How does genetic variation make a community more stable?

Normally, members of an average community have downfalls in their DNA. However, as each being is completely different, different imperfections affect different people. 

Now, imagine a community based on the offspring of two people. The imperfections passed on through the family line will grow stronger and stronger until a disease rare to normal human life strikes the family.

This is one of the reasons why incest is illegal (although still pretty gross even without that fact.)

Allow large amino acids through