Bittersweet Victory: Cockroaches Avoid Poisoning By Losing Sweet-Tooth

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This guy clearly hasn’t lost his sweet tooth. Photo credit: Neil Turner

Opponents of the theory of evolution frequently ask proponents the same handful of seemingly unanswerable questions, in an apparent effort to discredit the theory. Chief among them: “If evolution really occurs, why can’t we see it happening right now? ” This may seem like an unreasonable request; after all we typically imagine a species taking on new forms over time periods that are just a tad longer than the average human lifespan. Take for example the polar bear (Ursus maritimus), which took somewhere in the region of 150,000 to 200,000 years to evolve from the brown bear (Ursus arctos). Trying to demonstrate such slow progress in an 80 year lifespan would prove difficult to say the least. But trying to answer the question on these terms plays into the denialist’s fundamental misunderstanding of how evolution occurs.

Goldfish

“You’ve got 10 minutes to turn amphibious or I’m calling BS on this whole Evolution thing.” Credit: Dean McCoy Photography

There appears to be an expectation, among the opposition, of an almost instantaneous transition from one clearly defined form to the next. I like to refer to this misconception as the Pokémon fallacy. In the Pokémon universe, one need only rub an animal with an evolution-stone for it to magically morph from one animal to its next “evolutionary” stage. The two animals/monsters typically bear some resemblance to each other, but are very clearly defined as separate “species”. The expectation of such dramatic changes in reality, would of course lead any rational mind to question the legitimacy of the theory. But nobody is proposing such a ludicrous theory. A Fish-achu doesn’t become a Frog-achu in a single step (See what I did there?). It takes countless subtle changes acting over vast expanses of time to produce such dramatic differences. But here’s the good news. We absolutely can see these subtle changes occurring right now.

A recent study performed by North Carolina State University revealed that certain populations of the German cockroach (Blattella germanica) have lost their sweet tooth. Why? Because it was killing them. For the past 30 years or so, the predominant method for controlling cockroaches has been to serve them a poisoned bait that was just too tasty to turn down. These delectable baits typically included sugars such as glucose and fructose. After just a few years of munching on poisoned desert, a new trait came to dominate these populations: glucose aversion.

A German cockroach,(Blatella germanica). Photo credit: David Monniaux

A German cockroach,(Blatella germanica). Photo credit: David Monniaux

Glucose is a phagostimulant for cockroaches (when they detect it they are prompted to feed). Much like our taste-buds, cockroaches possess receptors that can differentiate between sweet and bitter foods. Normally when a cockroach encounters glucose, the sugar-gustatory receptor neurons (GRNs) are stimulated, while bitter foods like coffee stimulate the bitter-GRNs. However, in glucose averse cockroaches, while glucose still stimulates the sugar-GRNs, it also stimulates the bitter-GRNs and suppresses the responses of sugar-GRNs. So while glucose still retains some of its phagostimulatory properties for these cockroaches, its deterrent properties win out. Try to imagine a situation whereby sugar stimulates your bitter taste-buds instead of the sweet ones. You probably wouldn’t have as much trouble leaving that tub of Ben & Jerry’s in the freezer if it started to taste of vinegar.

These glucose-averse cockroaches grow and reproduce more slowly than their wild-type counterparts, but in environments where glucose-baited controls are utilised, glucose-averse cockroaches have the competitive advantage. As these glucose-averse individuals tend to outlive the sugar-lovin’ “normal” cockroaches they will pass this heritable trait onto the next generation, eventually leading to a population dominated by glucose-averse individuals.

Such examples of rapid evolutionary response to anthropocentrically-derived, selection pressures are not as rare as you might think. For example, the widespread use of Monsanto’s herbicide Roundup (Glyphosphate) has led to the emergence of so-called superweeds. In the mid 90s Monsanto released several GM crops that were resistant to glyphosphate. This allowed farmers to spray their crops with the herbicide; killing weeds without harming the crop. However, glyphosphate-resistance has since emerged in at least 20 of the species Roundup was designed to control.

Both of these examples are the product of our failure to anticipate a rapid evolutionary backlash to pest control, but it’s not all bad news. We can potentially exploit a species’ natural capacity to adapt in conservation efforts. Take for example the chytrid fungus (Batrachochytrium dendrobatidis). Chytrid is devastating amphibians on a global scale, but we may be able to save some species, simply by breeding them in captivity and releasing them into chytrid-infested habitats until such time as fungus-resistant individuals emerge.

While the changes observed in each of these cases are subtle, the documented benefits to the species are anything but. As a result of their respective adaptations, each species can now survive and reproduce under circumstances that would otherwise have led to their demise. It is via the accumulation of such subtle adaptations, over time, that new species emerge. So the next time someone asks you why we can’t see evolution occurring right before our very eyes, correct them. Tell them that we can see it. Some people just aren’t looking hard enough.

Source: Wada-Katsumata, Ayako, Jules Silverman, and Coby Schal. “Changes in Taste Neurons Support the Emergence of an Adaptive Behavior in Cockroaches.” science 340.6135 (2013): 972-975.

Photos: Credits as per Captions

Neil Turner’s Flickr Stream

Dean McCoy Photography’s Flickr Stream

David Monniaux’s Flickr Stream

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