Des clés pour comprendre : la notion de compromis évolutif (evolutionnary trade-off)

TTX is a toxin produced by newts that protects them against predators like snakes. 

Recall that newts* face an evolutionary tradeoff. Newts that make too much TTX have less energy to produce offspring*, but newts that make too little TTX will be eaten. TTX production is costly for newts, but it turns out that TTX resistance is similarly costly for snakes. By measuring the speeds and TTX resistance levels of many snakes, the Brodies discovered that the cost of TTX resistance is slower average* crawling* speed even before eating a newt. A snake that is non-resistant can slither away quickly in normal circumstances, but will be killed if it eats a toxic newt. A resistant snake, however, will survive if it eats a toxic newt, but will slither* slower in normal circumstances. So snakes face an evolutionary tradeoff, too. Too much resistance results in a much slower snake that is more likely to be eaten by snake predators, but too little resistance would mean death for any snake that tried to eat a newt. Thus, we'd expect snakes to evolve just enough resistance to eat a newt but no more so.

*Vocabulary : newt = triton ; offspring = descendants ; average = moyen ; to crawl = to slither = ramper