Like a secret ingredient to a signature recipe, "survival of the fittest" is a crucial part of the theory of evolution. The fittest individuals survive to mate and pass on their genetic lineage, and the weaker creatures fail to pass on their wimpy genes.

But if that's how it works, where do all the runts in nature come from?

A new study of red deer populations, detailed in a recent issue of the journal Nature, suggests that a genetic tug-of-war related to sex may be responsible. When red deer search for mates , each sex instinctively looks for different qualities. Males seek out females that will produce the biggest, toughest sons, and females seek males that carry a genetic blueprint for the best offspring-creating daughters.

It turns out, however, that the genes that make a good male aren't the same genes that make a good female.