In my understanding, the evolutional function of berries is to be eaten and pood out somewhere else, so that the seeds of the plant spread. Is this so? Then why are some berries poisonous?

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    $\begingroup$ A berry may be poisonous for one species, but not for others. See also: ask.metafilter.com/131461/Dont-they-want-to-be-eaten $\endgroup$ Sep 10 '12 at 13:38
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    $\begingroup$ I have no clue whether or not it is true but maybe it is for the same reason why pathogenic bacteria are pathogenic. The berries are poisonous so that they can kill the host and that in turn is the mechanism of plant spread. $\endgroup$
    – bobthejoe
    Sep 11 '12 at 3:23
  • $\begingroup$ A great explanation, and gorgeous pictures, on the Kew blog this week: Why Fruits Are Poisonous $\endgroup$ Aug 30 '13 at 10:53

While poison affects not every organism equally, plants did develop some poisons to avoid being eaten. However, if you look at the great multitude of so-called secondary metabolites, most of them are poisonous to either viruses, bacteria, fungi or other microorganisms, or insects, or even other plants. Plant evolution just hasn't had time to adapt to humans.

So, if some substance from plants is a poison to us, it's accidentally so, and the target was another organism. Examples: nicotine is first and foremost an insecticide, nematistat, and herbicide; aconitine, atropine, caffeine are insecticides; many essential oils (causing allergies in human) are antimicrobial and so on.

This has also to do with the fact that nerve cell physiology has not much changed since they developed in the first multi-celled animals, and poisons for insect nerves have at least some effect on human nerves.

  • $\begingroup$ All along I thought that the lack of poison and the presence in other berries is due to co-evolution between berry eaters that don't digest the seed wall(advantageous to the plant) and berry eaters that can digest the seed wall(not advantageous to the plant). $\endgroup$
    – niobe
    Oct 29 '16 at 20:08

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