When you think of a trait that confers positive fitness to an individual specimen.

then you know that the gene that generated that trait also has a likelihood of being passed down.

but what else is passed down in addition to that gene is whatever process bore out that gene in the first place

For example, let's say there is a rodent family of 10

and one of them has a mutation that makes his brain size plastic, i.e. it can grow either large or small.. rather than be constant like his peers.

assume he survives this mutation and reproduces

then his progeny, lets' say a new set of 10, all have highly varied brain sizes.

let's say 9 of the dumbest ones die and the smart one lives

what is passed down from there is not JUST the smarter brain

but ALSO his father's "plastic" brain gene.

so in addition to passing on a smart brain, he also passes down the process that got him the smart brain in the first place, plasticity.

and in addition to thatn

he also passes down the trait that allowed his father to have a plastic brain, let's say, neoteny (longer gestation period, perhaps)

So what you see is that evolution through DNA is extremely cumulative

when it's survival of the fittest, you don't just get the fittest trait handed down, but about four billion years of history that delivered that trait.

Another thing that's interesting about evolution

is that your existence is thanks to the existence of literally trillions of parents and parent's parents, surviving while as their peers died off.

there is 4 billion years of history in your DNA

another interesting thing to note, is that we have about maybe 10,000-100,000 ancestral parents who were anomolies.

You know how sometimes families will have one mentally-handicapped person, or someone who is deformed perhaps. Well, that person actually sometimes reproduces, and passes down those traits.

well somewhere along the line, one of our parent's parent's parent's parent's parent's ... was a weird-ass kid.

Like one of our ancestor's was the first Homo sapien in a Homo habilis family.

like imagine that family in africa

they have one kid who takes forever to mature, or had a way-too-early pregnancy... and his head is really huge, and the family doesn't know what to do with it... but he gets to reproduce anyways, and has kids who are all the same, and survive oddly enough.

So imagine, in the long-line of ancestors that mated to produce you, there is a slice of them that were like "retarded" compared to the rest of the group.

crazy when you think about it.