Do they serve a purpose beyond letting us know if we enjoy eating something?
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First things first. Evolution does not have a plan or purpose. Animals can and do evolve traits that might be beneficial or harmful. Animals may even evolve traits that have no impact on them at all. The actual mutation is random, there is no reason why animals evolved taste buds in the first place.
The real question is, “Why were taste buds an advantage for the animals that did evolve them?”
I think you already have the answer, but you have to go a little further.
Specifically, you mentioned
> beyond letting us know if we enjoy eating something?
Well there you go, but on an evolutionary time scale, we should think about it a little differently. Instead of, “I like toneat things that are good for me”, it is more, “Things that are better for me taste better”
Imagine a brand new batch of organisms with the first ever taste buds.
What do they like to taste? Who knows, they never have tasted anything before, but for the sake of the example, let us assume that each creature likes different things. Their taste buds are a little different and brains wired a little different, so some like sweet things, some like bitter things, etc etc.
The creatures who like sweet things, begin to eat more fruit. Foods packed with more sugar and energy than grasses or leaves, and so they have more energy, are more fit, and more likely to survive.
The creatures who like bitter things end up eating a lot of poisonous foods (many poisons and compounds that are harmful do taste bitter). Those creatures get sick, or worse, they may die from their tastes. They arent able to compete with the other animals for food, mates, or living space. They have a harder time fighting off predators.
So, the creatures who enjoy foods that are better for them, will be more likely to survive and reproduce than the creatures who had taste buds that made bad food appealing. (An important note here is that evolution didn’t “pick” winners here. Although we live in a world where sweeter plants were more high energy foods, it could have been the case that bitter foods were high energy.)
Eventually, the creatures who like bitter things die off because the mutation hindered them from thriving.
Evolution by itself does not favor one thing or another. Change can, and does, happen in completely harmless ways. Look at humans being left or right handed. They are different, but niether one is better than the other, so we have a world full of right and left handed people.
The thing that caused many mammals to have taste buds was selective pressure COMBINED with mutation. Evolution requires both to exist. Mutations create differences and Selective Pressures pick winners.
But then why are their people who really enjoy bitter things, or foods that are bad for you?
One of the amazing things about humanity is our non-evolutionary adaptations. You like eating bitter things that would naturally be poisonous? Congrats, instead of eating poison, here is some bitter candy. Evolution is still happening, taste buds, like anything else in our body are not fixed, different people will still have different tastes. However we as a society have a lot of tools that removes the selective pressure, so no single mutation becomes dominant.
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