Runt of the litter

Why is there a runt of a litter.Generally due to competition from a large litter.
What do we mean by a runt . This term is used to distinguish the smallest animal in the litter. The origin of the word was established in England in 1501.. They are noticeable as they are small and find it hard to compete with other siblings . Very interesting fact is that pig runts can very often get eaten by sibling under times of stress .. A lot of people chose the runt of the litter because they feel sorry for them . But after reading a few blogs it seems there are 3 scenarios . If the runt animal was a runt due to position in the womb and blood supply was ok it should be fine … if the pup was conceived at a different time close to the others it should be fine but if it is a genetic problem well the chances are slim.
Some people suggest alternating the puppies to the mothers nipples so they all get a fair share and a suggestion also was to bottle feed . The runt should be put right on the mom's nipple and sit there and watch the pup nurse. You should not seperate it from the rest of the litter
The question asked also is should we let natural selection take its course as any genetic impurities can be passed onto consecutive generations..