When biologists think about the evolution of life, they think about climbing mountains.

To understand their alpine frame of mind, imagine a biologist studying the fish in a lake. Each fish may be born big or small. Fish born at certain sizes may be more likely to survive and reproduce than others. Each fish may be aggressive or shy. Again, their aggressiveness may determine their odds of having babies.

To picture all of this, it’s very helpful to imagine a landscape. Each point on that landscape is a different combination of aggression and body size. They’re like the longitude and latitude on a map. Each combination leads to a particular level of reproductive success. Picture that success as the elevation of that