Is there such a thing as biological degeneration due to inbreeding (incest) among garden plants?
For instance, if the quality in a bed of calendulas diminishes somewhat year-after-year, could this be due to too such genetic causes, or is soil depletion the likely only cause? If genetic degeneration is a relevant issue, are there specific signs when it occurs or recommendations how to avoid it?
Incest, well, not really, is it. No, it doesn't happen, well, not real degenerative changes, in the manner that happens with humans if they breed with close relatives, and not in plants like calendula grown outdoors where they are in the open and free to reproduce as they will. Sometimes there may be slight changes - perhaps a flower arrives on, say, a dahlia or some other flowering plant which is different in some way from all the others. It may have a colour break where all the others are one colour, or maybe the petals might be somewhat frilly when the others aren't - when this happens, breeders may select that one and try to breed from it in hopes of creating a new variety of a particular plant.
Where trouble occurs is where it where it always occurs in both flora and fauna - when breeding is restricted, usually by man, under controlled conditions, and plants such as F1 hybrids are created. Many hybrids don't have good vigour, or don't retain the characteristics they were bred for, or are, in fact, sterile. You might call this in breeding, which is a term that can be applied to humans and animals too, and it often leads to significant problems.
There is, though, a good level of success with crossing and recrossing plants to get new varieties, but it's trial and error. Many 'faulty' crossed versions of plants will have been rejected before one is finally selected as being good enough to put on sale.
In the case of your 'bed of calendulas' example, environmental conditions will usually be the culprit - the soil is exhausted, or there's a build up of pathogens/insects which affect that particular plant, a common problem with mono culture of vegetables.
It could be in part due to natural selection, rather than degeneration from inbreeding. I know Yellow Pear tomatoes have been grown since at least 1805, and most of that has probably been from inbreeding. They're still pretty nice tomatoes.
It could just be that the climate and conditions in your area favor certain traits, or the less colorful flowers attract less pests (such that they may survive and seed more). If you're letting them go to seed on their own, things like that can happen. If, however, you save seeds from the best plants every year, and plant them yourself, you'll probably see better results.
However, I'm not 100% sure if degeneration actually does occur with calendula like that, or to what degree.
There is such a thing as hybrid vigor, though.
