, a few hundred winged stowaways) hundreds of miles down the road.
In this case, humans are eroding the Drosophilia population structure and connecting subpopulations with bursts of new immigrants. This mixes up mutations and adaptations that otherwise would have arisen and evolved in isolation for hundreds of fly generations.
Flies aren’t the only creatures we ship around the planet. Our global crop monocultures have replaced local, diverse crop varieties, leaving us with little genetic insurance for changing environmental conditions ahead. Salmon hatcheries in the Pacific Northwest have inadvertently mixed up a coastline of diverse subpopulations, introducing maladaptive traits to locally adapted populations.
We’re not just in the business of genetic reshuffling. Often our activities — the construction of roads and cities, the replacement of forests with farms — fragment once-continuous plant and animal populations. Newly minted subpopulations may be too small to stand alone and dwindle into local extinction. If they do survive, they may be forced through genetic bottlenecks, in which traits present in the few founding individuals are locked into the future population, for better or for worse. And small population fragments may adapt and evolve to microscale environmental differences in ways that the original, larger population could not.
That’s more in the vein of Luke’s work. He studies populations of frogs and salamanders broken up by agricultural fields and cities. Some species thrive in their human-modified environments: anyone who’s listened to a midsummer frog chorus in Manhattan’s Central Park can attest to that. Others aren’t so lucky — there’s a reason we use amphibians (and their loss) as indicators of ecosystem health.
What will these new population structures mean for evolution and the rate at which organisms evolve into new species? In the case of Drosophilia, salmon and corn, will one dominant genotype rule the world? Will its suite of human-loved traits spell lasting success or ecological doom? And when we splinter populations, will we increase genetic diversity or facilitate its loss? Will we drive creation of new species or erode away resilience from existing ones?
The answers will differ for every organism. In part, they’ll depend on how rapidly evolution can happen. One day, we may wonder if we can adapt fast enough to keep up with our own modifications of the world. For now, we’re left to ponder what right we have to change the course of billions of years of evolution — or whether, as products of evolution ourselves, we ought to ponder such ethical conundrums at all.
Luke and I return from our dumpster dive with a grand total of two flies. The weather’s misty, and there’s a chill in the air. I can tell Luke thinks we’d have more luck finding salamanders in these conditions, but only outside city limits.
Contact Holly with comments, questions and offers of fly samples via email at hollyvm “at” stanford “dot” edu.