repens will not weather major environmental change without some impact. We should expect to see more of these examples, particularly as climate change begins to affect species more significantly.
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In fact, environmental change caused by humans and their urbanized landscapes is a prime driver of modern natural selection. This classic example of natural selection in action illustrates that evolution is going on around us today and that it’s not restricted to so-called “wild” populations. Over many generations, non-functional hydrogen cyanide genes became common among city plants, while remaining rarer among populations in rural areas. Those plants were less likely to freeze to death and more likely to pass copies of those broken genes on to the next generation than plants that produced hydrogen cyanide. These colder temperatures seem to have given an advantage to city-dwelling plants carrying broken hydrogen cyanide genes. With less snow cover, plants in cities experience colder temperatures than plants in rural areas do (though on average, temperatures are warmer). While cities are generally warmer than surrounding areas, in part because of this, less snow accumulates in cities - and snow is actually an insulator it keeps the ground that it covers from experiencing as high or as low temperatures as does the air above it. However, a well-established side effect of hydrogen cyanide strongly supports another idea: for physiological reasons, this compound makes plants that contain it more sensitive to cold and vulnerable to freezing. This could potentially explain the pattern in hydrogen cyanide production that the researchers observed. One hypothesis is that city plants experience less herbivory, so the advantage of warding off nibbling insects is outweighed by the energetic cost of that protection. It requires extra energy that could otherwise be put towards growth and reproduction. In general, producing defensive chemicals involves a trade-off. Some aspect of city life must be making life more difficult for plants that produce hydrogen cyanide, selecting for plants that carry broken versions of the genes - but what could it be? The pattern was remarkably consistent from city to city regardless of how large the urban area was. When researchers sampled almost 10,000 clover plants from 20 cities and their outlying areas, they found that the plants closest to the city center were most likely to have “broken” hydrogen cyanide defense systems, with this trait less and less common (and hydrogen cyanide more and more common) the further out of town the plants grew. However, if either of the genes responsible for producing the two chemicals that form hydrogen cyanide is broken, so is the entire defense system. Many white clover plants ward off herbivores with hydrogen cyanide.
In fact, this mechanism of defense is so common among plants that herbivores have evolved various strategies for dealing with the toxic chemical, allowing them to chomp away happily. While a single bite of clover won’t kill a cow, some insects are repelled. When an herbivore bites down on a plant with a working defense system, two chemicals are released from different parts of the plants’ cells, mix together, and react to form hydrogen cyanide, a bitter compound that disrupts an animal’s ability to use oxygen and can be fatal. This adaptation takes the form of a noxious cocktail. repens became a city weed, it also lost a common plant adaptation that offers protection from herbivores.
The big issues – Pacing, diversity, complexity, and trends.Macroevolution – Evolution above the species level.Microevolution – Evolution within a population.Mechanisms: the processes of evolution – Selection, mutation, migration, and more.The history of life: looking at the patterns – Change over time and shared ancestors.
An introduction to evolution: what is evolution and how does it work?.
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