The Brodies still had not demonstrated that the snakes and newts coevolved. Only when two or more species evolve in response to each other do we call it coevolution. The Brodies provided evidence that newt toxicity evolved and that snake resistance evolved, but they needed more evidence to conclude that newt toxicity evolved in response to snake resistance and vice versa. To do that, they studied the tradeoffs between the costs and benefits of toxicity and resistance.

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.

The Brodies used these tradeoffs to test the coevolution hypothesis…