Wayne: What a beautiful series of questions. Hope that I can give a suitable response.
How do I go about selecting for a trait that doesn't already exist in a population?
First of all, I don't dwell in a black/white world. I live in the rainbow, or on a gray-scale. By that I mean that my plant breeding philosophy is that there are thousands or tens of thousands of genes in a plant, and that each one tends to have an incremental influence on the overall shape and function of the plant. Some more than others, but none of them exist in isolation. So my plant breeding philosophy is based on the meme of promiscuous pollination, so that the genetics can constantly be rearranging themselves, so that all those little incremental differences can have the opportunity to recombine themselves in excellent ways.
I don't have to know the genetics of a plant to be able to observe how it grows in my garden... So I don't worry about pedigrees or alleles. I look at the whole plant... I also don't worry about "mutations". I don't know what those are or how they would be helpful to me. And just cause something new appears, I'm not going to call it a mutation. It might just be a rearrangement of previously existing genes.
Because I save my own seeds, year after year, the offspring tend towards having genetics that thrive in my garden. I don't need to know what those genetics are, or how they have the effect that they do. I just save seeds from the plants that are able to survive my growing conditions. If a species is not able to survive my growing conditions at all, then I might try changing my growing conditions, or try different varieties of the species until one of them succeeds. I have the most success planting genetically-diverse landraces. Because when I plant a landrace
, there are many many different genetic combinations, and something might succeed. Next best, is the second generation from hybrids, because they have lots of different genetic combinations.
I often write about my frost/cold tolerant tomatoes. I am not selecting for winter-hardy (USDA Zone 4) tomatoes. That is far outside of the genetic ability of tomatoes to survive. What I am selecting for is tomatoes that can bounce back from an unexpected spring frost, and for tomatoes that can thrive in spite of the intense nighttime radiant-cooling experienced in my high-altitude desert garden. So I'm selecting for something just a little bit outside of their comfort zone.
To specifically address powdery mildew. It is a fungal disease that is often spread by aphids. So how might a plant population deal with it? Perhaps by having longer hairs on the leaves so that aphids are less attracted to it... Perhaps there are many more hairs... Perhaps by making chemicals in the sap that are poisonous to aphids, or that make the plant less attractive to them... Perhaps by developing a thicker or tougher leaf which makes it more difficult for aphids to feed... Perhaps by having more waxy leaves... Perhaps by having more chemicals that interfere with the fungi's ability to spread or reproduce... Perhaps by cutting off sap flow to infected areas. Perhaps by growing faster than the fungi is able to spread. Perhaps by fruiting quickly before the plant is killed by the fungus. Perhaps by growing better in temperatures that are too cold/hot for the fungus or the aphids. Perhaps by releasing pheromones that attract aphid predators. Perhaps by some combination of all these, and/or by modifications in many other traits which I can't even imagine. Squash originated in warm-humid areas: perfect conditions for powdery mildew. Therefore I believe that they inherently contain the genetics necessary to deal with mildews. If any particular variety has lost that ability because it was grown with crop protection chemicals for generations, then perhaps that isn't a variety that I want to be growing on my farm.
I don't worry about the specifics of how the plants are going to solve the problems, I just grow the plants and let them figure it out for themselves. It helps if I give them enough genetics to have the tools to solve the problem. That's why I like to grow the offspring of hybrids, and why I like promiscuous pollination, and why I like growing landraces. Landrace
have a lot of genetic diversity so that it makes it easier to try lots of different genetic combinations. If I'm growing a commercial (inbred) variety, the seeds are more or less clones of themselves. I can expect approximately the same outcome from any plant, and from year to year. If I plant a packet containing 300 seeds from a landrace, it's like trialing 300 unique varieties. The offspring of hybrids aren't as diverse, but they can be useful.
A few years ago, I planted about 15 seeds from a pink bean along with hundreds of varieties of other dry beans. Today, that pink bean is about 1/3 of my bean population. Most of the other varieties disappeared within a few generations. Pinto beans were another variety that thrived for me.
When I first started growing cantaloupes, I planted dozens of varieties. The first year, none of the varieties produced a ripe fruit
. Some of them produced green fruits with viable seeds. The second year, a couple of plants produced more fruits than all the rest of the patch put together, and they got ripe. The third year, the offspring produced bushels, and bushels of ripe fruit.
I could provide one example after another of these kinds of results: obtained by growing a number of varieties, and selecting among their promiscuously pollinating offspring for plants that thrive on my farm.
The climate is not separate from the weather, from the soil, from the farmer's habits, from the insects, from the bacteria, from the molds, from the birds, from the sunlight, from the dust, etc. I live in the rainbow: Everything has an affect on everything else. By being better adapted to my habits as a farmer, the plants are better able to adapt to the soil. By being better adapted to the soil, they are able to handle the insects more reliably. Etc, etc, etc, etc... I think of plant breeding as a mesh network. Everything is connected to everything else. I can't change anything without affecting everything.
If I plant a variety, and it fails in my garden it is permanently gone from my garden. No second chances. If it don't grow here, I'm not going to bang my head against a wall trying to get different results next year. Sure I grew runner beans 5 years in a row before I harvested seeds. But I grew different varieties every year. And it's a species that is expected to grow here. It's not like I was trying to grow oranges.
It's not chance that I find varieties that work, or rather it's not -only- chance. I am consistently playing the genetic lottery. Eventually I may find something that works. With spinach, about half of the varieties that I buy from a glitzy seed catalog will do well here. With tomatoes, about 95% will fail. With moschata squash the failure rate is about 75%.
Third year. Magical year.
Edit to add: I do a lot of successive approximations.. For example, if I receive a corn that is too long season for my garden, I might replant seeds of the earliest of the early. After a few generations of doing that, the corn may be ready to harvest weeks earlier than the original. If the skunks eat the lowest cobs from the patch every year, and I plant the survivors, then eventually, the cobs end up being higher off the ground so that they are out of reach of the skunks.