Not unless you inbreed to much. I remember hearing awhile back that the person who actually first found endlers said that even the “pure” endlers may not be pure since there’s guppies in those waters and they breed so easy.
When I was a kid, my great aunt gave me a bunch of guppies. (She'd probably been colony breeding them for close to 20 or 30 years, my step dad said he remembered her having them forever in one of those steel framed aquariums.) They were small finned sort of "peacock" colored males with various shaped tails and not very colorful females. Having looked up different type guppies online recently, since my own kids have become interested in fish, I believe the ones Aunt Hazel gave me were probably half endler or better. I had a few guppies that I got at my LFS around that same time (I was about 10 or 11 years old). Had some yellowish albino females and a handsome neon yellow albino snakeskin male and a few reddish females, they all bred together just fine and they ended up being more hardy after the infusion of endler type guppy in my colony. Unfortunately, I ended up selling them all when we moved from OR to MO when I was in the 6th grade.
Guppies and endlers breed easily together without any problems. It really depends on what your goals are to crossbreed or have actual guppies. Or that you'd like to breed randomly.
Just a note overhere: There are so many wildguppies or wildtype guppies that are short finned and have got nothing to do with endlers.
Like it's been mentioned before overhere by Platylover, it's questionable how pure wildcaught endlers are. For yes, a number of the known locations of wild endlers are areas where also guppies occur. So, natural hybrids occur as well and maybe (if the conditions allow it) they will form a new strain (population) at some point.
As being a true wild guppy and wild endler fan, even I have to admit that some wild endler strains do show some guppy traits. Which makes you think of the pureness. Some good wild endler examples are staeck and campoma nr.9 that show guppy traits as well. But there are more wild endler strains that are questionable.
But despite of the things mentioned by me overhere, there's a difference in DNA between guppies and endlers as being wild versions. But they're related to eachother.
Not only because of the fact that they're related will allow them to crossbreed but also their sexual organs are compatible with eachother, which makes a genuine copulation possible. If species are related to eachother but the sexual organs aren't compatible, a copulation will never take place in a natural way. Even if it seems that a male of the other species is chasing the female and points his gonopodium at her direction, no copulation will take place.