I gave a presentation on "Planning for Seed Saving" last night to my county's Master Gardeners -- some of my most favorite people! Among other things, I talked about legal issues (patented seeds) and biological aspects that can affect choices gardeners might make about saving seeds from their own crops to replant in subsequent years.
One aspect of the process that I mentioned is removing plants that are showing undesirable traits from the garden, to keep those plants from cross-pollinating with the other plants that have traits you want.
If the plants with less desirable traits flower and pollinate the "good" plants, then those less-good traits likely will appear in your next generation of the crop. Removing "rogue" plants helps keep the next generation of the crop productive and wonderful, so this practice makes total sense.
However, even when we are trying to maintain a variety with its original traits, in selecting which seeds to save for the next crop, we sometimes make choices that change it anyway!
After the talk, one of my seed-saving friends shared her experience with a Southern pea she'd been saving and replanting. Each year, she'd saved "the prettiest" seeds from the crop to replant. After about a decade of saving pretty seeds, she found the original packet that she'd started with, and it had seeds in it.
She dumped out the seeds and found -- in a grand surprise -- that they looked very different from the seeds she'd saved from this summer's crop of what was supposed to be the exact same variety!
Gardening is never boring.