Yes and yes.
'Weedy' Amaranthus are varied natives across North America; A. retroflexus is quite ubiquitous
Several years ago I started with one 'Red Garnet' amaranth plant--it seeded itself all over my yard, and its progeny have done the same.
Every year it is a surprise where they will come up and what they will look like.
This was a 2013 garden hybrid
Here are some pics from this year
I usually pull the green ones, but I let this one go and it has purple flowers
The green one and the red one in this pic (click to enlarge) both came from the same seed head sowed here last fall.
They are obviously cross-pollinating with something else.