A priori natural selection: the universe selects solutions before making them real, in the sense of existing absolutely rather than existing as one solution in a superposition of solutions. This applies to the laws of the universe, life itself, the building blocks of life, genera, species and mutations of species. Solutions are subsequently subject to a posteriori natural selection - survival of the fittest - but that determines which solutions are eliminated, rather than which solutions exist in the first place. Prior to survival of the fittest, there is arrival of the fittest.
Life is naturally abundant: the tendency of the universe to ever greater variation results in life being abundant in numbers of individuals, variety and complexity of species, and environments in which it occurs.