I should say that I agree with the answers given by <PERSON> and <PERSON> - evolving is possible and will happen, but very slow. On the other hand I haven't seen some parts which I see as important in their answers, so I would like to add to the said above and before me the following:
To <PERSON>'s answer: there is no such thing as a "purely cosmetic" modifications in evolving. If a color of feathers/hair/skin is changing (differ from the one parents/the main part of population have) it is due to either random mutation or adaptation to the surroundings (in the last case most probably the feature will acquire in one of/both parents but in a less expressive manner or it might not acquire at all, check this link, part №4 http://www.biology-questions-and-answers.com/mendel.html).
This said I'd like to add to PipperChip's answer that skin color is a result of color pigment presence intensity in the skin cells, which means that in cases where there is no need for skin to have a certain amount of the pigment it will change the color to a less expressed one. To put it simple - if you take a group of white skinned ppl and put them all in the equator area they will morph firstly into darker individuals after several generations being constantly exposed to intense sunshine/UVR (ultraviolet radiation) and after several more generations they will acquire black/brown skin tone. Same goes for black/brown skinned ppl and for the first group if they are moved to less exposed to UVR area – after several generations they will become/go back to white skin tone(d). For skin tone explanation (in case you’re not familiar with this) just read about melaninum/melanin.
What I mean is that mentioned Drow would not keep the skin tone of ebonite black, as it is in FR universe, unless magic.
Also evolution for isolated group of any species is slower comparing to the not isolated group of the same species due to not having same amount of factors to influence evolving process.
Adding to Yours “Edit 2”:
When we are talking about mutations in DNA code caused by parents age it is true. 762
But you should understand that in over 90% of cases when such mutations acquire they are not beneficial, but damaging ones. For example the trisomy of the 21st pair of chromosomes causes down’s syndrome (easy brief article on wiki will let you know more about it if needed without going too deep in genetics). The reason for this is that cells being old have some mutations of their own that appeared during individual’s life, and now not all of them work always as they were supposed to. This brings up a problem of incorrect transcription, cooperation of parental DNA chains and sometimes in addition of extra parts to the final code (three instead of two chromosomes of the 21 pair type as in the example) and thus – deviations of different scales and harshness in the developing individual.
When we are talking about beneficial mutations/modifications they might sometime happen spontaneously, but this event has a very low chance of happening and in cases when there are no other individuals with same feature or the feature is recessive (not dominant), which would help to keep new feature in the further generations, even the most beneficial mod might be lost for the species.
That’s why if we are talking about colors of butterflies, for example, that live in the industrial area, most probably after couple generations they will be primary of grey/black color (in case those are the most often met colors of their surroundings), even though they used to be white or any other bright color in the first place with small variation towards black/gray. The reason for this is that all the well seen due to being bright butterflies will be eaten by birds meanwhile the grey/black ones have better chances of survival.
This example goes more to what's been said in the first place, but the main mechanism is same.
Thank You for Your attention. All the best. 429