a very non stupided question ...
the whole point is its a dynamic , interedendant system that every creature lives in , the whole system IS a filter , and getting through is the only measure of good / bad / indifferent
an single mutation to a creature can be benificial and harmful and neutral and a given point in that creatures liniage .
eg a creture may mutate so that it hasd a dark blue spot on its tail , seem s to have no effect on creature
time passes , a new periditor has evoled near by that happens to have eye that are very good at spotting deep blue , and so this preiditor nearly kills off our creature by hunting .
more time passes and the pressure from the preiditor has selected those creatures in the population with that can feed in the darker hours of the day around dawn and dusk , this population has sleceted for vision that sees better in the dark , now this so happens that for individuals that have good low light vision the deep blue spot is a good way spotting fellow creatures ands becomes important trait in finding a mate , and hence breeding .
thus we have a mutation being all 3 types over a prolonged period , set along side changes to perdation , enviroment and breeding condistions .
it has been said if you rewound evolution and re run it would you get the same results ... and the answer given is maybe not .. you would end up with a lot the same but the steps and stages could be diffrent .
as the measure of benifit / harm can only be seen after the creaturte has survived or died out , hindsight is the only tool .