'adaptation' or just 'heritable deformity'

[leilani]

I think we too often assume unusual morphological features are benificial adaptations when they may just as well be detrimental heritable deformities.

Here is a possible example .... N. fusca, Sarawak

I can see no advantage to the characteristically narrow lid .....

[Dave Evans]

Dear Sam,

Probably a lot of attributes are neither positive or negative. They don't have to help the plant, but at the same time detrimental features would not be selected for so I rather doubt this N. fusca characteristic could be considered a deformity... A deformity of what, per-se? How does it hurt or reduce the plants' abilities?

[leeb]

There are a number of highland Nepenthes that have infundibuliform pitchers, reduced, narrowed or reflexed lids, sticky pitcher fluid and a pale upper pitcher colour.
These species include at least two unrelated groups and occur in Sumatra, Sulawesi and Borneo.
It looks like these features are being selected for; we would need to know more about what they are trapping in order to know why.

LeeB.

[leilani]

Probably a lot of attributes are neither positive or negative. Dave Evans

We call attributes "positive" or "negative" according to how they fit our explanatory narrative. The typical causal story goes something like this: First we assume that the reduced structure of the lid serves some function such as "enhanced water collection capability" or "increased ease of access for flying insects". These seem reasonable and simple and we like that in our stories. We would then go on to suggest that this plant had, via 'selection', evolved a reduced lid for this purpose.

... detrimental features would not be selected for ....Dave Evans

It looks like these features are being selected for ..... leeb

The big question is "Selected how?" .... by what mechanism? The answer is, of course, "Natural Selection" and "survival of the fittest". Both of these are insightful and powerful concepts but, none the less, have never left me feeling truly satisfied with the story. After all, neither concepts seems to work in the short term and it is only when coupled with an expanded metric (millions of years) that they begin to offer explanation at all.

... we would need to know more about what they are trapping in order to know why. leeb

You have already assumed a causal model (our favorite kind) and would seem to have a story in mind. All you need to do now, and you seem prepared for the task, is work backward building a explanatory narrative to support your assumption. There you have it .... a good story that fits comfortably into our orderly library.

I think we, both lay person and scientist, tend to construct these stories around the data we collect in order to co-relate discrete data and compose a comfortable causal explanation. N. lowii has evolved to capture bird droppings, N. albomarginata has evolved to capture termites, N. ampullaria has evolved to capture leaf litter, etc.

On one level I have no trouble with any of the examples as stated above. N. lowii has "evolved" and it does "collect bird droppings" (I am not sure "capture" works with droppings and vegetable material in the same way as it does with insects.)

But there is often more going on here. The introduction of a causal element: "N. lowii has evolved to capture bird droppings." This is where I get uncomfortable. Is the "to" in the above sentence intended to simply mean the evolution of this plant has "resulted in" its collecting bird droppings? If so, then the only (trivial) problem I would have with it is that, to be consistent, it cannot be seen as an end-point but rather as a way-point. (The only true terminus in evolution is extinction).

On the other hand, the "to" in the above sentence is very (more) often used, together with "selection", to support our causal hypotheses. There is an implied willfulness in this case ("N. lowii has evolved (in order) to collect bird droppings.") that I think is inappropriate.

It seems to me that Nepenthes pitchers have evolved into collection (entrapment) mechanisms that are about as non-specific as they could be and I wonder if most of the speculation we indulge in regarding specific prey or collected matter are not just spinning a narrative that re-enforce our desire for simple, complete and orderly story telling.

[stevestewart]

[quote author=leilani board=evolution thread=3214 post=9781 time=1251616040It seems to me that Nepenthes pitchers have evolved into collection (entrapment) mechanisms that are about as non-specific as they could be and I wonder if most of the speculation we indulge in regarding specific prey or collected matter are not just spinning stories that re-enforce our desire for simple, complete and orderly story telling.[/quote]

Sam,

I think many species of most bio-organisms including Nepenthes have very specific mutations that are only utilized in their natural ecosystems. Whether those systems exist as they would have without all current mechanisms in their current form today, is a no brainer. (NO!) It is also true (and obvious) that none of the current natural systems will remain unchanged for all time.

I always like to think of Nepenthes bicalcarata for co-evolutionary species "mutation" examples. There are more organisms involved in this Nepenthes species than one species of ant. Bacteria, Cyanobacteria, other arthropods and native animal species all play important roles, as do horticulturists, biologists etc... ;D.

Our understanding of the systems involved in nature are limited, and a nice complete story makes for good reading, and is the only way to get these "stories" published by educational institutions. Sensational stories often need to be more obscure (impossible for the commoner to fact check) for publication.

When I first read about N. bical. I only had a book by Bailey, "Cyclopedia of Botanical Plants" accounting Burbridge's story of how the "strong sharp spines helped the species protect it's contents from the thieving habits of the small lemur Tarsius spectrum". This book also mentions the ants that hollow out the curl of the tendril forming a myrmicarium.

Having grown this species in Florida, without it's natural partners, I found the plants to be very attractive to ants in general, as well as all species of Florida animals that are attracted to ants, nectar, and the smell of decay and whatever the smell (bouquet?!) of the flowers is. None of the animal species in Florida where I grew N. bical. hollowed out the tendrils. Many species of animals were caught by the plants, including many arthropods, reptiles, amphibians and an occasional small rodent.

I believe these plants became general opportunists because of the unnatural environment they were being grown in by me. I also believe the amount of prey captured by my plants was to some extent too much for the plants to deal with when they became mature. I was forced to remove young pitchers that caught prey prior to hardening, because of black rot that would spread into the plant from the pitchers.

Time and natural selection would have to alter this Nepenthes species if I had continued to grow large numbers of them, and the environment of Florida, or increased cold tolerance of N. bicalcarata ,had allowed the species to become naturalize here. I still grow several plants that were in sub 32F /0C. and recovered.

If this post isn't example enough, a non rambling straight forward, know it all story, is easier to digest. All life is opportunistic, and needs to mutate for the sake of survival.

Understanding every adaptation in Nepenthes is impossible, because of our limited understanding of all aspects of Nepenthes evolutionary history.

Take care,
Steven Stewart

[leeb]

Sam,

sorry if my statement seemed overly teleological.
I am aware that people compose narratives and even "just so stories".

Also that evolution is a continuous process with no defined endpoint; the differences in the shape of the lids of the species I mentioned shows this.

However when I see members of different groups of Nepenthes converging on a similar pitcher and lid morphology I start to suspect that convergent evolution is at play, this implies that there is a selection pressure that these Nepenthes species are experiencing and that if we look closely we may be able to discern what it is.
Having multiple species experiencing this selection pressure makes it more likely that we may be successful in this endeavour, as we can compare the environmental factors that they experience.

I think to not attempt to do so is a mistake; we could be overlooking something interesting.

And while I agree that most Nepenthes species may be generalists in their choice of prey I also believe that some have evolved to become specialists; N. albomarginata being a particularly good example; while it can catch a broad range of prey it appears that most of it's nutritional requirements are met by catching social termites as these far outweigh all other prey.

LeeB.

[leilani]

I think many species of most bio-organisms including Nepenthes have very specific mutations that are only utilized in their natural ecosystems. stevestewart

A very good point and your example of N. bicalcarata and it symbiotic relationship with particular ants drives it home. It may very well be possible that the reduced lid of N. fusca, Sarawak serves some purpose in it’s natural habitat that would be unrecognizable outside of that environment.

I believe these plants became general opportunists because of the unnatural environment they were being grown in by me. Stevestewart

My first inclination is to say that the plants did not “become” anything. They were merely exposed to a different (“unnatural” and perhaps, too rich) feeding environment.

The idea of Nepenthes exhibiting new environmental adaptations, extending their tolerances or otherwise changing the patterns they exhibit in their natural habitats, when grown in the novel conditions of a horticultural setting, sounds like a fascinating topic but, for now, I want to go in another direction.

... when I see members of different groups of Nepenthes converging on a similar pitcher and lid morphology I start to suspect that convergent evolution is at play, this implies that there is a selection pressure that these Nepenthes species are experiencing and that if we look closely we may be able to discern what it is.

I would say that there is certainly evidence of convergent evolution in the examples you cite. (We would, of course, need to clarify what we mean by “convergent evolution” but, at the moment, I’ll leave it at that.)

What I don’t see is your suggested conditional .... if, “ ...convergent evolution is at play ...” then, “ ...there is a selection pressure that these Nepenthes species are experiencing ...”

This sounds like it is playing to a narrative.

I’m not at all sure what “selection pressure” you might be referring to or that “environmental factors” serve (in any exclusive or fundamental way) to explain “convergent evolution”.

And while I agree that most Nepenthes species may be generalists in their choice of prey I also believe that some have evolved to become specialists; N. albomarginata being a particularly good example ... leeb

This is exactly the example I was hoping for.

I finally received my copies of Stewart’s new books and the narrative coming from Merbach’s studies of N. albomarginata and termites has apparently established itself well enough to prompt the heading “The Termite-specialised Trap of Nepenthes albomarginata”!

I have been skeptical of the narrative surrounding this study from the first and re-reading it in the form presented in Stewart’s book only further enforces my feeling that the information gained from these rather simple studies is being blatantly spun to fit into a pre-conceived story line.

... while it can catch a broad range of prey it appears that most of it's nutritional requirements are met by catching social termites as these far outweigh all other prey. leeb

quoting Stewart's new book .....

"Moran et all, (2001) traced isotopes to demonstrate that N. albomarginata derives nitrogen primarily from the trapping of termites whereas in other (sympatric) Nepenthes species, nitrogen is primarily acquired from other prey types, particularly ants." Pitcher Plant of the Old World, Volume I

Talk about spinning backward narratives .....

At one time I grew some of my N. veitchii in a highland greenhouse with a particularly large population of sow bugs. At times, the N. veitchii pitchers would fill completely to the rim with these creatures while other species would capture none or few. What would "traced isotope" studies tell me about derived nitrogen in that particular population of N. veitchii as compared to the other species housed in the same greenhouse?

Maybe, I just don't know enough about the details of the studies by Merbach and Moran but, right or wrong, it appears that this narrative will now be around for a long time.

[leilani]

My guess it that Merbach et al had a very specific narrative in mind long before conducting their studies which they then, consciously or unconsciously, tweak, bend and arrange into a support construct for their hypothesis.

I don't really think their conclusions would stand up if analyzed and subjected to more rigorous and 'blind' ( .. free of pre-conceived conclusions) testing. When we start with a conclusion and work backward we tend to find what we want to find, give weight to what supports our conclusion and play down or ignore data that works against our argument.

I am not questioning the integrity of Merbach et al as I am sure that these gentlemen are highly educated and dedicated scientists. What I am suggesting is that we need, scientists in particular, to not forget that it is human nature to slip into this kind of support selective theorizing.

[leeb]

Sam,

convergent evolution could be random chance, but when I see it occurring in several features I believe it is more parsimonious to believe that a selection pressure is operating.
The particular pitcher plants in question all have narrow lids, a pale colour in the upper pitchers, a wide infundibuliform shape, and sticky pitcher fluid.
The chances of all these features evolving in the same direction by chance seem much smaller than the chance of them being selected for.
Unless you are going to suggest that they are all genetically linked and evolve as one feature, which to me seems unlikely.

With respect to the studies on N. albomarginata, at least the abstracts of some of the papers are available on google scholar.

I read one of the papers years ago and from memory the discovery of Hospitalitermes filling the pitchers was discovered more than once in more than one location, and other prey in pitchers without Hospitalitermes was rare.
And pitchers with Hospitalitermes had the trichomes removed.

So it was not a one off spacially limited occurrence like your pitchers filling up with sow bugs.

The isotope studies merely backed up the field observations, giving corroboratory evidence.

LeeB.

[leeb]

Sam,
here is a link to one of the papers.

aob.oxfordjournals.org/cgi/reprint/88/2/307.pdf

They mention several named authors having noticed N. albomarginata catching Hospitalitermes.

LeeB.

[leilani]

I read the paper you cited. First, let me say that this sort of analysis is not something I am very familiar with and I would certainly not be in a position to comment on the methodology, mixing models, etc. employed. Just the sophisticated methods of analysis would, in most peoples minds, already give an air of authority and academic rigor to the study. The question, however, is “how does this support the hypothesis that N. albomarginata “specializes” in termites?”.

I may be wrong here, and once again, showing my ignorance, but as far as I can tell, this study only serves to quantify the fact that N. albomarginata, in the habitat studied, catches more termites than N. rafflesiana in the same habitat. It confirms, through chemical analysis, already established observations. Plants that capture more termites than ants will derive more of their nitrogen from termites than ants. Am I really missing something here?

How does this study support the suggestion that N. albomarginata “specializes” in the capture of termites. As far as I can tell, it does not. It merely quantifies one data set.

I think that first we need to settle on the meaning of “specializes” in the phrase “N. albomarginata specializes in the capture of termites.” This is not made explicit in either the research papers of Merbach et al nor in Stewart’s retelling of the tale in his new book. Simply, I will take it to mean to “direct to a specific end or use” with the implication being that the primary purpose of N. albomarginata pitchers is the capture of termites rather than as a more generalized trap. Not to say that the traps of N. albomarginata do not capture other insects but to suggest that termites are the “intended” victims. That these particular traps lure termites through some termite specific mechanism.

The suggestion, implicit at times and explicit in Stewart’s retelling, is that the tricomes found in the band beneath the mouth of the pitcher have evolved to serve specifically as termite lures or bait.

I am not sure I can hang this on Moran as he, quoted in Stewart, say at one point that “all contacts seemed to happen by chance.” but Stewart is quite explicit in his acceptance of this hypothesis ...

“Nepenthes albomarginata is the only species of carnivorous plant that offers its tissue as a form of bait to lure prey to its traps.”Pitcher Plants of the Old World, Vol. 1, pg92.

Merbach did establish that termites will and do eat these trichomes when they encounter them but not, as far as I can tell, that they served as lures or bait. In fact, in the very sentence following the above quotation, Stewart quotes Moran as saying that the foraging termites would often miss pitchers “less than 1 cm away”. This does not sound like a strong case for the suggestion that these tricomes act as lures or bait to me.

It seems to me that these studies have established at least two facts: 1. termites (in this habitat) compose the largest percentage of catch for N. albomarginata” and 2. “termites find the tricomes of N. albomarginata tasty”. Neither of these facts seem sufficient to support the presumptuous suggestion that N. albomarginata has evolved to specialize in the capture of termites.

OK, it would not be fair to just pick at this hypothesis without at least offering an alternative story explaining the, at times, very large concentration of termites in the diet of N. albomarginata. This actually seems quite simple to me. ...

First, as noted by the authors of these studies, termites compose perhaps the largest group of insects in the environment under study and might reasonable be assumed to be found in larger concentrations in the plants pitchers than other insects.

Second, I would suggest that the large concentration of termites captured in N. albomarginata pitchers has more to do with the dynamics of termite logistics than with the efficacy of N. albomarginata pitchers. Consider, from Stewart’s book, the following ...
“The marching masses of arriving termites would push the first individuals onto the peristome and many would fall into the pitcher, being successively replaced by the termites behind.” Pitcher Plants of the Old World, Vol. 1

This is a classic example of “herd-behavior”. The vast majority of the termites caught are not lured and trapped so much as pushed and herded to their death by their own dynamic.

[leeb]

Sam,

I found the cited paper by Kato, Hotta, Tamin and Itino in Tropical Ecology 6 p. 11-25 which discusses N. albomarginata prey spectra in Sumatra online and have also seen what Clarke said about N. albomarginata in his Nepenthes of Borneo book.
It appears that both in Borneo and Sumatra where N. albomarginata prey has been studied more than 50% of the prey is termites, and where the particular termite has been identified it is Hospitalitermes species.
In the same environments other Nepenthes were not found to trap this termite in any numbers.
This termite forages in columns, and specialises in eating fungae.
However it will also eat the trichomes on N. albomarginata.
Whereas N. albomarginata does not attract the termites from a distance, any termite crawling onto a N. albomarginata pitcher and finding the trichomes recognises it as food and starts to remove the trichomes.
I am not sure how or if the termite workers are able to signal other workers of the presence of a food supply but in any case in the studies it was found that once the termites found the trichomes more termites moved onto the pitcher to remove them and this continued until the trichomes were all removed.
In the process large numbers of termites were trapped in the pitcher.

So we have a Nepenthes that produces a structure (the trichomes) that other Nepenthes don't; that are a food supply to a termite that forages in large numbers in the areas where it grows, and as a result where studied has been found to catch there termites in greater numbers than any of its other prey.
And furthermore no other Nepenthes in the same areas catch these termites in anything like the same numbers.

This looks like a specialisation to me.

LeeB.

[Dave Evans]

Aug 29, 2009 21:07:20 GMT -10 leilani said:

Probably a lot of attributes are neither positive or negative. Dave Evans

We call attributes "positive" or "negative" according to how they fit our explanatory narrative. The typical causal story goes something like this: First we assume that the reduced structure of the lid serves some function such as "enhanced water collection capability" or "increased ease of access for flying insects". These seem reasonable and simple and we like that in our stories. We would then go on to suggest that this plant had, via 'selection', evolved a reduced lid for this purpose.

<snip>

I think we, both lay person and scientist, tend to construct these stories around the data we collect in order to co-relate discrete data and compose a comfortable causal explanation. N. lowii has evolved to capture bird droppings, N. albomarginata has evolved to capture termites, N. ampullaria has evolved to capture leaf litter, etc.

But Sam,

What narrative? Someone said they think N. lowii looks like a toilet and someone else asked, "But for what?" Someone else said, "Birds" and someone else said "Shrews". But no one had a certain answer. Now we do. N. lowii is specifically evolve to act as a tree shrew feeding station complete with a toilet. The shape of the pitcher is exactly the shape require be the tree shrews for them to be able to eat the material from the over sized lids. They stay at one pitcher for a while and often drop a load before moving on their way. Specific shrews follow a specific path or circuit of pitchers.

The pitchers have these unique characteristics: Extra strong pitchers and tendrils to support of the weight of the shrews. A modified peristome which had ridges for the shrews' paws to grip. The over-sized lid is positioned so when a tree shrew is feeding from the lid, its bottom is positioned directly over the constriction leading to the septic tank, I mean bottom of the pitcher.

[leilani]

Hi Lee,

Thanks for taking the time (and having the patience) to continue discussing this with me. I want to keep to the point but the following comment calls for a momentary diversion.

So we have a Nepenthes that produces a structure (the trichomes) that other Nepenthes don't .... leeb

I have often wondered why the band areas below the peristome have received so little study. (I think such studies might be useful in understanding the developmental history of the pitcher structure.) Except for N. albomarginata few, if any, writers have had much to say about, or even note, these structures.
I think most people assume, as you (and Clarke) state, that N. albomarginata is the only species that exhibits this band beneath the peristome. However, unless I am mis-interpreting what I have seen then, these bands, although often hidden beneath the fold of the peristome, are not singular to N. albomarginata.

Below, for example, are a series of pictures of the band beneath the peristome of N. bicalcarata.




Note the “tricome” structures in the banded area .....




In the following picture you can see that many of the “tricomes” have either fallen off or been removed from the upper portion of the band.


You will notice that “tricomes” are also found in abundance outside of the banded area but this is no different from N. albomarginata.




_________________________________________

And furthermore no other Nepenthes in the same areas catch these termites in anything like the same numbers. leeb

Just why N. albomarginata, in the habitat studied, catches more termites than sympatric species of Nepenthes might be attributable to a number of factors. It is noted by Clark that the species of termite most often found in the pitcher of N. albomarginata are a “surface foraging” variety which might help to explain the discrepancy between the numbers caught by N. albomarginata and N. rafflesiana. (It would be interesting to know if these large termite catches are distributed evenly among ground and aerial pitchers or if they are primarily observed to happen in the lower ground pitchers.)

Of particular interest, regarding my hypothesis: that these large catches of termites have more to do with termite behavior than with any specialized trapping strategy by N. albomarginata, is the observation of J. Moran, noted by Clark (Nepenthes of Borneo, pg. 38), that “N. rafflesiana pitchers occasionally catch large numbers of termites”.

It is noted by Clark that that “some of the Nepenthes species studied so far seem to be rather inefficient traps” and I would assume that he considers N. albomarginata to be among these. Most reports seem to show that N. albomarginata gets by on a very small catch except on those occasions when it becomes the object of obsession for an army of foraging termites. When this happens the plant reaps the benefit of the termite mob

I think it is important to consider the dynamic of these large termite catches in order for us to better determine just what is going on here.

“After opening, many pitchers observed in Brunei caught very little prey for well over three months, then, over the next couple of months, caught large numbers of termites which sometimes filled the pitchers entirely. The contents then became putrid, slowly decomposing over a period of weeks. Some pitcher which were overwhelmed by prey died, but most seemed capable of surviving the decomposition of the termites.” Nepenthes of Borneo”, Clarke (1997), pg. 38

It would seem that we might even question whether these occasional “bonanza catches” are necessarily a good deal for the plant. An extraordinary abundance of potential nitrogen for the plant but at what cost.

Termites and ants are eusocial insects and cannot rightly be considered as individuals in the same way as most species of potential prey.

“To watch a single ant apart from the rest of the colony is to see at most a huntress in the field or a small creature of ordinary demeanor digging a hole in the ground. One ant alone is a disappointment; it is really no ant at all.” Bert Holldobler and Edward O. Wilson, Journey to the Ants, pg. 107

Testing the specificity of N. albomarginata catches.

It would seem to me that the observations and experiments conducted by Merbach and others have been ill designed to determine if N. albomarginata really produce “termite specific” traps. At best, the field experiments so far conducted show that N. albomarginata catches more termites, in its natural environment, than any other creature. This is not the same as saying that “N. albomarginata pitchers specifically target these termites”. The natural environment does not provide a level playing field for this kind of quantitative catch analysis as its insect potentials are heavily slanted by the larger abundance of eusocial termites (and ants). Also, to further level the playing field we would want to eliminate the “black swan” of the “bonanza catch”. In order to prove that the pitcher of N. albomarginata are specific in their intended catch a more controlled experiment would be necessary. One possible test would involve N. albomarginata being exposed to an environment with equal numbers of multiple species of potential prey (species from their natural habitat or not) over some period of time. A post mortum analysis of the dead might then reveal some specificity of attractant or catch. Such an experiment would be free of the “bias of the mob” and the “bonanza catch”.

[leilani]

Hi Dave,

What narrative? Dave Evans

I was referring to the psychological (maybe physiological) methodology humans seem to employ in order to retain and relate discrete bits of data. We create narratives all the time in order to fill in the blanks.

Suppose I show you a series of still pictures of me all, apparently, taken the same day. In the first picture (8 am) I am standing in front of my house, in the second (10 am) I am standing in front of the Hilo Airport, in the third I am at the Honolulu Airport (11 am) and in the last at the San Francisco Airport (7 pm).

There you have it ..... four discrete bits of data. Nothing in any of the pictures tells you anything about the other except that I was present in all of them. You would then build the causal narrative ..... you fill in the blanks in such a way as to make sense of the data.

"Sam left his house in the morning and drove to Hilo where he caught a plane to Honolulu and then flew on to San Francisco."

Your relate the data in a way that makes sense to you and is consistent with your other knowledge. You might know that I live roughly 30 miles from Hilo, you might know that Hilo is roughly an hours flight to Honolulu, that it is about 5 hours of flight time to San Francisco and that Hawaii time is three house behind the west coast at this time of year. The narrative you create may reflect the reality of how I got to San Francisco or it may not. I might have taken a cab, I might have transfered planes, I might have used a mode of travel totally unknown to you, the pictures might have been taken on different days or I might have made a side trip to Maui that you could never know about by simply examining the four pictures I showed you.

Creating narratives is not bad or wrong ..... it is essential. What is important is to realize that we are filling in the blanks and that, even when the overall story seems to make sense we may still be missing something critical ( .. like my trip to Maui).

Regarding N. lowii, the story seems to make sense but I will admit that I have not had the opportunity to study this in depth. For the moment I would like to stay with N. albomarginata and termites.