Good thread. Here are my comments:
Empirically there are plenty of "mother" plants out there that have been maintained for 15+ years and are still going strong with no evidence of genetic change.
Empirically, there are any number of "clone only" strains that represent clones of clones of clones to the 10th or 15th generation or more, and the offspring appear to be indistinguishable from the parents.
So yes, it definitely is *possible* to maintain individual plants for many, many years without issues, and its also *possible* to take clones from clones from clones for many generations without "genetic drift". But just because this "can" be done, does NOT mean that it always works every single time, or that its impossible for lines to go "bad" over time!
The environment a plant is grown in, and good old fashioned dumb luck can play a role here.
Now to address a few concerns from this thread:
How would the dna of the plant change by taking cuttings?
Just taking cuttings does NOT change the DNA. The DNA in a piece obviously is the same whether its attached to the parent plant or not!
The issue is "mutation".
In essence even though DNA replication is 99.999999% faithful, its not 100%. When you are growing entirely new plants from clones, you're literally building tens of millions of new cells, each one copied from previous cells. Over many many cell replications, over many years or many generations, small errors in DNA sequence (called "mutations") can accumulate.
These can be simply copying errors, or they can occur from mutagens in the environment (like UV light), or for other reasons. When the happen to the DNA of a mature organism (in this case a plant) they could be called "somatic mutations".
In practice, many of these DNA copying errors will be harmless and undetectable. They're "there", but since they don't affect anything you can see, you don't know they're there. Some of these errors will kill the cell that has them, ending the mutant line, and again, for practical purposes being undetectable.
Some small number of them might even IMPROVE a plant. (Ultimately all variety of plants comes from random mutations).
But more likely that being helpful, some of the accumulated errors will hurt the plant in some way, by reducing yield, vigor, potency, infestation resistance, or by causing weird growth, etc.
In the "wild" these sorts of mutations wouldn't matter. Any mutation that actually hurt a plants survival or was otherwise negative would get weeded out by natural selection in a few generations. Constant natural selection for the "fittest" plants ensures that weaker genetics are perpetually being discarded. Even under artificial selection of human breeding this would be true too. Negative mutations leading to lower potency, vigor, disease resistance, etc, would be (hopefully) weeded out by breeders.
But in the context of a plant where there is NO ongoing selection, ie where someone is DELIBERATELY maintaining the same plant for decades as a mother plant, or maintaining the same genetics by taking clones from clones from clones, etc, accumulated mutations MIGHT eventually reduce the quality of the genetics. So THAT is the problem this thread is about.
This is "genetic drift", and it explains why a plant that is a clone of a clone of a clone to the 100th generation or more *might* not be the same as the original parent. The further you get from the original parent, the more likely you are to have accumulated mutations. The same process explains why inbreeding the same line to itself for many generations without external selective pressure can also sap vigor.
The likelihood of this happening can be reduced in a number of ways: Keep plants away from mutagens, to the extent possible. Again, to the extent possible, try not to unnecessarily increase the number of generations. Don't perpetuate clones/lines that appear inferior to or deviant from the parent.
Also how is cloning like making a copy? It's more like cutting something in half and having it regenerate itself with no "replication" of any kind done, just natural plant growth.
In natural plant growth, cells divide. That's how the plant grows, obviously.
Every time a cell divides into two cells, its DNA gets copied from one copy to two.
So that's where the DNA replication occurs.
Is new growth on an old plant as old as the rest of the plant despite the fact that its new?
Of course not.
New growth is new growth. All the tissue is new.
The question is whether or not the DNA present in the new growth is the same as the DNA from older parts of the same plant.
Again, even though in theory the DNA from every single cell in a plant "should" be identical, in practice, mutations can occur and it isn't always true that every part of a plant is genetically identical to every other part.