The spider ball python is a mutation or "morph", meaning that a specific gene is different from the normal or "wild type". In this case whatever gene it is that is different effects the all the different aspects of the python that characterize it as the spider morph, this could be anything from a defected enzyme or regulatory protein, but it all comes from the single mutation.

Natural selection controls the genomes that exist within a population, and may select for a mutation that offers a survival advantage. If the spider mutation had a selective advantage, they would be the wild type and we would call them normals.

Spiders do make great morphs to keep though, I love their colors and the wobble gives them lots of personality, they are great feeders and make beautiful combos too.