Rapid urban evolution

July 24, 2016 • 2:15 pm

By the title above I mean “evolution in animals that adapts them rapidly to urban areas,” not “changes in cities.” The former is the subject of a piece in today’s New York Times by Menno Schiltzhuizen, “Evolution is happening faster than we thought.” (Schilthuizen is a professor of evolutionary biology at Leiden and also works at Netherland’s Naturalis Biodiversity Center.)

Schilthuizen’s point is that evolution can work very quickly, contravening the conventional wisdom that evolution is always very slow. His examples involve animals and plants adapting to urban environments, where all of a sudden you’re thrown into a radically different habitat, one in which selection can be quite strong. All of a sudden you encounter noise, pollution, high temperature, cement, and so on. He gives a fair number of examples of “rapid urban evolution,” and here’s one that I liked:

French biologists have been studying a daisylike weed called Crepis sancta [JAC: that link didn’t work for me, but this one does], which normally produces two kinds of seeds: heavy ones that fall to the floor, and light seeds that drift in the wind for long distances. But in Montpellier, in southern France, C. sancta makes reduced numbers of the airborne seeds. Small wonder: The plants grow in pockets of soil on sidewalks, and any seeds that are carried on the wind are likely to land on concrete. The heavy seeds that land at the parent plant’s feet, on the other hand, are pretty certain to find a patch of fertile soil. So plants genetically predisposed to produce more heavy seeds have been favored by urban evolution.

Of course a captious biologist like me has to pick a few nits, but I’m not going to say “I don’t want to nit-pick, but.  . . “, for people who say that actually do want to nitpick! However, Schilthuizen’s piece is quite good, and I learned about some nice examples from it; only I want to add two points to his message:

First, we’ve known for decades that evolution can be quite fast when there’s a drastic environmental change or when an animal or plant invades a new habitat. Since the rate of evolutionary change is directly proportional to the “selection differential” (the difference in a trait between the mean of the original population and the mean of population that leave offspring in the new environment), an increase in the strength of selection will naturally speed up evolution (all things equal).  And we’ve seen this happen in human lifetimes. The classic work is Peter and Rosemary Grant’s finding that the beak size of medium ground finches (Geospiza fortis) in the Galápagos increased 10% in only one generation when there was a drought in 1976. That was because only big species of plants survived, and those bore bigger seeds. Only those finches with larger beaks could crack the seeds and get food, so there was strong selection for big beaks. A 10% change in one generation is a very strong rate of evolution.

I give more examples in Why Evolution is True, and John Endler, in his book Natural Selection in the Wild, gives several hundred. We’ve seen herbicide resistance evolve in plants, insecticide resistance in mosquitoes, and of course, antibiotic resistance evolve in microbes. So it’s not quite true, as Schilzhuizen says, that “recently, we have come to understand that evolution can happen very quickly, as long as natural selection. . . is strong.” We’ve known that all along, and have seen it. What’s novel about Menno’s piece is that he’s demonstrating strong selection due to urbanization. (I’ll let you read his piece to see the other nice examples.)

The other issue is whether changes we see are really genetic changes as opposed to developmental alterations or learning that occur in a new environment. The changes in Crepis noted above are truly genetically based—as are reductions in “nervousness” in blackbirds discussed by Menno.  The way to demonstrate that is to rear individuals from urban and nonurban environments in a “common garden” (the same environment, preferably an intermediate or “neutral” one), and see if they maintain the differences we see in the wild. If they remain different, then the character differences, such as the seed weight in Crepis, must be due to true evolutionary-genetic differences among populations. (I define “evolution” as most of us do—as genetic change in populations.)

But I’m not familiar with all the cases discussed, and it may well be that some differences we observe among populations might be either learned (in the case of animals) or developmental. For instance, in the first half of the 20th century blue tits and great tits suddenly begin piercing the aluminum  tops of milk bottles and drinking the cream. (Remember the days when milk was delivered to your door? I do.) This was a learned, not an evolved, response, and spread through populations quickly. Now if you’d taken blue tits from a truly rural population, and compared them with an urban population, you’d see a difference in behavior. But that was learned, not evolved. To see if it was evolved, you’d have to hand-rear blue tits in the lab and then see if, when faced with milk bottles, they behaved differently. I don’t think anybody ever showed that.

In the case of Anolis lizards (a case not cited by Schiltzhuizen), researchers found that lizards placed on small islands with more trees got longer limbs in a few generations. There was a paper published in Science about this, implying that this was true evolutionary change due to selection for better climbing ability. When I read the paper I thought, “They don’t know that—they never reared lizards from different islands in a common environment.”  Sure enough, when they did that, they found that the differences in limb configuration were purely developmental: that climbing on Tinkertoy “trees” made your legs longer. The authors had to admit that they hadn’t really found a case of rapid evolution.

This is all to say one thing: if you see a population change over time, that doesn’t mean it’s evolving. You have to test that using special experimental designs. And with that caveat, I recommend you read Schilthuizen’s piece.

Finally, here’s a great tit that has learned to pierce the aluminum caps of milk bottles. Why do they drink only the cream and not the milk? Because birds can’t digest lactose, which is present in milk but not cream! And cream, of course, is loaded with fat, which provides energy much needed by birds:


26 thoughts on “Rapid urban evolution

  1. No problem with the article but the picture’s of a great tit (Parus major) not a blue tit (Cyanistes caeruleus). Both species learnt the milk bottle innovation (see Aplin et al 2015 Nature for an experimental analogue using great tits).

  2. I wonder if any distinction exists between selection for variations already present in a population, and occurrence of genuinely new mutations subsequently selected.

    I’m not fully comfortable with the notion that shiny new urban-friendly mutations suddenly appear when useful. I’m not convinced beneficial mutations become more likely simply when environments change.

    1. Like you, I’m not comfortable with the notion that shiny new urban-friendly mutations suddenly appear when useful, and I don’t think Jerry is either. Mutations happen all the time: for example, a new human has on average 100+ new mutations. Many of these, in the environments in which they arise, are neither significantly helpful nor significantly harmful, so they can, with luck, drift to reasonably high representation in the population. Put members of this genetically diverse population into a new environment, and maybe some of the otherwise uninteresting alleles turn out to be useful: positive selection kicks in and they increase their representation.

      1. Maybe it’s a semantic problem? I find it hard to believe that during a drought, finches “just happened” to get mutations for larger beaks. I think it far more likely that there were already existing alleles for beak size variants, and those for larger beaks became more beneficial. Likewise, I don’t think peppered moths keep enjoying dark and light mutations fortuitously.

        So my question is, is there a terminology difference between selecting new mutations, and selecting among existing variation, or are both of these “evolution”?

        1. Who said that the mutations arose when the environment changed? They were almost certainly there in advance, perhaps hanging around via mutation selection balance. (i.e there were slightly deleterious but kept at a low equilibrium level by selection removing them versus mutation creating them).

  3. When I was a kid, I used to love opening a new bottle of milk at breakfast time, so that I could pour the cream onto my cereal. My parents would scold me and tell me that I should shake the bottle before opening it to mix the cream into the milk, but I always seemed to forget.

    Now, I drink skimmed milk (0.1% fat) and that trick no longer works.

    1. A couple months ago I bought some whole milk by accident, as opposed to my normal skim milk. Had some on cereal….oh my was that so good. Reminded me of when I was a kid.

      1. I remember my grandmother would use half & half on cereal. Now that is rich. Today I use one percent.

        One of the great technologies today is ESL milk. We started shipping it from the states to overseas destinations in the late 90s. I think the shelf life was getting some where close to 80 or 90 days.

      2. I drink whole milk most of the time. A little fat in the diet is not that bad, unless you drink gallons. One percent is unpalatable and tastes like dishwater. 2% is acceptable, but an unnecessary restriction.

        1. Yes, please ! A child of mine poured and handed me a glassful about a decade ago.

          I thought it was .that. of my youth ! Parents kept one grass – fed Guernsey or Jersey (richest cream !) for the kiddos and the 16 – 21 barn cats then.

          I never looked back after Zac’s reminiscing beverage ! I, too, researched the dairy – fat deal: one consumes a lot less of other saturated fats .if. there is some in one’s diet from, specifically, dairy fats. This I have found to be true for me at the least.

          I love it ! In smoothies and especially with chocolate syrup: this is righteously rich ! and also nostalgic as was Daddy’s drink in addition to his farm – fresh mocha lattes in the 1950s and 1960s which he secretly prepared & shared with me, a three – year – old through my teen years, inside the earliest morning mists and dews afore Mama ever arose to know !

          And m’weekend favored: the Dude’s swill = white Russians !


  4. Hi there, over here in the UK I work on London underground and we have distinct species of mosquito that spend their entire lives underground and have started to speciate. I think that entomologist have noticed differences between populations on the Victoria line and ones on the Piccadilly and District lines.
    Pickens molested

    1. I would say it is.
      I don’t see it in the shops and supermarkets – not that I want it – but you can buy it in the so called Farmers’ Markets around the capital at weekends.

  5. Thanks for the post, it helps to keep from ‘wandering’ around and cement my understanding of this , if you get my drift.
    And I do remember milk being delivered, in fact I had a schoolboy job doing just that in the summer holidays.. 1200hrs till 0600hrs.
    Two cream, two homos (homogenised milk)here! we would call out in the still of the morning with a silly schoolboy smirk.
    But there has been ‘Urban’ decay of a lot of services, dry-cleaning, clothing, grocery and vegetable bus (the old guy would get you herald his arrival by knocking on the doors in the street and pay with a treat) bread, ice cream man, morning and evening papers, Rawleigh’s health care products, local doctor and all to your door.
    It is, not only birds and small flowers that have adapted to change with the urban sprawl.

  6. I had a milk round when I was at school in the 70’s. Customers left out plastic cups for us to put over the bottles if they had a particularly troublesome bird in their garden. One lady insisted that it was a robin who was the culprit in her case.

  7. “Urban Evolution” — that would’ve made s great title for a Marvin Gaye or Gil Scott Heron album. I could see it sitting atop the Billboard R&B charts.

  8. Wasn’t there a case of evolved zinc tolerance in weeds growing under galvanized barbed-wire fences?

    1. Yes, and under electrical pylons. And there are several famous cases of plants growing on old mine tailings that have developed resistance to normally toxic metals like copper and zinc.

  9. A nice little article on a subject very dear to me. It’s this line that I hope continues to be emphasized: “With urban environments expanding all over the world, wildlife and biologists alike are starting to treat the city as a true ecosystem.” From a conservation point of view, it’s also important to get the general pubic to view the urban environment as a true ecosystem, not some human-only place where we don’t have to consider our impact on other species, or care about coexisting with them.

    Why do some species thrive in an urban environment and others do not, even when those species are very similar ecologically? This is something I want to understand better in shorebirds and gulls. If we understood this a bit, then we could perhaps design urban elements to encourage or discourage the nesting of certain species. What easier way to conserve and monitor a species than to provide a home next to ours?

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