One of the theories how organisms switched from RNA to DNA is due to viruses. Viruses have a pretty wild range of their genetic diversity. Single strand DNA, double strand DNA, positive sense single strand RNA, negative sense single strand RNA, double strand RNA. We’ve also probably got viruses as a permanent part of our genome from some ancestor species.
I think they’re pretty cool. Also, they do respond to outside stimuli, otherwise they’d be completely inert.
They respond because they have to do things like inject the genetic material into the organism once it latches on to whatever on the cell surface. That doesn’t occur in the host, it occurs in the virus.
It’s been a while since I took virology, but I feel pretty confident that something occurs in the virus due to an external stimulus.