Answer: What you describe is not all that uncommon with daffodils - it happens with beds of iris, too. The plants are not actually changing colors or reverting to their species color (rather than the hybrid colors), but are gradually being replaced. Daffodils reproduce by offsets and the strongest bulbs will reproduce reliably while the weakest (usually the hybrids) will die out after a few years. Evenutally you'll end up with a bed of the strongest offsets - in this case the yellow ones. If it were a bed of iris, you'd eventually end up with a purple color. It all has to do with survival of the fittest. Hope this sheds some light on the subject!
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