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A lot of people think evolution on Earth happens a little something like this.  A slimy creature crawls out of the ocean and hundreds of millions of years later, its descendents peel themselves off the sofa wondering how they managed to binge-watch all five seasons of Breaking Bad in less than three days.  In other words, evolution is a long, slow process, but it's not always that way.  Some species can rapidly adapt to cope with fast-changing environmental conditions like human encroachment, invasive species, climate change, you know, things that are 100% our fault.

Biologists call this high-speed adaptation 'evolutionary rescue', though that kind of makes it sound awesome when really, it's how some species are able to respond in times of severe population stress.

Take cliff swallows, for example.  That's a species of swallow that builds nests on cliffs, except that lots of them don't do that anymore.  Today, many cliff swallows make their nests on bridge supports.  That, of course, means there are large populations of cliff swallows living next to roads, and birds that live next to roads face a strange new enemy.  They're fast, they're hard, they're cars.

But it seems like cliff swallows are quickly adapting to this threat.  In fact, since the 1980s, the number of road-killed swallows has declined, even though there are more birds than ever nesting next to roads, and that's probably because populations near roads have rapidly evolved shorter wings.  Biologists studying the birds have found that road killed cliff swallows tend to have longer wings than the not-road killed variety, a little over 5% longer on average, and they think that's because shorter wings give the swallows an advantage against vehicles.

Commercial fishing operations catch a lot, a lot of fish every year.  In fact, they catch so much that species like salmon, cod, and herring have all evolved quickly to avoid extinction.  Basically, they're now living fast and dying young.  You see, human fishing tends to favor larger specimens, so many species evolved to be smaller overall or are becoming sexually mature at a younger age, or doing both of those things, and in some populations, the change is really dramatic.

Like, the average adult fish is 20% smaller than the species used to be and lives only 75% as long.  The advantage of this is pretty obvious.  Smaller fish are less likely to be harvested by humans, so fish that stay small or reach sexual maturity sooner can pass their genes on before they end up someone's dinner.  Fishes, well done, and these changes can happen really quickly.

For example, after they were overharvested in the 1920s, chinook salmon took only about 30-40 years to get roughly 25% smaller, and that's great for them, unless we start letting people catch smaller fish to keep up with the demand. 

When the London Underground was built in 1863, workers inadvertently created a tidy, isolated habitat for the world's favorite bug: the mosquito.  Standing water would collect in the tunnels, which made a perfect breeding ground for the bloodsuckers.  Humans discovered this the hard way during World War II, when the tunnels were used as overnight bomb shelters and the mosquitoes took to feasting on those people.

It's bad enough when you're underground trying to avoid getting exploded without being covered in bloodsucking bugs, but what's actually weird about this is that they were biting people at all, because the mosquitoes that live in the London Underground are a subspecies of Culex pipiens, a species that above-ground, usually feeds on birds.  So in the eight decades or so it had spent apart from its kin, the London Underground mosquito has switched to preferring the blood of mammals, probably because rats are a bit more common than pigeons in the new habitat, and that's not the only difference between the underground mosquito and the species it evolved from.

Unlike the above-ground version, the London Underground mosquito doesn't hibernate in the winter and the females don't need blood in order to lay their eggs.  In fact, the two types are now so genetically distinct that they can't mate and produce offspring, so the metro mosquitoes are arguably a new species, not just a sub-species.  Anyhow, whatever you want to call them, researchers think it only took a few hundred mosquito generations for this new pest to evolve, so like, yay, evolutionary rescue, I guess?

Okay, all sorts of garbage and pollution get dumped into the sea, but one species may have evolved a trick to survive this refuse onslaught.

Turtle-headed sea snakes seem to be adapting to chemical pollutants through an evolutionary phenomenon called industrial melanism.  In other words, they're getting darker.  All members of this species of Australian sea snake have black and white stripes, or they did, until recently.  Around 17 years ago, researchers noticed that the turtle-headed sea snakes living in the Noumea Lagoon in New Caledonia were mostly black instead. 

They get that color from the pigment melanin and it just so happens that it not only gives black animals their color, it's also very good at binding to heavy metals.  Heavy metals are substances like zinc, lead, and arsenic, which can accumulate in animals' tissues and being toxic, eventually cause their death, and the waters around New Caledonia are full of heavy metals from nearby nickel mining activities and industrial run-off.

Scientists actually collected shed skins from New Caledonia snakes and found higher amounts of zinc, nickel, and lead, so they think the snakes are protecting themselves from these pollutants by storing the heavy metal they ingest inside their melanin-rich skins.  Then, they can shed that skin, effectively ridding themselves of the dangerous substances, which is probably why researchers have found that blacker snakes shed their skins about twice as often as their striped relatives.

Speaking of oceans full of garbage, let's talk about garbage patches.  There are large areas of the ocean where our trash collects thanks to water circulation patterns.  The North Pacific Garbage Patch alone contains about 79,000 tons of plastic in an area about the same size as the state of Alaska, and that's a massive amount of plastic, but it's not as massive as it should be.  Scientists say they're finding about a hundredth of the plastic they expect to find in the ocean.  

The garbage patches don't appear to be getting any bigger, though they definitely should be.  Now, no one is entirely sure why this is, but some scientists think it's because of the rapid evolution of microbes that eat plastic.  The mere existence of all our trash could be acting as a strong selective pressure driving the evolution of molecular pathways for chopping up plastics, and plastic-eating microbes have been documented in other places. 

Scientists were recently able to isolate a type of bacteria called Ideonella sakaiensis which was nomming on plastic outside a bottle recycling facility.  Considering the kind of plastic garbage they eat has only been a thing for around 70 years, they evolved really quickly, so it's possible that that's happening in the oceans, too, but this is a guess.  Research is needed to confirm plastivores are a thing and can account for the missing plastic.  It could be explained other ways, too, like maybe this plastic is just sinking as it's colonized by marine organisms, and recent research has suggested some plastics aren't as everlasting as we thought.  I'm rooting for the microbes though.

Human beings cause all kinds of problems for nature all by ourselves, but sometimes, we go above and beyond by introducing plants or animals that wreak havoc on our behalf, and that's what happened to an isolated population of Edith's checkerspot butterflies on a cattle ranch in Nevada. 

In their natural habitat, these butterflies lay their eggs on maiden blue-eyed mary plants, a native species of wildflower, but in this particular location, ranchers introduced an invasive weed called English plantain.  Turned out the butterflies that laid their eggs on plantain actually fared a bit better since the plants lived longer, so the butterflies quickly evolved to prefer this new host plant, and things were going swimmingly until the cattle moved way.  When that happened, the fields were almost immediately taken over by grasses, because you know, they were no longer being eaten by cows, and this grassy vegetation shaded the plantains, so the caterpillars living on them didn't get the warm sunlight they were used to.

The poor butterflies simply weren't able to switch back to the wildflowers fast enough, so within a couple years, the isolated population of Edith's checkerspot butterflies went extinct, and this whole process from the introduction of English plantain to the extinction of the adapted population of butterflies only took around 100 years.

Elephant tusks are highly sought after in the ivory trade, so the presence of poachers gives tusk-less elephants a profound biological advantage, which is probably why African elephants without tusks have become a lot more common in some areas.  During Mozambique's civil war, elephants were poached in large numbers for their ivory and meat in order to help sustain the fighting forces and pay for weapons.  Since tuskless females were more likely to survive, lacking tusks went from being a relatively uncommon trait to a common one possessed by up to a third of young female elephants, and a similar thing has happened elsewhere on the continent, like nowadays, in South Africa's Addo Elephant Park, up to 98% of female elephants never develop tusks.  

Incredibly, this has all happened over a period of a couple generations, and that's probably because about 2-4% of females already possessed the tuskless trait.  Sadly, those tuskless females were pretty much the only ones that survived long enough to breed and that allowed the mutation to sweep through the population very quickly, despite the fact that elephants reproduce extremely slowly compared to all of the rest of the animals on this list, but while being tuskless is great when it comes to avoiding poachers, it's important to note that female elephants in this species had benefitted from having tusks up until now.  

They're helpful for finding food, among other things, so while losing their tusks might seem like a win for rapid evolution, it's not an unqualified one.  It might be tempting to think that we can like, stop worrying now about endangered species because this high speed evolution is gonna swoop down and eat up all the plastic and save the elephants, but evolutionary rescue can't save every species.  Some simply aren't capable of adapting quickly because evolutionary change happens over generations, not years.  

Even hyperspeed adaptations like the ones we just talked about generally take ten to 100 generations.  Basically, the more babies you have and the quicker you have them, the faster evolutionary changes can spread through a population.  So we're talking about hundreds of years for species like polar bears to make big changes that might help them survive.

Sometimes, rapid evolution can happen faster, like if a species already has an adaptation that helps them respond to a very bad situation as we saw with the tuskless elephants, but for the most part, it occurs in animals that breed fast and have lots of offspring, and even then, evolving quickly can totally backfire.  Just ask the Edith's checkerspot, so while examples of warp-speed adaptation are fascinating, we shouldn't see evolution as some superhero that's gonna save this planet.

That is still on the folks who caused the problems in the first place, which is us, though I am thankful for many of the wonders that were brought to us as we did it.

Thanks for watching this episode of SciShow.  Don't forget to subscribe to get all of our episodes delivered into your YouTube inbox.  You can also subscribe to SciShow Psych and SciShow Space, where we talk about those topics in exclusion of all these other ones.  If you were surprised to learn that evolution can happen so quickly, you might learn a lot from our episode on evolution misconceptions, so we've linked to it for you to click on and watch it.  I hope you enjoy.  

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