So I understand that fruits are a mechanism for spreading the seeds, and that berries and nuts are sub-types of fruits. But how exactly does it work? How does a banana and a walnut accomplish this?
(I know that evolution is not an entity with a purpose. The form of the question is shorthand for “what is the evolutionary advantage that the mutations causing these traits have given the plants”.)
The advantage is that the plants gets its seeds spread farther than either gravity or wind would disperse them, and by “packing” those seeds in something animals consider tasty the animals will tend to take the seed packages (fruits/nuts/berries) and not so much of the rest of the plant. In conjunction with this, parts of the plant like leaves and stems may be bitter, astringent, or otherwise non-tasty so as discourage eating of those bits. As bonus, in some circumstances (elephant dung, horse dung, etc.) the seeds are even deposited in a big ol’ steaming pile of what is, essentially, fertilizer.
Once in awhile this backfires. Chili peppers have capsicum to discourage mammals from eating them (they “want” to be spread by birds, which don’t perceive capicum as “hot” in the same way mammals do) but then this one formerly African ape decides it likes chemical heat in its food. The story has a happy ending, though, as that bald ape has learned to cultivate plants and thus, despite the capsicum not working as intended the ape has now spread the seeds of that group world-wide.
Yes, okay. But I still have many more questions.
So all seeds can survive digestion in animals? If the purpose is to be eaten by animals, why do nuts make a protective shell, thereby making it difficult to be eaten. What about coconuts? Do most fruits cater to a particular group of animal, or are they indifferent?
I think you’re also missing the crucial point that cultivated bananas and walnuts, which have been carefully bred to maximise their attractiveness and utility to humans, are very different from their wild ancestors. Cultivated bananas, for example, have small or non-existent seeds. Clearly, this doesn’t meet the evolutionary purpose of using the fruit to entice animals to ingest and spread the seeds, but the plant no longer needs to do that, since there’s a species of ape that’s quite happy to ensure its survival.
Coconuts are a special case of dispersal by water; they can float for extended periods and the husk helps them resist salt water. The coconut seed can also go into an extended “hibernation” while it’s floating around on the ocean blue, only reactivating when it encounters fresh water.
You might want to read this article as an introduction to seed dispersal techniques: Seed dispersal - Wikipedia
Not all seeds in all animals. In fact, some seeds are adapted to different animal digestive systems. The chili peppers I mentioned, for example, reliably survive a trip through the bird species that typically eat them but can be destroyed by many mammalian guts.
In some cases the seeds need protection to survive a trip through a gut. Cows, for example, have quite the digestive system which involves several grinding steps, soaking in acid and enzymes, and quite a bit of time. Any seed that needs to survive that gauntlet will need more protection than something taking a trip through a chickadee. Elephants will swallow whole seeds and nuts you or I need to crack open and take apart. Trained science types can make a good guess as to the size and type of critter that is “supposed to” eat a fruit or seed based on a size and outer covering.
Other seeds have protective layers to survive harsh environments (like coconuts) or require some stimulus to be released, such as certain pine cones that require exposure to fire before they release their seeds.
Most are “targeting” a specific group, and as I noted with the chili peppers will also sometimes have traits that discourage other groups from eating them.
I don’t know if this also applies to walnuts, but for some nuts, like acorns, squirrels bury them during the fall, and dig them up during the winter for food. The squirrel may not find all the acorns it buried, or may die before digging them up. Now the oak tree has a bunch of its seeds nicely planted all around it. Win!
Meanwhile the hard shells protect the kernel from casual consumption by other organisms, and from environmental damage factors.
Okay! That seems to answer all my questions. Thanks to the posters. I hope some other people can also learn something from this thread, and it’s not just me who was clueless about this topic.
You mean, you hope you have helped disperse the seeds of knowledge more widely? 
Wait!
Are you suggesting coconuts migrate?
Not at all. They could be carried. 
Damn, I can’t believe I missed touching on that reference in my reply. I’m getting old.
Fruits are “designed” to be eaten. The plant provides the animal with food, in order to get the animal to move the seed to another location. As has been said, fruit seeds survive passage through the digestive tract and germinate where they are deposited. In fact, some seeds won’t germinate unless they have been passed through a digestive tract and the seed coat weakened by stomach acid.
Nuts, on the other hand, mostly don’t “want” to be eaten. They have a hard coat that discourages most animals from eating them. However, some are dispersed by seed hoarders like squirrels and some woodpeckers which may not return to retrieve some of the nuts they cache.
Most fruits are designed to appeal to and be dispersed by a particular group of animals. Small sugary berries are mostly designed to be eaten by small birds. Their seeds may not withstand digestion by larger animals. Large oily fruits like olives are designed to be eaten by larger birds that will carry them further.
So what do nuts “want” then? As I understand you, they don’t mind so much being eaten by squirrels, but it’s not their main thing. So what is their main thing? Is it being sturdy while being blown away by the wind, so that when the shell degrades to release the seed, it is hopefully far away.
Colibri,
Have to nitpick your last post. Can we change the word “designed” to “evolved”? I believe most Dopers know what you meant but in the interest of fighting ignorance, lets not give ammunition to the anti science crowd.
Respectfully
Capt
Hard-shelled nuts probably are mostly dispersed by squirrels or other rodents hiding them and then failing to recover him. They’re too heavy to be moved far by wind or water. However, unlike fruits they make it as hard as possible for the squirrel to eat them, while still not so difficult that it’s not completely not worth its time. And only a few specialized gnawers like squirrels or nut-crushers like wild hogs can get at the seeds.
That’s why I put it in quotes. The anti-science crowd is impervious to evolutionary explanations, so I don’t think it much matters here.
Upon re read, I missed the first quotes, thank you.
Capt
It’s to the squirrel’s advantage when some acorns get missed as well, in that their great-great-great-…grandchildren will have more oak trees to feed them.
As another specific example, you know how tomato seeds have a sort of slimy coating? That coating enables them to survive a trip through the digestive tract of a medium-large mammal. They probably didn’t evolve that in response to us specifically (they originated on a different continent from us, after all), but we’re digestively similar enough to whatever they did evolve in response to that they like our guts just fine. As a result, one plant you’ll very often find growing in the vicinity of sewage treatment plants is tomatoes, from all of the seeds that have gone through humans and into the sewer.