When I encounter scientific creationists, I often hear the following argument: “If all living things have arisen through evolution, why have so few evolved to be big, strong, and smart? Evolution predicts that adaptive qualities will always be favored over non-adaptive ones. So why are so many species small, weak, and stupid?”
Perhaps because size, strength, and intelligence aren’t all they’re cracked up to be. Everything has a cost. This is true for even the most desirable of qualities. At the very least, a ‘good’ quality will drain metabolic resources away from other uses, which may be more necessary for survival.
This point has been recently made in a New York Times article that featured interviews with two biologists: Reuven Dukas at McMaster University and Tadeusz Kawecki at the University of Fribourg:
"We use computers with memory that's almost free, but biological information is costly," Dr. Dukas said. … Dr. Dukas argues that learning evolves to higher levels only when it is a better way to respond to the environment than relying on automatic responses
.… Dr. Kawecki suspects that each species evolves until it reaches an equilibrium between the costs and benefits of learning. His experiments demonstrate that flies have the genetic potential to become significantly smarter in the wild. But only under his lab conditions does evolution actually move in that direction. In nature, any improvement in learning would cost too much.
"Humans have gone to the extreme," said Dr. Dukas, both in the ability of our species to learn and in the cost for that ability.
Humans' oversize brains require 20 percent of all the calories burned at rest. A newborn's brain is so big that it can create serious risks for mother and child at birth. … Dr. Kawecki says it is worth investigating whether humans also pay hidden costs for extreme learning. "We could speculate that some diseases are a byproduct of intelligence," he said.
Zimmer, C. (2008). Lots of Animals Learn, but Smarter Isn't Better. New York Times, May 6, 2008