Animal Behavior/Coevolution

Coevolution and Evolutionary Arms Races

Coevolution describes the relationship between species where each exerts selective pressures on the other, thereby affecting each other's evolution. It includes the evolution of a host species and its parasites, predator-prey scenarios, or examples of mutualism evolving through time. As in any complex scenario, explanatory attempts will tempt us to consider ad-hoc explanations that are intuitively appealing but in many instances spectacularly wrong. There is no alternative to using such explanations as functional hypotheses and to subject them to rigorous experimental examination. Moreover, we need to look beyond the current status and consider the likely paths such a system of co-evolution may have undergone in the past. Our final consideration needs to go to the end points of this arms races. Has it ended in a stalemate rather than with the demise of one of the protagonists?

The Rough-skinned Newt and the Common Garter Snake

Newts are generally slow-moving animals with few defenses. Forming a major component in the diet of Garter Snakes a system of co-evolutionary adaptations appears to have given rise to a spectacular case of an evolutionary arms race . Rough-skinned Newts, common on the western coast of North America, produce extremely high amounts of Tetrodotoxin, a potent blocker of voltage-gated sodium channels in nerve cell membranes. With levels of toxin far exceeding what is needed to kill any other conceivable predator, it is only ingested by garter snakes with a high degree of immunity to the toxin throughout much of the newt's range. In the early stages of this interaction, some newts with an ability to excrete tedrodotoxin may have gained a relative advantage over their conspecifics. With predation pressure selecting for higher levels of poison in newts, snakes with some level of resistance to the poison were deriving a relative advantage. Such resistance emerged through a mutation in sodium channel structure, which retain much of its functionality yet made it less susceptible to a block by tedrodotoxin. Continuing to select for higher levels of toxin on one hand and increased tolerance to it in snakes, the present system may have emerged. The system's progression has, however, entered a stable end point when further increases in immunity to the toxin could only be achieved with a concurrent decrease in neuronal functioning.

At present, newts appear to have won the upper hand. Individual snakes with the highest levels of resistance to the toxin are also characterized by a reduction in motor abilities. Although able to handle even the most toxic newts, this ability is hard-won as it renders them at much greater risks of predation themselves.

In some areas snakes may have prevailed in the evolutionary arms race between predator and prey. Surprisingly, snakes in several geographic locations have developed such extreme resistance to TTX that newt production of the toxin cannot keep up

References

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