Can anyone explain biologically why two different specimans of San Pedro would have very different alkaloid profiles? Some are "very powerful"....and some barely so. Same genus, same species. Even if you try to match them up with size, age, growing conditions, thickness of green layer, etc....some are still more powerful???

I can't answer that question, but my solution to it is to get a whole bunch of San Pedro cactus, chop it up, dry it out, and then extract it. Make a kilogram batch of dried powder from a variety of SP and then you'll be able to standardize your dose. I have found it is difficult to gauge the effectiveness of even cactus extracts that were cleaned up to a decent level of purity. My habit has been to dose liberally.

Last week I went to a local cactus vendor who has acres and acres of cactus and the winter did a job on many of his San Pedros. They were all dead and dying. I told him that I would like to try to revive the dying SP and he gave me a ton of free SP. In one part of the field there was a giant garbage heap of dead cactus with a lot of dead and wilted SP. Needless to say, I walked out of there with the mother lode! I am set for a while.

I bet you're gonna be succesfull. these cactusses have the incredible power to rise from the death. If you put a 'dead' cactus in good soil and make sure it's not too wet or too cold or water it if it looks like it's realy dehydrated, than you have a big chance of reviving it.

But back to the topic.
sanpedro cacti are very unreliable indeed.
peyotes are much more consistent. They are more expensive as well, unfortunately.
If you combine peyote and sanpedro in a soup, you can be sure that it will give a good quality trip and you can use less peyote.
I do prefer peyote though. A few big fists of peyote will give you one of the best psychedelic experiences you can have.

Genetics... Its as simple and as complicated as that.

The seeds are born with a pre determined genetic code that roughly dictates how fast the plant grows, if it readily branches out, how much of a certain chemical is produced etc etc and that remains true from seed to plant to clone and so on. Seeds are often quite different from their respective motherplant tho and often a new powerful batch of offspring can come from a weak mother.

Ive found these plants to be as variable as cannabis