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It seems kind of anti-productive in terms of survival for a plant to produce an addictive chemical as that plant will constantly be sought after by animals that ingest it. In this instance, I'm looking for a possible general & inclusive answer here that would describe most plants that make this. Not a specific instance (although if provided as an example would be a plus).

To appreciate the scope of this is terms of number of plants producing potentially addictive compounds - see this compendium:

compendium of botanicals reported to produce toxic, physchoactive or addictive compounds

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    $\begingroup$ Compounds like nicotine and caffeine are poisonous, especially to insects. $\endgroup$ – canadianer Aug 4 '15 at 19:28
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    $\begingroup$ The supposed addictive properties easily could be random side effects, unrelated to whatever function the chemical performs in the plant. Remember that evolution does not plan ahead. $\endgroup$ – jamesqf Aug 5 '15 at 4:27
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    $\begingroup$ @canadianer There was an article in science magazine some months back which said that bees develop a stronger memory of flowers that contain caffeine in their nectars. $\endgroup$ – WYSIWYG Aug 5 '15 at 4:30
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    $\begingroup$ @WYSIWYG That's quite interesting. I was also thinking that such compounds, being addicting, could aid in seed dispersal by higher animals. $\endgroup$ – canadianer Aug 5 '15 at 4:35
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    $\begingroup$ Well, see. Coca or tobacco plantages house billions and billions of plants. That's a huge success in terms of Darwin! $\endgroup$ – phresnel Aug 5 '15 at 8:36
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It's a matter of perspective. Most of the chemicals that are addictive to us humans (particularly alkaloids), and may be addictive for some other animals as well, are also insecticides. Lots of plants that we consider poisonous are good food for other species, and lots of plants that insects would consider poisonous are treats for us.

This is a great example of the aimless nature of evolution. The plants that could successfully defend themselves against insects stabilize on a solution that happens to be bad for them in certain ways. Although, you would be hard pressed to find a better way to guarantee reproduction than being addictive to humans.

Background reference

Also of interest

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  • $\begingroup$ Good answer, gonna keep this open for a couple more but I like this $\endgroup$ – rhill45 Aug 4 '15 at 21:19
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    $\begingroup$ No addictive products produced by plants can be found to have exclusively detrimental effects on the plant, they all have mostly positive effects, and plants can minimize or maximize a single chemical at their disposal, over some dozens of generations, because it's only a single chemical. Chemical changes are the most usefual and evolutionarily cheap method for plants to ward off or attract animals, easier to adjust than color, shape, hairiness, stinging cells, size, and so on. $\endgroup$ – com.prehensible Aug 5 '15 at 8:12
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    $\begingroup$ Indeed. Even if they are generally toxic to the plant's system, they simply store them in vacuoles. $\endgroup$ – jzx Aug 5 '15 at 8:18
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    $\begingroup$ + 1 for "Although, you would be hard pressed to find a better way to guarantee reproduction than being addictive to humans." $\endgroup$ – lorless Aug 6 '15 at 10:53
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    $\begingroup$ Add to that most plants are not nearly so addictive in their natural forms as they are when processed. So, the problem probably self regulates to an extent until such time as we decide to break out of the natural system and cultivate it. $\endgroup$ – Jason Aug 6 '15 at 17:44
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As someone commented earlier, chemicals such as nicotine and morphine were products of evolution meant to repel animals. It is explained in more details in this article here.

Evolutionary biologists studying plant–herbivore interactions have convincingly argued that many plant secondary metabolites, including alkaloids such as nicotine, morphine and cocaine, are potent neurotoxins that evolved to deter consumption by herbivores.

But it seems that those same chemicals produce adverse effects to what they were originally intended for:

For example, one or more plant alkaloids have been identified that interfere with nearly every step in neural signalling. Targets include neurotransmitter synthesis, storage, release, binding, deactivation and reuptake, ion channel activation and function, and key enzymes involved in signal transduction.

Paradoxically, the same properties invoked to explain why common drugs like caffeine, nicotine and cocaine are toxic are also those invoked to explain why these compounds are rewarding. It is therefore important to stress that these and other addictive drugs appear to have evolved only because they successfully deterred, not rewarded or reinforced, plant consumption.

For example, let's take a closer look at nicotine. This compound is not present at all times in the plant, instead it is produced as a reaction to a trigger.

Nicotiana attenuata is an important model species for the analysis of plant–herbivore interactions involving nicotine. It is a domesticated North American tobacco plant that is attacked by over 20 different herbivores, ranging from mammalian browsers to intracellular-feeding insects. These attacks elicit a battery of defensive responses, including nicotine production.

Nicotiana has therefore evolved to allocate chemical defences strategically by concentrating them in the most valuable parts of the plant, such as young leaves, stems and reproductive organs, and by modulating its production according to the type of herbivore and extent of leaf damage.

This last example concentrates on nicotine, but it makes it easier to grasp how plants might use the production of such a chemical as a mean of protection.

Also, depending on the substance and the creature which consumes the plant, one might witness different outcomes. I found interesting information in this less detailed text here:

Defensive compounds from plants, like nicotine and cocaine, usually target nervous system components in insects. These components include proteins that have important roles on the insect’s physiology, which may include specific receptors, ion channels, enzymes, etc. In most cases, the defensive chemical kills the insect by interfering with one or more of these proteins; in other cases, the chemicals just make the plant distasteful for the insect, and therefore, the bug will leave the plant alone.

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I am providing an example which somewhat contradicts the points mentioned in the other answers regarding toxicity of alkaloids to insects.

Caffeine is a stimulant and is toxic at high doses (also for humans) but at low doses it has a stimulating pharmacological effect on the organism. The same principle applies to insects as well. A study by Wright et. al (2013) has revealed that caffeine in the nectar of some flowers, enhances the bee's memory of that flower (a reward, in general).

They have also mentioned that:

Two caffeine-producing plant genera, Citrus and Coffea, have large floral displays with strong scents and produce more fruits and seeds when pollinated by bees (8, 9)

However, caffeine tastes bitter and bees would reject nectar (sugar solutions) containing high levels of caffeine (>1mM).

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  • $\begingroup$ Arguably, caffeine acts as a stimulant at all doses, which is why it is toxic at high doses. $\endgroup$ – March Ho Aug 5 '15 at 4:51
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    $\begingroup$ It's also arguable that everything is toxic at high doses - including oxygen and water. $\endgroup$ – jamesqf Aug 5 '15 at 18:00
  • $\begingroup$ Caffeine has an overall positive effect? That's not what I heard: New research from Johns Hopkins Medical School shows that performance increases due to caffeine intake are the result of caffeine drinkers experiencing a short-term reversal of caffeine withdrawal. By controlling for caffeine use in study participants, John Hopkins researchers found that caffeine-related performance improvement is nonexistent without caffeine withdrawal. (found at linkedin.com/pulse/… ) $\endgroup$ – Mason Wheeler Aug 5 '15 at 19:48
  • $\begingroup$ As for bees developing a preference for addictive drugs, that's actually turning into one of the great tragedies of our age: part of the reason honeybees are dying off so catastrophically in the last few years is because of the introduction of new pesticides derived from nicotine, which, in addition to being highly toxic to bees, is also highly addictive, so they just keep on coming back for more until it kills them. Entire colonies worth of them. $\endgroup$ – Mason Wheeler Aug 5 '15 at 19:54
  • $\begingroup$ @MasonWheeler Caffeine and Nicotine has been existing for several thousands of years. $\endgroup$ – WYSIWYG Aug 5 '15 at 20:48
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Short answer
The appearance of psychoactive compounds in plants has nothing to do with their addictiveness in man.

Background
Psychoactive plants were there long before humans. The question therefore should be: "Why would humans evolve brains that exhibit addictive propensity to poisonous compounds abundantly available in nature"? The answer is: because our brain evolved in the absence of addictive substances.

One has to realize that addictive compounds are grown and processed. When humanoids evolved over millions of years, for the larger part there were no means to grow, harvest and process coffee beans, tobacco and coca leaves. Moreover, the most addictive drugs like injectable heroin and smokable cocaine (crack) are chemically purified. Methamphetamine and many addictive opiates are purely synthetic. In their native form, coca leaves and poppies are far less addictive, because in the raw form they lack the dopamine rush. Instead, the chewing of raw plant materials like coca leaves produces a mild high with a slow onset, and a mild offset. The sudden dopamine rush is what evokes the blissful euphoric state chased by heroin, crack and meth addicts, while the dreadful crash associated by these purified drugs is one of the strong motivators to seek for another hit. Also note that tobacco is heavily processed through a curing process before it is sold. The raw, woody tobacco products are far less likely to cause addiction.

Moreover, note that many of the addictive stuff originates from the Americas (coca) and Asia (opiates). Humans came from Africa. Africa is one of the continents with very, very few drug containing plants (Qat being the exception - a mild drug).

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  • $\begingroup$ It's a good response but not what I'm looking into. It's pretty obvious that purifying cocaine and heroine make a much more addictive version. And the disease model of addiction...another topic. I was trying to question a natural selective process in which these compounds have apparently developed a survival niche in a sense $\endgroup$ – rhill45 Aug 5 '15 at 13:02
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    $\begingroup$ @rhill45 - and that line of thought is exactly what I counter here. I understand it is not what you want to hear, but I deemed it appropriate to share my view. Others have explained why plants produce drugs, I explain why the appearance of those plant-based drugs have nothing to with addictiveness in man. $\endgroup$ – AliceD Aug 5 '15 at 13:05

protected by WYSIWYG Aug 5 '15 at 8:10

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