There are many types of plants which produce chemicals which can be beneficial in treating human illnesses, or at least alleviate painful symptoms.

Why would they do that?

Is it just random chance, like some plants being poisonous, some having no effect, and some having beneficial effects? However, for poison there are evolutionary benefits, as it might reduce the chance of animals eating the plant.

Many plant species produce tasty fruits for the purpose of having animals eat them and disperse the seeds. However, for most plants used for medicinal purposes, the leaves, roots or flowers are used, the cultivation of which doesn't seem to be useful for the plant itself.

  • $\begingroup$ Fruits serve a purpose, they contain seeds which are dispersed by the consumer. There is generally no benefit to having leaves and roots eaten, any effects on consumers are likely to be coincidental... I think that's the general principle - that may be wrong and there will probably be exceptions so I'm not posting it as an answer $\endgroup$ – rg255 Feb 22 '16 at 22:15
  • $\begingroup$ Also note that not everything evolves for a reason, there are four mechanisms of evolution - only selection leads to adaptation (ie gives things a reason to have evolved) see illustrations here biology.stackexchange.com/questions/5007/… $\endgroup$ – rg255 Feb 22 '16 at 22:30

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