The key realization is that there is no Evolution Fairy. There’s no outside force deciding which genes ought to be promoted. Whatever happens, happens because of the genes themselves. Genes for constructing (incrementally better) rattles must have somehow ended up more frequent in the rattlesnake gene pool, because of the rattle. In this case it’s probably because rattlesnakes with better rattles survive more often—rather than mating more successfully, or having brothers that reproduce more successfully, etc.
