evolution - Why are there so many medicinal plants?

Question

Quite a few plant species can be used for medicinal purposes wiki. As an example, Filipendula ulmaria is rich in acetylsalicylic acid (aspirin).

An allele that produces a substance which is beneficial for a predator should not get fixed in the population! Why are there so many drug-like plants?

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Thoughts

Here are some (intuitive, unclear or far-fetched, non-exhaustive and non-exclusive) hypotheses I can think of:

1. 
   

   Because plants want their seeds to get ingested in order to propagate

   
   


2. 
   
   

   Is it because there are so many substances that affect our homeostasy that many plants are toxic and many are healthy just by chance. Containing healthy substances are not adaptations but by-product of evolution.

   
   


3. 
   

   The substances that are beneficial for us actually evolved to repulse predators. These substances are toxic at high dosage and therefore are efficient against predators that are small or eat a lot of plants (herbivorous). These substances at low dosage might actually have a benefical effect. For example a substance that make blood thicker is very toxic except if you eat just a bit of it while bleeding.

   
   


4. 
   

   We (primate or whatever taxon you want to consider) evolved in order to get advantage of the surrounding envirronment. Sbstances that were neutral became beneficial. The advantage of being sensitive to various products cause that, by chosing our food source, we can heal. Therefore, by evolving sentivity to various substances, our behaviour can act as a reinforcement to immunity (and other anti-illness system). If this is true, we might expect that the frequency of presence of a plant species affect the probability that out homeostasy get affected when eating it.

   
   


5. 
   

   Lineage selection. Lineages that produce substances that are active in predators body in a way or another are undergoing many various selection forces because of these substances. Therefore they get a higher speciation rate than other lineages.

   
   
   


6. 
   

   Keeping some non-herbivorous species in large population size (by helping them) is the best way to keep herbivorous species in small population (because of territory competition, predators relationships, etc... )

   
   

Answer

Perhaps the question may also be phrased, "Why is it common for plants to produce chemicals that possess pharmacological or toxicological effects in man and animals?", and to that question it is often reasoned that plants, being sessile and otherwise defenceless food sources for predators, produce compounds that affect the physiology of animals in such a way as to make it not beneficial for a predator to consume them, which is close to your 3rd hypothesis. Remember that pharmacological benefit is largely anthropocentric, and relative at that. For instance, a plant compound that lowers blood pressure could be used as treatment of hypertension, but if the plant were to be consumed in its native state in appreciable quantity, it could put the eater into shock, cardiac arrest, etc.

Most pharmacologically active compound from plants comprise a class of compounds known as secondary metabolites: metabolic intermediates that are not appreciably involved with functions of growth, respiration, etc. (often interpreted as evidence for a defence mechanism).

Try this wikipedia article and see what you think: http://en.wikipedia.org/wiki/Plant_defense_against_herbivory

The major classes of secondary metabolites in plants are: terpenes, which are polyisoprenoids, phenolic compounds, including the polyphenolic compounds, which are a popular study in natural products research, and alkaloids (along with other nitrogenous compounds).