Originally Posted by benbaaa
(1) The way our DNA replicates when we reproduce is very, very accurate, but but it isn't perfect. The small copying errors lead to variations in the physical and mental attributes of the offspring.
(2) Some of these variations are helpful to the survival of the offspring, some of the variations are neutral, and some may be detrimental.
(3) Variations which are helpful to the survival of the organism stand a greater chance of being replicated, because the organism reaches sexual maturity and reproduces, in turn passing on those helpful variations. These variations will become, over time, more prevalent in the population. Variations which are unhelpful to the survival of the organism won't, because the organism may die before reaching sexual maturity.