Gravity is a "natural law" that we defined by observing that things fall down instead of up, and measuring how that invisible force appears to us (the speed of objects falling, the directions of objects attracted to each other, how it relates to other forces like electromagnstism, etc.)
Evolution, the way it applies to the theory of evolution, is a "natural law" that we have defined by observing that things can change in form over generations, and measuruing the nature of those changes (the colour of fur, the length of a leg, the pattern of spots on a wing, etc.)