Why Do Humans Act UnNaturally?

Assumption:  Humans are the highest order of all species.
Assumption:  In order to get to the top, humans would need to conduct themselves in a manner that was consistent with both nature and Darwin's evolutionary theories.
Assumption:  In a naturalistic word view, it is impossible for anything to act other than under the laws of nature
Assumption:  A very clear understanding of evolutionary theory is that species will act selfishly and want to replicate themselves.
Exception:  There seems to be an inclination on the part of some life to stop replicating or reduce this natural inclination when they are feeling squeezed by the density of population.
Humans who are not squeezed are purposely reducing replication.  The humans who have the least amount to fear from density, food shortage, shelter issues, are the ones who are most likely to "unselfishly" agree to reduce replication.
Thus this intentional decision to not reproduce would seem to be both unnatural and potentially devastating to the most highly advanced of the most highly advanced.  (Sorry for the eugenics).  The very most successful, regardless of race, or any other observable trait, are the very ones who are most inclined to forgo reproduction.

Separately, the question of why do humans act unnaturally with regard to unselfish behavior of any kind?  Why would very, very large percentages of the human population believe that the unselfish approaches to life offered by Jesus and others would be the "best" way to live.  Even atheists and followers of other religions point to the beatitudes as ideal.