AskScience: Got Questions? Get Answers.

To start, I'm a biologist and am absolutely NOT questioning evolutionary theory. What's been bothering me though is when people ask the question "Why did Trait X evolve"? What they're asking of course is "Why was Trait X advantageous?". Usually someone comes up with some logical reason why Trait X was advantageous allowing everyone to sit around and ponder whether or not the explanation is reasonable. If something doesn't come to mind that makes more sense, the explanation is usually agreed upon and everyone moves on. Ok cool, but we know of course that not all traits are propagated by natural selection. Some are propagated by genetic drift. Some traits may not confer a particular reproductive/survival advantage, they could be neutral, or just not mal-adaptive enough to be selected out of the population.

So, outside of inductive logic, can we ever have empirical evidence for what factor(s) caused Trait X to be selected? I can sit here and tell you that a particular bird evolved feather patterns to blend in with its surroundings, thus giving it the adaptive advantage of avoiding predators, but this may not be true at all - it could be sexual selection or genetic drift that caused the trait to persist. While some adaptations selective pressures may be so obvious that it would be perverse to withhold provisional assent, many are not so obvious and we should be cautious assigning causation when only correlation may exist.