‘Culex molestus’: What the London Underground mosquito species says about us

Humans are undoubtedly the living species that has had the greatest impact on biodiversity. Human activity is behind the extinction of many species. At the same time, humans have contributed to altering the natural evolutionary process of many others. With the dawn of agriculture and livestock farming, humans initiated a process that has given rise to many new species that did not exist in nature.

One example is artificial selection. Grass spikes that did not shed their seeds when ripe were easier to harvest, and so we created wheat and barley. The more docile goats were less likely to escape and became domesticated. Larger seeds produced more abundant fruit. And so, generation after generation, we shaped the organisms that feed us. Corn, for example, wouldn’t exist without us. Its wild ancestor, teosinte, has so little in common with a modern ear of corn that we wouldn’t even recognize it. Another example is the dog. From a wild wolf, we’ve created all the current dog breeds. This process, quite distinct from natural selection, has led to some detrimental traits. Short-legged dogs are carriers of achondroplasia, a genetic abnormality. And dogs with short snouts, like bulldogs, develop respiratory problems.

Sometimes humans influence evolution indirectly. As human activity creates new environments, some organisms can find new ecological niches more favorable than their own ecosystems. Cockroaches are a tropical insect that, thanks to the heating in our homes, have been able to colonize colder climates. Rats have found an ideal ecosystem in our sewers, not to mention seagulls, which long ago ceased to be fishing birds and have become specialists in landfills.

Sometimes this can be much more subtle. One example is the mosquito Culex pipiens, common in many temperate regions of the world. A population of these mosquitoes had adapted to life in the subway, where the abundant puddles allowed them to reproduce. On the surface, these mosquitoes prefer to feed on plant nectar; in contrast, those in the subway had a blood-sucking diet, feeding on unsuspecting passengers. Thanks to the enclosed environment and the stable temperature year-round, the subway species had lost the seasonality of its relatives that lived outdoors.

It remained to be seen whether these variations in behavior or appearance that had been studied actually constituted a new species. When researchers tried to hybridize the subterranean species with the surface-dwelling one, they encountered a strong reproductive barrier. Hybrids were very rare, and the few that were born were non-viable. The reproductive isolation between the two populations had led to a speciation process, and they could now be considered two different species. We tend to think of biological evolution as a very slow process that requires thousands of generations, but this is not always the case. Here is an example: in the little more than 160 years that the London Underground has existed, it has given rise to a new species of blood-sucking mosquito, specialized in biting public transport users. Not surprisingly, this new species was named Culex pipiens molestus. A similar process has been observed in the New York and Moscow subways.

The most surprising thing was that further study of the new species revealed it wasn’t homogeneous, but rather divided into subspecies. Mosquitoes typically live in tunnels, and the microenvironmental conditions vary (temperature, humidity, human density); the mosquitoes on each subway line evolved separately, favoring even greater genetic diversification. In this way, the London Underground map has become a large-scale evolutionary experiment, where each subway line has defined a different subspecies from a pioneer species that still lives outside. It’s very likely that some of these subspecies will eventually become independent species. It would be interesting to see how they’re named. I propose naming them after the station where the first specimen is identified.

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