In Cities, Wildlife Evolves Astonishingly Fast
In Cities, Wildlife Evolves Astonishingly Fast
Animals, plants, and insects adapt to the extreme urban
environment—and even to specific subway lines.
In Sendai, Japan, carrion crows put walnuts on roads so
that vehicles will drive over them, crushing the shells and allowing the birds
to get to the food inside.
By SIMON WORRALL PUBLISHED MAY 5, 2018
Most naturalists turn up their noses at cities, regarding
them as anti-nature—sterile wastelands of concrete and steel. But evolutionary
biologist Menno Schilthuizen, author of Darwin Comes to Town, takes the
opposite view: Urban environments are in fact “powerhouses of evolution,” where
animals as diverse as blackbirds and bobcats are adapting to their new
surroundings, with startling results.
Speaking from his home in Leiden, Netherlands,
Schilthuizen explains why mosquitoes on the Piccadilly Line in London’s
Underground are genetically different from those on the Bakerloo Line; why
cities accelerate evolution in ways Darwin could not have imagined; and why sex
in the city is helping urban blackbirds evolve into a new species.
Despite its name, the London Underground mosquito is not
restricted to London. It’s the name that has caught on for a species that lives
in human-constructed, underground spaces all over the world—including the
Amsterdam Metro—and cellars and basements all over the world.
It is called the London Underground mosquito because it
affected people who hid there during the Blitz in 1940. What’s special about it
is that it seems to be a species that has evolved very recently. Its ancestor
was culex pipiens, a common mosquito that lives above ground, feeds only on
birds, and forms large mating swarms.
This new species, culex pipiens molestus, is different.
It feeds on human blood, mates one on one, and the female doesn’t require a
blood meal before it lays eggs, as an aboveground mosquito does. On top of that
is the fact that it has recently evolved: Because the mosquitoes are confined
to separate subway lines, they don’t mix and so have specific genes, which may
be an adaptation to conditions in that particular subway line. The only way
they would become a unified species is if they all changed trains at Oxford
Circus. [laughs]
Most naturalists regard cities as sterile wastelands. How
did you get interested in urban ecology and why do you call cities
“evolutionary powerhouses"?
I got interested in ecology and evolution in cities
because cities are, in many ways, extreme environments. Cities have urban heat
islands; they are polluted and noisy; they have artificial light; and there’s
concrete everywhere. Because the environment is so different, some subspecies
disappear, but others adapt to the new conditions.
The evolution of species also occurs faster because new
mutations, which give a species the ability to survive in that extreme
environment, will spread very rapidly. This is what we call HIREC, or
human-induced rapid evolutionary change. We see that in cities and also in
other environments where humans create a new habitat or ecological situation.
In those places you see very, very fast evolutionary adaptations, which can
take place in the space of decades or even years.
That contradicts a core tenet of Darwin’s theory, doesn’t
it? That evolution is a very slow process that unfolds over millions of years.
I think Darwin underestimated the speed it can happen,
particularly with species that have numerous generations in a short space of
time. Generation time is the evolutionary clock speed, so if you have multiple
generations per year you can accumulate evolutionary changes much more quickly
than humans can, for example, which have one generation every 20 years.
An example is the Hawk’s-Beard plants in the French city
of Montpellier. A French researcher named Pierre-Olivier Cheptou studied the
seed science of these weeds that grow in little patches of soil around trees
that are planted along city streets. These plants make two kinds of seeds, big
seeds and small ones. The small ones have little parachutes that they use to
float on the wind. In the city, we found that these small-sized seeds are
disappearing. That has to do with the fact that in the city those drifting
seeds will probably land on tarmac and will not be able to germinate. So the
plant’s genes invest more in the heavy seeds, which fall to the floor and
germinate at the foot of the parent plant. Those genes have an advantage in the
city, which has resulted in a change in as little as 10 to 20 years. So, really
fast!
In recent years, studying the DNA of urban species has
yielded some startling results. Explain why bobcats in Hollywood are different
from those living north of the 101 freeway.
Fragmentation in cities is a common theme. In urban
ecology humans create all kinds of barriers, like roads and highways. North of
Los Angeles, the bobcat population is divided by two very large highways, which
bisect the area where they live. These barriers cause something similar to what
happens to mosquitoes in the London subway lines, whereby evolution is
restricted to the areas cut off from other populations.
In Los Angeles, there was a mange epidemic caused by
exposure to mites, which resulted in high mortality. But high mortality also
means strong natural selection and in the section of the bobcat population that
is cut off by the 101 freeway, genes evolved to make these bobcats more
resistant to mange. In other words, the species in that particular area evolved
a change to be better able to deal with this particular disease.
In Japan, crows in the city of Sendai have found a very
clever way of cracking walnuts. Explain how this works and why urban creatures
tend to be what you call “fearless problem solvers.”
The crows are a good example of an animal that is a
fearless problem solver. I spent some time in the city of Sendai and have seen
those crows with my own eyes. They have been feeding on walnuts for a long
time, which they drop from a great height. At some point in the 1980s, the
crows discovered that it’s easier to take a walnut and put it in front of the
wheel of a slowly moving vehicle, like the cars at this driving school. The
habit of putting walnuts in front of car wheels then spread to other crows in
the city.
This is an example of the cleverness we see in many urban
birds. It is not evolution, in the sense that there is not a gene for putting
walnuts in front of car wheels. But there are genes for personality traits,
which have to do with problem solving, curiosity, and being tolerant of people.
Urban birds and mammals all over the world constantly find new ways of making
use of the human inhabitants of the city. There are new foods, resources, and
nesting opportunities. Wild animals are shyer, because curiosity kills the cat.
But in cities, you do see that mammals and birds are evolving different
personalities.
You compare the urban blackbird to Darwin’s finches, in
terms of its importance to the study of city ecology. Unpack the parallels for
us.
Many examples I’ve given up to now are of evolution
within a species. There’s a change, but you don’t see a new species. The
blackbird is one of the few cases where it seems there is in fact a new species
evolving. Blackbirds started colonizing cities about two hundred years ago, in
Germany and Italy. The urban streak then spread throughout Europe and into
Asia. Before that time they were shy forest birds.
City blackbirds have changed in so many different ways
that you can say, like Darwin’s finches, that they have adapted to a new
ecological niche. City blackbirds have shorter beaks; don’t migrate anymore;
have different stress responses; start breeding much earlier in the year; and
sing at a different pitch.
All these things prevent them from crossbreeding with
forest blackbirds, which is also a crucial step in producing a new species. So
the blackbird is a good example of a real urban species that is in the process
of evolving, similar to the Darwin finches evolving in the Galapagos Islands.
Toward the end of the book, you describe some fascinating
new attempts to harness urban ecology in our cities. Tell us about some of
these, including Japan’s satoyama revival.
Satoyama is a term for the traditional Japanese natural
landscape around villages, this mosaic of rice fields and bits of forest,
managed by local populations. That concept is now being revived in the
mega-cities of Japan as local communities begin to set up urban farms, or build
little vegetable plots and orchards right in the heart of the city. That’s one
way to become aware of the fact that an urban ecosystem exists in the first
place and that we are a part of it.
Urbanites can also become part of studying the evolution
of city ecosystems. Here in Holland, we have introduced an app called Snail
Snap, which allows people to take pictures of the garden snail. We’re studying
the colors of the shells they photograph, and we’re seeing that, inside the
city, snail shells are lighter in color. This probably has to do with the fact
that in the city you have this urban heat island and snails may have a higher
chance of overheating in the summer. By having a lighter shell, they can
reflect more of the sun’s heat and survive better. We’re hoping that this kind
of citizen science project will expand all over the world to help study urban
evolution.
There is also a big trend in urban planning and
architecture to include ecology in buildings or city design, by having green
roofs or green walls, and pockets of vegetation in or between buildings. It’s a
good idea, although I think it could be improved by having a better
understanding of urban evolution. Many of the plants planted on green walls and
roofs are simply picked out of the catalogue of a garden center. But in the
city, there are many wild plants that are in the process of adapting to urban
conditions. So I would argue it would be better to simply provide the space or
soil and let these areas be colonized naturally by the plants that are already
growing in the city.
What about us? Are humans evolving in cities as well?
We also are living in an extreme environment, even though
we’ve created it ourselves. Our ancestors never lived as we do now, in dense,
artificial mega-villages. We could adapt to that evolutionarily, but for
evolution you need what are called differential survivals. Some people with
certain genetic characteristics would need to survive better or have more
offspring than other people. But that is one way we differ from wild animals
and plants that die in large numbers in cities, whereas we tend to survive, as
there’s not so much death as in the past, fortunately. But that also means our
evolution is slowed down, even though it could be good for us to adapt.
There is some evidence from the study of the genes of
skeletons in graveyards in very early cities that people’s immune systems were
already adapting to deal better with infectious diseases, which spread much
more easily in cities than among sparse populations in the countryside. That
will still be going on in cities today. But for the rest, we’ll just have to
wait and see.
Are you optimistic about our future on Earth?
I’m optimistic in the short run. In the long run, I’m not
sure if we can maintain this huge population. It’s a completely unprecedented
situation that the Earth is in nowadays, with one species dominating the
ecosystem in such a dramatic way. This has never happened before. So we don’t
have any examples from paleontology to predict what’s going to happen. If you
look at smaller ecosystems, where one species dominates, that’s usually not a stable
situation. So, in the long run, I think we might become less dominant. [laughs]
But how that’s going to happen, I don’t know.
This interview was edited for length and clarity.
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