When a product is a good fit for the market's needs, we call it Product-Market Fit. The product Uber fit our needs to conveniently get around a city.
In biological evolution, when genes are a good fit for the environment, we call it Gene-Environment Fit. The genes in plants and animals were a good fit for the newly oxygenated atmosphere during the Cambrian explosion.
In memetic evolution, we can think about Meme-Brain Fit. Memes need a home to live. That home can be technological storage, like paper or hard drives, or it can be biological storage, like our brains.
Let's say were trying to store some new meme, like a catchy song. Where would it live? It would need to live in the short-term memory of the brain. Here is how our brain stores short-term memories:
We store images in the visuospatial sketchpad, our mind's eye. We store sounds in our phonological loop, the mind's ear. We store events in our episodic buffer. From there, memories can get put in long-term memory.
A catchy song would get stored in our phonological loop, our mind's ear.
Catchy songs are "fit" for our mind's ear. Religious symbols are "fit" for our mind's eye.
Ideas need to fit somewhere in the brain.
Just as genes seem to "learn" the laws of optical physics in order to create an eye, so too do memes "learn" the laws of neurobiology in order to be stored there.
But memes can live on other information layers too, like books and bits. And they need to transfer between minds, through sound waves or the internet. We could also think of specifics like Meme-Text Fit or Meme-Internet Fit. Most generally, there is Message-Medium Fit. (I will call it Medium-Message Fit to conform to McLuhan's order.) The message needs to be a good fit for the information medium available to it. A new medium like the internet creates new messages (like hashtags) that are fit for it.
Finally, just because a meme is fit for our brain doesn't mean it will actually outcompete the other memes that could live there. Just because a hashtag can be posted to Twitter doesn't mean it will outcompete the rest of the hashtags there. We could call this Medium-Message-Environment Fit. #FridaysForFuture is a good fit for our current environment of the climate crisis.
As a counterexample, genes only have one information layer type to live on. There's only one medium, DNA, so there is only Gene-DNA Fit. Genes don't live in lots of different homes like memes do. This is why we just think of Gene-Environment Fit and forget about the whole "medium is the message" thing.
All together I'd want to write a less pithy McLuhan statement like:
The medium determines the message space, and the environment determines which messages are successful.
Or a more pithy one like:
The medium is the message, and the message is the environment.