Loss of protective characteristics in plants

Faith & Science Conversation
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Hi, I recently watched a BBC series about evolution on islands (Nature’s Wonderlands). In the one about Hawaii, the presenter said that a particular type of mint plant had lost its protective characteristic of smelling of mint as produced by a certain chemical, because the mint plant no longer had to produce it as there were no herbivores to eat the plant, so it did not have to protect itself.

My question is - how did the plant come to ‘know’ that it no longer had to protect itself from being eaten, and therefore gradually stopped producing the particular chemical? If youre interested, this part of the programme is towards the end.

Many thanks, Peter

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Hi Peter,

Welcome! Thanks for coming and sharing your question here. I hope we Forum folks can be of service to you as you explore this and other questions you may have.

I’m not a biologist, but I like to play one on TV. :slight_smile: (I’m a linguist, but I enjoy talking about biology avocationally here.) This is one I question I may actually be able to field, but I’ll let other people correct me if I’m wrong.

Mutations just happen. They happen randomly with respect to fitness. That is to say, some mutations are beneficial, some are detrimental, and a large number are also neutral, neither good nor bad for fitness.

After mutations happen, natural selection occurs and it is this that decides whether an organism survives and reproduces and dominates.

So let’s imagine some other place… let’s say a deciduous forest in Pennsylvania. Lots of herbivorous white-tailed deer there. Let’s imagine that the mint mutates in PA to lose its mintiness. Will it survive and reproduce? Well, perhaps for a time, because deer might be smart enough to recognize it and steer clear. But the point is that eventually, one of the many herbivores in that ecosystem will take a bite, and they’ll realize it’s not so nastily minty anymore, and then that variant will wind up disappearing before it takes over. The mutation will result in negative fitness, and it will naturally get weeded out.

Contrast this with Hawaii. There, the same mutation happens, again randomly. This time it has no effect on fitness. It doesn’t “know” anything. But the environment fails to shape the destiny of this variant to destroy it. So it survives and reproduces. Perhaps it naturally takes over (could be a dominant gene), or perhaps it has an advantage over mintier mint because it doesn’t have to commit resources to creating its mintiness.

Does this make sense?

The same could be said of island-bound flightless birds (e.g., kiwi), humans’ inability to create Vitamin C, and all kinds of other such mutations.

Hope this helps. Feel free to ask follow-ups, of course.

Merry Christmas,
Andrew

[Edit multiple times for typos.]

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In my garden I grow a tree from central America called Wigandia caracasana. It is notable for its enormous umbels of purple flowers in the spring but alas also for the irritation which its huge leaves can cause. Interestingly the substance secreted by its leaves is only produced on leaves growing within several feet of the ground -just where it might draw interest from herbivores. Above that the leaves are safe to handle.

Now I don’t think the plant ‘knows’ it needs protection for its lower leaves, but those which had it would have a greater capacity to thrive than those that didn’t. Likewise I don’t think the plant ‘knows’ the substance isn’t needed for its upper leaves; however since plants that expended the resource didn’t receive any added benefit there is no reason for it to have been selected for.

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Hi Andrew, thanks for the reply. This is the sort of thing that I thought probably happened, but your explanation is NOT how the biologist framed it. Here is exactly what he says:

‘Even plants can loose protective characteristics in the safety of an island. It has lost what it didnt need, which is the protective chemicals that protect most mints from being eaten by herbivores. It hasnt gone to the trouble of making those chemicals anymore because it didnt need them in the past, though it needs them now because of pigs, sheep etc’ (hence why it’s kept in a greenhouse).

So he certainly does not give the impression that it was due to randomness that this plant lost its mintiness. This is why I get confused, evolutionists are often unclear as to what they think ACTUALLY has happened, and they use language that sounds as if the evolutionary mechanisms are ‘directed’ when in truth as I understand it evolutionists do not believe that. Though personally Im not so sure. And Merry Christmas to you!

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Thanks for that example. But per your final sentence, how does the plant know there is no ‘added benefit’ to having the irritating leaves above a certain height. Is evolution that ‘intelligent’?!

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I don’t know this particular story, but there are two possible (and potentially overlapping) biological explanations here.

  1. One possibility is that the plants have lost the capacity to make the chemical, due to lack of selective pressure on its continued production. This is a standard, classic Darwinian evolutionary process. In the presence of herbivores, production is selected for, or perhaps more likely, absence of production is selected out of the population (via the guts of herbivores). Without the herbivores, the occasional variant that can’t make the chemical is not selected out of the population. Then, more interestingly, the story sounds like one in which the non-producers took over the population. If that is the case, then the most likely explanation is that there is a cost to making the chemical, and so in the absence of herbivores, its production turns into a burden instead of an advantage. The plants that don’t make the chemical enjoy a slight advantage and take over.

  2. The other possibility is that production of the chemical is turned on in the presence of herbivory (or in the presence of the herbivores themselves) so that it is only employed when needed. There are cool interesting examples of this kind of interspecies interaction in both plants and animals. In fact, for example, plants emit chemical signals at different times of day (in response to light) and detect and respond to vibrations induced by leaves being munched.

In neither case does the plant need to “know” anything.

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This is just careless wording. He is talking about a population of plants over generations, not a single plant with a will to survive and control over its mint scent production.

The mutation or the naturally occurring variation in the population that results in a certain plant not being very minty would be random. But the mechanism of selection that acts on the genetic variation is not random. It is “directed” by factors in the environment that make one random mutation or gene variant more beneficial than another and more likely to proliferate in the population over time. So, in that regard, adaptations are a response to the environment (ability to get food, avoid predators, survive adverse weather conditions, etc.) or they give some other reproductive advantage (being attractive to mates, or allowing more offspring reach maturity, for example). The adaptations themselves aren’t random.

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Yes, I’m fairly certain this is a bit of anthropomorphism, but @sfmatheson will no doubt confirm better than I. (Unlike me, he actually is a biologist…)

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Okay, a couple of additional comments.

I just read about the Hawaiian mints and the story is VERY interesting. It’s just another explosive radiation in one of the most isolated ecological situations known to humans. There are other examples but it seems the Hawaiian mints are among the most prolific radiations we know. Thanks for bringing this topic up! I’m glad I read more about it.

The sad part is that the mints are endangered and many are already extinct. According to the recent genomics papers I found, the radiation is really hard to study because it happened so fast–the colonization happened less than 5 million years ago, from populations in North and/or South America. Given that this is a process that happened over a few million years, and not over a few centuries, it is likely that the Hawaiian mints cannot make the anti-herbivory chemicals, and so my first hypothesis above would be by far the most likely.

The other thing I would add is that if plants can adjust their chemical signals after they hear the crunching of herbivores and can time their chemical signals to achieve the best effect, and then if they can set elaborate traps to put pollen onto insects or to actually eat insects, then… they’re pretty smart, and arguably smarter than most Americans. So there’s that.

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It was unfortunately worded, perhaps excusable as a metaphor in which “the plant” is the population. I agree that the sense of agency and/or purpose that the scientist is using is potentially confusing, but IMO it’s only confusing to people who don’t know anything about evolution or to those who… want to be confused. Maybe that scientist peppered her/his words with teleological excesses the whole time, but in general this kind of language when talking in brief to a lay audience shouldn’t be such a big deal.

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This is an interesting thread! Thank you very much for posting it. As a family physician, I’m reminded of the the rise and fall of mutations in plasmids that confer resistance to different antibiotics. Pressure from overuse of a certain kind of antibiotic can cause most bacteria to be resistant, but as it cost the bacteria extra energy to manufacturer resistance, if you remove the pressure, often bacteria will lose the resistance again. I’m not sure how often they retain the plasmid and pass it on, or if they ever lose it or just don’t activate it. However, I know that bacteria can share plasmids in some cases among themselves. Biologists on this Forum can correct me, however.

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That is indeed the modern classic Darwinian story. The ubiquity of resistance plasmids means that the outcome of antibiotic use (not just overuse) is the selection of resistant subpopulations. Clinically, this is usually fine, since the goal is to reduce the size of the population so that the immune system can take care of business. This is why antibiotic therapy is so different conceptually from anti-cancer chemotherapy–in most cases we can’t rely on the immune system to finish the job when attacking cancer. Anyway, back to antibiotics, in a molecular biology lab, plasmids are used as tools to clone and amplify DNA, exploiting the antibiotic resistance genes they carry. You simply insert your DNA of interest into the plasmid, then induce bacteria to incorporate the plasmid. You then select for the bacteria you want, using the antibiotic as the selection agent.

But I think that HIV resistance (meaning resistance of the virus to antiviral therapy), within individual human bodies, is an even more dramatic example of selection/resistance, with the cost/benefit analysis playing out in therapeutic strategy.

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Hey, I’m an American so I can tell you you’re absolutely right about that.

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Thanks. If you ever get a chance to talk more about HIV, I find that fascinating. Coming from West Africa where it grew very quickly, I made my residency term paper on HIV management from the primary care setting (it’s certainly less important in the US but I was planning on going back to Africa) and found the initial medications like AZT and their research trials fascinating. I did my “fellowship” in rural medicine in Johnson City, TN, where Abraham Verghese based his book on HIV work “My Own Country,” and learned from some of his former coworkers, which made it more interesting yet.

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On a side note, I wonder if you or @sfmatheson would happen to know the answer to a question I have about all this. When it comes to the mint losing its protective chemicals since there is no longer any predation -or- when the tree I mentioned only develops the protective chemicals at the needed heights, is there some sort of entropy at work which works to withhold resources unless there is a selective pressure at work to reinforce them. I guess the alternative would be to either imagine that the loss of the adaption would be entirely random or else to posit divine guidance.

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I don’t know of any need to invoke a particular “sort of entropy” to explain anything in biology, but I am probably not understanding the question. My best guess is that you are wondering whether evolution is constrained from exploring design space when there is no “need” (or “selective pressure”) to do so. I think the answer is no, in principle, but often yes in practice, simply because even minor costs associated with adaptations will be visible to selection.

I do think that it is easy (and an error) to fail to consider the mere existence of variation. We know about variation, can measure it, etc., and this means that all sorts of potential adaptations, however tiny, are currently in existence in populations, and no one (AFAIK) is asking about whether entropy is somehow withholding resources. Hope that helps.

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Thanks. I find it interesting that you say plants may be ‘smarter’ than we think. I watched another programme a while back about fungi, and if I remember correctly, the underground root systems have been likened to a neural network. Perhaps Prince Charles was right all along, we really should talk to plants!

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Yw. So is that why we are encouraged not to keep using antibiotics (imo one of the most important human discoveries) so that even if bacteria become resistant, if we cut back on our usage those bacteria may no longer be resistant, at least for a time?

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I dont think that’s fair. Such programmes are for lay people like myself, not trained biologists or evolutionists, and I certainly dont want to be confused. I actually want to understand! Using such inappropriate language is, well, inappropriate. As someone with a background in physics, it’s like me saying the ball continued in it’s straight line motion because it wanted to and had no need to deviate from its course!

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Hi, it’s a lot easier to have a conversation if you include the text you’re responding to.

I assume you are responding to my comment here:

I agree that the sense of agency and/or purpose that the scientist is using is potentially confusing, but IMO it’s only confusing to people who don’t know anything about evolution or to those who… want to be confused.

If you thought that the scientist actually meant that plants make decisions, and that this is a description of evolution, then… either you don’t know anything about evolution (that’s not an insult at all) or you are actually claiming to be “confused” by these obviously metaphorical comments on a lay-level program. What I’m saying is that if you are genuinely confused by language like that, you don’t know much about evolution.