Ahimsa: As others have pointed out, the Garden was not a place that put any kind of environmental pressure on Adam and Eve. Regardless, the entire formulation of evolution as selfish is ill-defined. Selfishness as a moral quality is quite different from “biological selfishness”, and, in fact, “biological selfishness” (the biological urge to preserve one’s life and/or genetic code) more often lends itself to selfLESSness, and cooperative behavior. Selfishness as a moral quality is an inherently disordered and defective inclination to better oneself at the expense of those around you.
For an example of the peculiar qualities of “biological selfishness”, let’s just take wolves as they are socially very similar to humans. In a wolf pack, ONLY the alphas breed, they are monogomous, and they are given top priority by the pack. Being a non-alpha, to say nothing of being the lowly omega, means you will eat less, will not get to procreate, and you’ll get smacked around by those above you in the hierarchy if you annoy them. At first glance this looks like classic “survival and betterment of the fittest”, except for one glaring oddity: every single wolf is descended from a pair of alphas, who were descended from a pair of alphas, and so on back as far as we can recognize the species. The species pre-selects that only the strongest will breed, and therefore only the strongest pups are born, yet not all those pups will be alphas. The biology is “selfish” in that it preserves the survival of the species by preserving the smartest and strongest, but functionally this means that the non-alpha members of the pack must behave in a selfLESS manner, cowering to their betters, giving up the better portions of food, despite the fact they they can only have been biologically descended from strong, intelligent, powerful alphas. The selfish biology of the alpha genetic traits are the selfless behaviors of the omega wolf.
Selfishness in the context of sin is a wholly different thing, as it represents a destructive force. Selfishness as sin is a self-interest that damages the social bonds, and is rarely biologically favored in social animals (such as wolves and humans). The omega wolf who does not cower before her betters is killed by the pack, or at the very best ostracized from the pack and forced to live a solitary existance; she does not generally become the alpha. Similar results can be seen in all social animals, including humans (we kill, or at best incarcerate, those who profoundly violate our social structures). One might argue that this defective selfishness is manifested in the alphas, who cower to nobody and demand the best portions of food, and hold exclusive, monogomous breeding rights, but then you must remember that every single wolf, from omega to alpha, comes from the same alpha breeding stock going back thousands of generations. If destructive selfishness was a positive trait for alphas, then it would have been past down to all levels of wolf a long time ago, shaping the entire species by now, and it has not.
To sum all this up, the “evolved selfishness” would not cause, or even necessarily allow for, destructive selfishness, as they are completely different traits without any real relation. When destructive selfishness pops up in evolution, at least in social animals, it’s a negative trait that hampers survivability, and therefore is not a quality of “evolved selfishness”, which is a survival benefit and actually enforces selfless behavior in social animals. So, even asuming your revised statement, Adam and Eve would not have been placed in an environment that favored the kind of destructive selfishness that is sin in a moral context. Rather, they introduced destructive selfishness by virtue of their moral agency, and with the prompting of Satan.
Given the Catholic belief that the Original Sin affected all of Creation, it’s quite likely that the disorder of destructive selfishness occaisionally popping up among otherwise selfless animals such as wolves was a result of the actions of Adam and Eve as well, and thus “decay” entered the world through sin.