>>938837747 (OP)
Reproduction can have errors, mutations, and mistakes. These can increase with environmental contamination.
There are natural processes that eliminate the defective individuals in a species. If they are not culled from the group they may breed their defects into the species contaminated the genome.
The same applies to behavior, if unsustainable and damaging actions are not suppressed in a group it gets "approval" by being silently uncontested, and escalates into negative habitual standards.
Also modern medicine "saves" those who would naturally be excised from spreading their genetic and behavior contamination to a whole species.
This is where humanity is at, fortunately still at small percentages, but they are loud and destroying humanity from within.