Pretty much every species has this capability. Of course magnitude of variation varies from case to case and variation is more frequently in some species. You might consider choosing a particular cultivar and contrasting that cultivar to its parent. Cultivars are cultivars because they possess some desirable trait not found or, in some cases, not consistently found in the general population of a species.

I'm not sure which of the infinite number of examples would be best for your use, but one group of cultivars that vary drastically visually from their parents are dwarf and miniature conifers. Other common examples would include daylilies and roses. You could pick a very elaborate cultivar of either of these and contrast that with a species version of either plant.

HOW ABOUT A MULE?

That's a hybrid, not a species!

I have several young plum trees in my back garden with what look to be thorns, although the parent plum had none. I think it bred back to a wild variety.

Also, I have noticed that mulberry trees are usually male or female, but sometimes of mixed sex.

Additionally, many varegated varieties of plants will send out a shoot that is plain green; gardeners are usually instructed to prune those off.

I would think some orchids or mushrooms would have different life stages that were quite different. There is even a plant that parasitizes others, and sends a blossom out through the skin of the parasitized plant.

Hope these help someone else to think of others... Good luck with your project.

sarahsflowers,
(I want to find a symbol of or a metaphor for "You don't necessarily have to be like your parents".)
In my opinion you are trying to establish a genetic-type example of a behavior-type relationship or absence of it; and I am unsure that it can be done convincingly. However, 'sporting' or 'bud sporting' may provide your example. A close look at a hedge of variegated hibiscus is sure to provide one or two examples of a shoot sporting green. But even this can be viewed as the plant reverting to its original coloration. I have been told that the camellia is quite rewarding in its ability to sport blooms of a different color but I have no first hand experience of this.
The more I think of it, the more I feel that the laws and postulates of Genetics tend to support more the statement that "the apple does not fall far from the tree".

Did someone say apple?

Plant a seed from a really good apple (just about any of the better know varieties). The resulting tree will likely produce barely edible fruit. The reverse is also true; plant enough apple seeds and, eventually, you will probably produce something better than the parent.

sarahsflowers
i don't know if you're using the correct terminology. species vs genus vs cultivar. you're right a cultivar is chosen for some desired trait. a species is a plant that when bred with another of the same species the offspring should be genetically the same. i think maybe what you're looking for is called a sport. many dwarf conifers do this, especially dwarf alberta spruce. one of it's branches will revert to the full sized tree. variegated plants will do this too. green leaves will start to grow and eventually you'll lose the variegation completely. or maybe you want an intergeneric hybrid. pardacanda might be a good example. it's parents are in the same family, iradaceae, which is why they can breed, but the plant is different than both of the parents, belamcanda x pardanthopsis. i don't know if this makes sense to anyone but me or not.

I think Sarahsflowers understood the terminology. I don't see anything about her request that would indicate otherwise. I do see some problems though with using sports or intergeneric hybrids. These would not meet the criteria set forth in the OP. A sport is not really an offspring and an intergeneric hybrid is somewhat predictably different than its parents because it's parents are different. It doesn't usually so much differ from its parents as it takes on certain characteristics from both of its parents (almost the opposite of the desired example).

You're wanting something that undergoes a metamorphosis, common in the insect world, as a butterfly or moth will produce a caterpillar. Or a fly or mosquito will produce a different larva.

Of course, in a way plants don't produce plants; they produce seeds, which are different from the parent plants.

Unfortunately for your illustration, in nature everything eventually winds up like its parents.

"Unfortunately for your illustration, in nature everything eventually winds up like its parents."

HUH????????????????????????

Not on this planet!