"The Principles of Biology by Herbert Spencer (1864) looked at biology in terms of themes, such as Function, Adaptation and Variation. In this book Spencer introduced the expression 'survival of the fittest', in the sense of 'the most appropriate to its environment'."
While survival of the fittest is a really catchy phrase and easy to remember its use as an explanation of Darwinian evolution has proven a disaster.  It gets all bogged down automatically in ideas like smarter or stronger or bigger should be better.  Those ideas have nothing at all to do with evolution.  They are secondary or tertiary effects.  What evolution is really about is not individual survival.  It is about reproduction.  That virus or bacteria or algae or fish or human that reproduces the most offspring is the evolutionary winner.  How it manages to produce the most offspring is not important.
So, let's consider a recent, well publicized example of evolution simply because it is easy to understand.  The original covid 19 virus changed within a few months to a new mutant form.  And it has done that several times since that first change.  None of these changes made the virus more or less virulent according to the best clinical data we have.  Those changes either had very little or no impact on getting around immunity regardless if that immunity was from a vaccine or from a prior infection.  They did not make the virus bigger or smaller or smarter.  What every one of these changes did do was make the virus more capable of transmission from one human to another human.  If you compare two viruses of the same species and one is more contagious that more contagious form with rapidly be the only one left in the wild.
It makes no real difference if the selection for reproduction is done by nature, as in my virus case, or by man in some selective breeding program.  In either case what is selected for will become more common.  So, you take a yard of 25 colonies of bees and rate each colony as to how aggressive they  happen to be.  The third that are most aggressive you go in and kill the queens and requeen with a queen raised from a docile colony.  It will not be long and your more agressive bees are going to be a lot less aggressive.  Now suppose rather than man doing such selection of who breeds you have that same yard and there is steady animal traffic every day.  Suppose it is Gnu migration season.  The aggressive colonies get busy stinging those animals every day resulting in hundred bees a day dying.  The docile colonies ignore the animals and spend their time making new bees and honey stores.  Come fall the aggressive colonies are weak with hardly any bees or stores and die over the winter.  In a few years you would have few if any aggressive colonies.  So, nature and man selected for the same trait and got the same result.  Some argue what man  did was not evolution yet what nature did was evolution.  Personally I fail to see the slightest difference.
Can we generalize the above easy example of reducing aggression by selective breeding programs conducted by either nature or man?  Will it work for any trait?  Sure it will work with some very important provisos.  It has to be easy enough to work before the species goes extinct.  As well over 99% of all species of animals that have ever lived went extinct so winning is not a sure thing by any means.  Some things, like aggression, are easy because they are genetically simple.  One or two mutations may be all you need.  Other things are way, way harder.  Take varroa resistance as an example.  It will probably take ten or fifteen mutations.  That is not some wild ass guess.  It is based on the speed of progress of a whole bunch of breeding programs.  It actually is often more accurate to say snails pace of progress.  I have heard claims it is impossible to put that many mutants in one animal, particularly by a bunch of breeders who know no genetics at all.  Yet, I can give example after example where man did exactly that with pigeons long before Darwin or Mendel were born.  And nature did even better.  She made a pigeon from a non pigeon.

Now, when you do a selective breeding program or when nature does the same what is going on at a DNA molecular level?  Well, DNA is really dumb stuff.  All it does is a pretty good job of copying itself and in the first version of the copy actually screws up a lot leaving errors.  So, life has evolved error correcting  machinery that checks that first copy and corrects errors.  Still, one error for more or less a few billion bases slips thru.  The vast majority of those errors do nothing at all.  Of the errors that actually do something most of them harm reproductive capability in some way.  But once in a while an error happens that allows that virus to transmit from one human to another better than the the parent form or causes that honey bee to be less aggressive and thus reproduce better.
Let me by clear.  Not one thing I said has anything to do with life expectancy or intelligence or size.  In fact there are crystal clear examples where a shorter life expectancy can potentially lead to higher reproduction rates.  In humans, having a copy of the sickle cell anemia gene will do exactly this if the person lives in malaria country.  In humans having an IQ of 90 versus 110 does not impact life expectancy but the lower number does mildly favor higher reproduction at least in today's USA.  I have had dogs that were really smart compared to other dogs of the same breed I have also had.  Yet both survived just fine and would have reproduced just fine had I not neutered them. 

Dick

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