We deal with the selection and hybridisation of plants and can tell you that when you grow plants for a long time in an environment that is not natural you are still applying a selection because the environmental factors, at the time of growing, are still artificial, like light, humidity, temperature, artificial wind etc.
These factors make it so that plants slowly adapt themselves to those factors. We do this also with animals. If we put, then, back in nature, those plants artificially selected and grown after many years, that population will no longer be ready for environmental responses because the weather ( outdoors ) can and does vary in many ways and at any given time. For example, a wind that is not constant, temperature fluctuation, seasonal changes, different latitudes, light intensity, different soil composition etc.
Then, practically, a population, even if it is and comes from natural environment but has been carried out in an artificial way, even for 5 – 10 years, will quite possibly have and carry “spoiled” genes from the artificial environment it was grown in, so what we will have in the end is no longer what we started from and maybe we don’t even realize it because we can’t record the changes over time but this happens nevertheless.
This happens also with us humans who spend more time at home in front of computers and are not the human beings we were even just thirty years ago.
Without taking into account spontaneous mutations, variations, genetic drift, the fact that only part of the population is taken away so part of the heritage of the gene pool travels and then another branch will develop from that original stock. Mother Nature has great numbers … It’s rare we grow 3000 plants all together but this is inevitable. In the end what people are interested in is the final product, the end result.
We can’t talk of diversity or absolute purity unless we grow very high numbers in natural conditions.