AUSTIN, TEXAS — City life can change the basic physical traits of its residents, down to what plant leaves taste like or how sticky a lizard’s toes are. The findings show that cities have become unintended experiments in evolution. That’s the process through which species change over time in ways that make them better suited to where they’re living.
Researchers reported their new observations here at the 2016 Evolution conference.
Kenneth Thompson described how white clover (Trifolium repens) has adapted to urban living. Thompson is a graduate student at the University of Toronto Mississauga in Ontario, Canada. Leaf taste helps defend this clover against becoming lunch for grasshoppers and other predators, he noted on June 19.
Like all living things, the plant's genes carry the instructions for building its tissues. And small differences in two of those genes let clover booby-trap its leaves and stem. Now, when bitten, it releases a warning burst of cyanide, a toxic chemical. A tiny, tiny taste of clover doesn’t kill animals. But its bitter taste can repel the spitting predator away so that it dines on some other plant.
Clover’s cyanide-making genes are more common in warmer locations. (That’s because ice harms clovers that carry these genes). And cities tend to be warmer than other areas. So Thompson expected to find more bitter clover in downtown Toronto than toward the outskirts. But he found just the opposite. That was startling because snails and other clover-eaters did about as much damage in both areas.
The same surprising pattern showed up in Boston, Mass., and New York City. But it didn’t occur in Montreal. The explanation, Thompson and his colleagues now propose, lies in what happens during winter.
Winter cold is a special threat for cyanide-carrying clover. The reason has to do with how the plant makes the toxic chemical in its cells. Cyanide is toxic to plant tissue as well as to its predators. When the leaf doesn’t freeze, clover leaves and stems aren’t at risk. That’s because cells store the chemical ingredients for making cyanide separately. Those ingredients usually don’t mix until an animal bites the plant, crushing and ripping cells.
Such a defense damages some plant tissue. But that damage is worth the sacrifice because it can save the rest of the plant. Freezing, however, breaks apart the safety system and sets loose the cyanide on its maker.
Snow insulates plants from the cold. But in Toronto, Boston and New York, the buildup of heat in city centers means that the snow cover is thinner. That’s more true downtown than at the cities’ outer edges. And areas with thinner snow means clover hiding underneath are more likely to get cold enough for ice to burst their cells.
So the closer to city centers that a clover plant grows, the less likely it is to set up a cyanide defense. Doing so could mean the plant would poison itself during hard freezes, Thompson concludes. Montreal, though, has such heavy snow cover that even downtown clover can stay pretty well insulated.
The clover study highlights one opportunity for studying how plants adapt to city life. “It’s basically a replicated evolution experiment,” says Jason Munshi-South. He works at Fordham University in Armonk, N.Y. By replicated, he means that each city offers a new chance to test whether the same results occur elsewhere. Comparing cities “is where we need to go,” he says.
Urban evolution “is a very young field, growing very fast,” says Kristin Winchell. She, too, is a graduate student in biology. While she studies at the University of Massachusetts Boston, her field work focuses on an agile lizard in Puerto Rico.
Called Anolis cristatellus, these reptiles have colonized dense cities despite the perilous open stretches between trees. In urban areas, The lizard’s legs tend to be 2 to 5 percent longer than those of their forest cousins. It's a finding that she and her colleagues reported in the May Evolution. And elaborate toe pads have more of the special scales that help these lizards skitter up walls.
On June 20, here, she presented her latest data on how this animal is adapting to city life.
Urban lizards may need all the advantages they can get to race around cities. The reason: They live in a more slippery environment. In the wild, the smoothest thing a lizard copes with is a leaf. They tend to be far rougher and easier to grip than a smooth metal surface.
Winchell showed videos of lizards dashing up slanting lab racetracks. Some clearly had better footwork than others. In one clip, a lizard darts up the slippery unpainted aluminum surface — and pauses. Then it shoots to the end. It beat out a lizard that also stopped and then steadily slid backwards .The more sure-footed lizards tended to be the city dwellers.
Differences between city and country lizards show up even in a generation raised in the same environment, Winchell reported. That might mean the two groups of lizards have somewhat different genes. Or the animals’ genes may be the same, but how the animals’ bodies use them might differ. This is called gene expression. If so, mother lizards might pass along these differences to their offspring. Winchell still has to test these two possibilities.
She has a way to go before putting together the whole picture of how cities change lizards. But however these stories turn out, they have a special pull: The adaptations they reveal all happened during human existence. And as Munshi-South puts it, the evolutionary history of city life “is one we [people] created.”
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adaptation A process by which living things become better suited to their environment through inheritance. When a community of organisms does this over time, scientists refer to the change as evolution.
anatomy (adj. anatomical) The study of the organs and tissues of living things. Or the characterization of the body or parts of the body on the basis of its structure and tissues. Scientists who work in this field are known as anatomists.
biology The study of living things. The scientists who study them are known as biologists.
cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, fungi, bacteria and some algae, are composed of only one cell.
cyanide Any chemical compound containing a pairing of carbon and nitrogen,. These compounds have had a number of industrial uses, from pesticides and the extracting of silver and gold from ore, to dyes and the hardening of metals. They also are deadly poisons.
defense (adj. defensive) A protective action, object (such as a shell or nest) or chemical response that can guard living things from harm.
DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
environment The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.
expression (in genetics) The process by which a cell uses the information coded in a gene to direct a cell to make a particular protein.
evolution (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed. (outside biology) A term to describe the changes that naturally occur in some system over time in response to changing conditions.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
generation A group of individuals born about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet -are referred to as belonging to a particular generation of humans. The term is sometimes extended to inanimate objects, such as electronics or automobiles.
genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.
graduate student Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).
lizard A type of reptile that typically walks on four legs, has a scaly body and a long tapering tail. Unlike most reptiles, lizards also typically have movable eyelids. Examples of lizards include the tuatara, chameleons, Komodo dragon, and Gila monster.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
predator (adj. predatory) A creature that preys on other animals for most or all of its food.
replicate (in experimentation) To copy an earlier test or experiment — often an earlier test performed by someone else — and get the same general result. Replication depends upon repeating every step of a test, one by one. If a repeated experiment generates the same result as in earlier trials, scientists view this as verifying that the initial result is reliable. If results differ, the initial findings may fall into doubt. Generally, a scientific finding is not fully accepted as being real or true without replication.
reptile Cold-blooded, land animals with backbones and whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles. Their young don’t live in water the way many amphibian youngsters do.
sacrifice To deliberately give up something of value in exchange for achieving something greater or more important — or in the hope of getting something of greater value.
species A group of organisms that share similar traits and ancestry, and can usually breed to produce fertile offspring. It is also the basic rank in a classification system called taxonomy. A species name is usually given with the next highest rank, the genus.
tissue Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
urban Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs. The development or buildup of urban areas is a phenomenon known as urbanization.
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K. Winchell, L. Revell and A. Puente-Rolon. Evolutionary effects of urbanization on the tropical lizard genus Anolis. Evolution 2016 (Conference). Austin, Texas, June 20, 2016.
K. Thompson and M. Johnson. Urbanization drives parallel adaptive clines in plant populations. Evolution 2016 (Conference). Austin, Texas, June 19, 2016.
K. Winchell et al. Phenotypic shifts in urban areas in the tropical lizard Anolis cristatellus. Evolution. Vol. 7, May 2016, p. 1009. doi:10.1111/evo.1292.
K.A. Thompson, M. Renaudin and M.T.J. Johnson. Urbanization drives parallel adaptive clines in plant populations. bioRxiv. Posted April 28, 2016. doi: 10.1101/034223.