People are using this as a cutting agent in heroin and cocaine (and probably Ecstacy), so I wouldn't chalk it up to "Darwin" as much as "Mexican Drug Lords slash Hells Angels". YMMV.

As an example, say you were to visit your local bar for some tequila, and after ordering your drink and consuming it you found yourself with blurry vision and feeling significantly more intoxicated than usual. Maybe someone slipped you GHB, maybe the bartender just cut your tequila with some methyl alcohol instead. Same thing, right? You don't know what you consumed is not what you wanted until after it is in your system, and what you consumed can kill you (faster) than what you were expecting. Fentanyl and derivatives are causing chaos throughout NA and it is largely a result of legalization of marijuana.

Huh?

I think he's saying that marijuana legalization has taken the profits away from groups that generally used it as a black market product. With that market shrinking, they are finding new products to supply the market or increase profits in existing ones, like heroin.

Fentanyl, carfentanil, other derivatives of synthetic opioids as well as hundreds of other chemicals are routinely used to cut heroin. Most of the narcotics dealers either have no idea what's in their product or the people creating them don't really know what they are doing, so you end up with "heroin" that kills people with one smash.

I have clients (mental health and addictions) who piss test clean for heroin, despite using daily. Some of their results come back literally 0% heroin, but a host of other stuff, mostly fentanyl at this point.

The solution is further decriminalization, but that's probably decades away for N America.