things vary (naturally - e.g. the moths)
some survive better (e.g. the moths)
the surviver's variation is preserved more than those which survive less (because the surviver survives more)
the net change is toward the better variation
it has "evolved" into the kind with the newer variation
it has another different variation
less than 100% of the versions of which survive better
only some of the new versions propagate
the net change is towards the preferred version of this 2nd variation
ditto, for a 3rd variation
ditto, for a 4th variation
ditto, for a 5th variation
ditto, for a 6th variation
ditto, for a 7th variation
ditto, for a 8th variation
ditto, for a 9th variation
ditto, for a 10th variation
ditto, for a 11th variation
ditto, for a 12th variation
ditto, for a 13th variation
ditto, for a 14th variation
ditto, for a 15th variation
it is now beyond similarity of the original state
repeat for 1000,000,000 years