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As far as seeds generation after generation becoming acoustomed to your local enviroment , This can be true but it has to happen on a semi-large scale to see any improvment. Otherwise you could be actually doing more harm than good to your genetic stock by cultering the "lower grade plants"
Hate to be the party-pooper, but I have a 10-foot tall 3rd-Gen Motherwort plant that's only meant to reach around 4ft and max at 6ft, that begs to differ
Ten feet! That's *THREE* midgets stacked lengthways!
Now, I know plant genomes can very greatly, and growing from seed means variability, but you really don't have to go commercial-sized by any means. Plant twice what you need (not a problem when you harvest your own, since most plants produce copious amounts of seed), then, at germination, clip the ones that're smallest, few weeks later, clip the slower growing ones (unless you're after small/space-saving plants for limited indoor space, in which case you might want to toss the tall ones) then when you're down to the number of plants you figure you need to grow-out to maturity simply taste/smell/etc and save the seeds from the plant that you liked best.
Could be you want a mild-flavoured Chilli, so you can save the weaker tasteing plants seeds, could be you want a monster basil plant - hence you save only the seed from the tallest basil each year. You get the idea, I could keep giving examples, but the point is that change is only a bad thing when you've already got a perfect plant, which most of us don't.
There doesn't need to be a mega-breeding program going on to be selective and create plants that successively become superior, since we're not trying to create new species. mm I'm meandering without any plan for this post.. the drugs are kicking in, my apologies.
My point is that the variability of open-pollinated seeds isn't some evil monster that ruins genetics, it's natures way of improving resisitance to bugs and diseases, growth, adaptability and all we have to do as gardeners is pick the healthiest looking plants. We're not neccessarily aiming to create designer F1 hybrid vegetables here, just tastier, faster growing food.