Why would a plant evolve to produce an addictive chemical?

Question

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

Answer 1

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

• Plant-insect coevolution and inhibition of acetylcholinesterase
• The defensive role of alkaloids in insects and plants
• Exploration of nature's chemodiversity: the role of secondary metabolites as leads in drug development

Also of interest

• Bees prefer foods containing neonicotinoid pesticides

Answer 2

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.

Answer 3

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).

Answer 4

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).