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Post by rainforest on Jul 25, 2009 11:34:16 GMT -10
The dodo was clubbed by whalers, seafaring men who clubbed them as nearby birds didn't even flinch. Their numbers were decimated in a short period of time, not killed for food by predators which needed their existence for survival. To wipe out an entire species is not the natural order, humans being the predator or not. A species cannot adapt to a situation if the means by which jeopardizes their survival falls on unnatural predation. To evolve with a predator is a way that a species can cope and adapt. As the predator evolves more skilled methods of hunting, so must a species evolve to cope with these pressures. Sardines lay much more eggs than can be supported by the sea, yet they produce vast numbers to also feed the system and keep it going. If the dodo's had time to cope with these predators, they might have been higher egg producers and not lave limited offspring counts. I imagine that laying few eggs that the species had very long life spans and produced enough so the food supply which they fed on was in ample supply.
M
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Post by marka on Jul 25, 2009 17:06:40 GMT -10
Humans were used as an example to show that young species can have a wide distribution. Ditto with grasses. And have wide variation.
I dont know how long tuberious rootstocks would take to evolve but i would think it could be quite rapid, its only a swelling of the root after all.
The mirabilis populations in Hong Kong are subject to light frosts, at least according to one grower i contacted there a while ago and looking at the typical climate of hong kong it seems very likely to be true. This is a sub-tropical / warm temperate climate, period.
Dave has a good point, southern china is not well known and you only have to look at the map to wonder what is out there. As i type this i'm actually in Nanjing and the day temp here is upto 40C, winter around here is -10, go a bit further south though and things get really interesting...
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leeb
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Post by leeb on Jul 26, 2009 18:23:04 GMT -10
Hi all,
according to Mullins and Jebb's studies based on DNA N. mirabilis is of hybrid origin and in a clade with N. ampullaria, N. rafflesiana and N. gracilis. These all share the characteristics of being of hybrid origin and remarkably widespread. Maybe they have some features that enable them to be distributed widely and efficiently. Given that they are in the same clade and remarkably diverse in appearance, the hybridisation event may be an ancient one. However given that each species only varies slightly over its range their dispersal events may be relatively recent. Study of the amount of their genetic variation in populations from different islands may give us an idea of where they originated and how long ago; the populations in the original range should have the greatest amount of genetic diversity, with less occurring as you move away from there. The Flora of China which is online notes that N. mirabilis occurs from 0-400m in S. and W. Guangdong and Hainan. An increase in altitude as you go north may limit it's northern expansion; but I don't know why it hasn't been reported from southern Guangxi which is between Guangdong and Vietnam where it also occurs.
A couple of other points; fossil grasses are known from the Cretaceous of India, and they became widespread during the Miocene. And the genus Geochelone for giant tortoises is polyphyletic and thus obscures their relationships; the correct genus for the Galapagos giant tortoise is Chelonoidis, and there were even bigger species on the mainland of South America before people arrived there, an example is Chelonoidis sellowi from the Pleistocene of Argentina. Giant species of Chelonoidis also occurred on Caribbean islands, but the North American giant species were in Hesperotestudo, the Madagascan and Seychelles species in Aldabrachelys and the Mascarene species in Cylindraspis. Each of these genera evolved separately fron different ancestors, there were other giant tortoises in different genera on mainland Africa and southern Asia. LeeB.
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Post by leilani on Jul 26, 2009 21:48:40 GMT -10
Welcome to the land of unrestrained speculation leeb! It's a real pleasure to have you with us. ;D I think most here have some familiarity with "Mullins and Jebb's studies". In my own case I am not sure just how much of it I understand. I have heard that this work seemed to indicated that the lowland species you mention are of "hybrid origin" but I am not sure what this means or how it is inferred by the study. Perhaps, you could enlighten me in words I would understand. How would the "hybrid" status of these species change any of the arguments regarding relative age? You say it might be and "ancient" or a "relatively recent" event. It would be tempting to suggest that it could have been some ancient "hybrid event" and the resultant heterosis in the lineage that has strengthened these populations and lead to their wide spread success as compared to "non-hybrid" species. I'm a little puzzled by your example of the tortoise. (I though "polyphyletic" was a dirty word among taxonomists and cladisticians.) Are you suggesting that Nepenthes might be polyphyletic? What exactly would this mean?
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leeb
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Post by leeb on Jul 27, 2009 12:52:18 GMT -10
Hi Leilani,
sorry to be confusing with the tortoises. Someone earlier had referred to the Galapagos giant tortoises as Geochelone and suggested they might have grown when they reached the Galapagos. I was updating their taxonomy because Geochelone sensu strictu has recently been restricted to a few species of Tortoise from South Asia and Africa; and some mainland South American tortoises were bigger than the living Galapagos tortoises so their ancestors may well have shrunk upon arrival in the proto Galapagos islands.
Nepenthes is definitely monophyletic; all modern species share a common ancestor. Mullins and Jebb infer a hybridisation event in the ancestry of N. mirabilis, N. gracilis, N. rafflesiana and N. ampullaria by comparing the DNA phylogeny for them derived from nuclear and plastid genes. Genes in the nucleus consist of two copies, one derived from each parent. Plastid genes, those in the chloroplasts and mitochondria in the cells are usually derived from one parent only. So in the event of hybridisation the nuclear genes will have derived from a mix of both parents, as a process called crossing over and recombination mixes the DNA from both parents into daughter DNA when seed and pollen is formed. The plastid genes however will only come from one of the parent species and as they exist as single copies recombination cannot occur. Therefore phylogenies derived from plastid genes will differ dramatically from those derived from nuclear genes when hybridization has occurred.
Until Mullins and Jebb publish more on their work the age of the initial hybridisation event in N. mirabilis's ancestry will be unknown; I was hypothesising that it might be ancient based on the disparity in the morphology of the species sharing this hybrid ancesrty.
However the species themselves and their increase in distribution seems to me to be relatively recent; otherwise such widespread species as N. mirabilis would have started to break up into many species in each of the separated populations throughout its range through both selection and random genetic drift.
LeeB.
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Post by rainforest on Jul 27, 2009 13:37:29 GMT -10
This is not always the case if a species is successful. Many cycads are very exacting to their prehistoric relatives and have evolved very little. The Coelacanth, an ancient fish has not changed dramatically from its fossil ancestors very much.
The example was of chelonoidis from the South American continent to different species on an island archipelago. Perhaps an island bridge or other aspect allowing this species to be established on both, and losing that land mass so species can establish even though no logical means of travel to these islands seems plausible. The example was as a means for an explanation for one species to become reestablished and colonize a new region. N. mirabilis didn't migrate, it was already present and established well before any of these suggested "dispersal" to took place. N. mirabilis is too well established in these regions for it to quickly colonize and establish itself in "new territory." I'd like to see another current example of nepenthes where new colonies have formed from a chance dispersal event. Their seeds do not migrate or travel far from where they were produced. Other than unnatural disturbances (from man), no natural disasters or event could create a new species from reestablishing itself from a chance seed dispersal, even if they were from migratory birds.
M
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leeb
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Post by leeb on Jul 27, 2009 15:37:32 GMT -10
Giant tortoises float rather well, contrary to expectation, and probably floated to the Galapagos which has never been attached to the mainland. TheGalapagos are a series of volcanic islands forming over a hot spot with older islands now below sea level to the east where they are being transported to the Peru-Chile trench down which they will be subducted.
Although some species look similar to their ancient relatives they still differ; the modern Coelacanth species are in a different genus than their ancient relatives. And there are many species of modern cycads, none of which is exactly similar to their ancient relatives.
Even if a widely dispersed Nepenthes species were to retain a common morphology its genes would be subject to drift and it would appear as a series of cryptic species when studied genetically.
According to all the phylogenetic studies carried out using DNA the modern species whose ancestors diverged from the rest of Nepenthes first are the far western Indian Ocean species, the next were the species with paniculate inflorescenses found in the east along with their relatives that have secondarily simplified their inflorescences, and then the next to branch off were the ventricosa group and then the tentaculata group.
There is no evidence of any ancient widespread species that has remained unchanged. Even the species whose ancestors branched off first have evolved enough to be easily separable species. Also from the evidence the ancestral Nepenthes species had a paniculate inflorescence, which N. mirabilis lacks.
LeeB.
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Post by leilani on Jul 27, 2009 23:49:01 GMT -10
Thanks for the clarification leeb. I want to be sure were using the same terminology. I understand this but wonder just what it is in the differing phylogenies than implies a hybridization event. From these quotes I take it that the assumption is that greater morphological disparity suggests greater age. This seems reasonable and fits well with the Darwinian model .... with selection and random genetic drift as the primary modalities of evolution. But even if we accept this does it really imply a parallel between the morphological disparity of a species and the time it has been around ( .. in other than a trivial way) or might there not also be long periods of relative stability (Gould). And, what is relative stability? Is it 1000 years or, as in the example of the Coelacanth, a couple hundred million years? __________________________________________ Ohh, how I miss the good old days when floating tortoises was an accepted form of family entertainment. ;D __________________________________________ Yes, but can we really expect these species to change according to the same timetable. With something like “genetic drift” I suppose we can calculate a certain amount of change based solely upon a time schedule of some number of years. But with “selection” this seems more problematic. Selection would seem to be opportunistic and not tied to any easily predictable schedule. Can we really expect species, even within the same family, to follow the same predictable schedule of change? I hesitate to ask but, does your understanding of evolution allow for any modalities of change beyond "selection" and "genetic drift"?
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leeb
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Post by leeb on Jul 28, 2009 11:07:59 GMT -10
Hi Leilani,
when hybridisation occurs the phylogenies derived from nuclear and plastid genes will be obviously different; the degree of difference depending on how unrelated the parent species are. When hybridisation hasn't occurred the phylogenies from the nuclear and plastid genes will be the same. Thus getting phylogenies from both types of genes is a good test for hybridisation. Now if we could find genes that were inherited from the seed parent and ones that were inherited from the pollen parent we could compare the phylogenies and work out where on the phylogenetic tree the ancestral species were that hybridised.
There are many forms of change other than drift and selection; for example founder effects in entering new habitats, hybridisation, and the rapid diversification that occurs when species enter very large new habitats with no competition or predation and initial low population density. Where the new habitat is ideal for the species its initial population population increases rapidly as all offspring survive and reproduce, even those that would normally be less than fit and not survive. This leads to a very variable population and when the new habitat is eventually filled up due to population increase and competition for resources sets in the population can then break up into a number of the most sucessful variants, forming a species flock. N. maxima seems to be at different stages in this process in Sulawesi and New Guinea.
Stabilising selection can keep a species morphologically unchanged for long periods, this is one of the reasons why stasis is so common in the fossil record. I just find it hard to accept that it would apply equally throughout the range of N. mirabilis or N. ampullaria on all the islands and continents they occupy, keeping all the populations so similar that they are recognised as the same species. I find it easier to accept that they have undergone range extension at least during the last million years or so, and more probably within the last few hundred thousand years. Genetic study of their populations looking at the amount of variation in each should give us information on this, allowing us to determine where they spread from.
And floating tortoises must have been accepted entertainment in the very old days, as they floated to the Galapagos from South America, to various Caribbean islands from South or Central America, to Bermuda from North America, and to the Seychelles and Aldabra from Madagascar. And also to the Mascarenes from Madagascar or Africa.
Ah fond reminiscences of past times.
LeeB.
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Post by leilani on Jul 28, 2009 15:35:32 GMT -10
Hi leeb, I was aware of these effects but what I was really getting at, and the reason for my hesitation in asking, is how you feel about the work of Lynn Margulis? I realize that the notion of micro-organism based symbiosis is difficult to apply to questions of larger life forms like Nepenthes but wonder how do you feel about the idea of 'captured' genetic material (from organelles and the like) becoming incorporated into the genetic makeup of larger ("colonial") life forms? Forgive me but since I have no credentials at stake and little to lose by being wrong I sometimes allow my speculation to stretch and even break with well accepted scientific 'facts'. This having been said, I now feel free to say the following ..... I, like everyone else, have often wondered about the carnivorous nature of Nepenthes. Just how did a plant come about developing such a radically different (secondary) method of sustaining itself? Trying to figure out most of the adaptations and evolution of Nepenthes is not much different from trying to follow the evolution of other families of plants but the origin of carnivority in this and related plants would seem, to me at least, to be a much harder to explain. There would seem to be no genetic precedent for such a revolutionary adaptation in these plants. Assuming that Nepenthes evolved from some non-carnivorous ancestor it would seem doubtful to me that it brought this potential with it. OK .... off the deep end! So radical does this adaptation seem to me that I wonder that it might not be the result of the incorporation of genetic material ( .. in some undefined process) from another life form ( .. perhaps bacterial). Has much research been done on the bacterial infauna of Nepenthes pitchers? Are there particular "families" of bacterial that are common to all Nepenthes? It seems to be generally accepted that the breaking down and consumption of captured prey is a cooperative effort with the bacterial infauna. I know this is "off the wall" but there seems to be precious few scientists willing to even speculate on the origin of this adaptation. There are a fair number of small, very specific studies of Nepenthes pitchers and how they work but little or no one willing to suggest an explanation of how such an radical adaptation might have come about in the first place. Anyway, throw your scientific integrity to the wind for a moment and tell me what do you think. I promise we won't let any scientists read it. Ohh, BTW ..... I find it a little difficult to accept the idea of "floating tortoises" having colonized all of these remote locations. Although they may float I wonder just how long can a tortoise survive at sea? And just how many tortoises must one float in order to get lucky and have a male and female arrive safely on the same remote island. Well, I guess it is no different from any of the other life forms that somehow arrived on these islands via the sea .... but it is a funny picture. ;D
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leeb
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Post by leeb on Jul 28, 2009 18:35:03 GMT -10
Okay, lets deal with the simple stuff first. Tortoises can go without food for some months, that is why they were collected as a fresh food supply on early sailing ships. Simply store them on your ships, don't bother to look after them except supply them with water, and kill and eat them as required. So a floating gravid female could float to an island, then lay eggs and hey presto a new population of tortoises. Perhaps even more likely a catastrophic event like a tsunami or a storm surge could wash a number of tortoises off the mainland or large island and the currents could carry them to a new island together.
Now for the more difficult. Lynn Margulis was obviously right about organelles like Mitochondria and Chlotoplasts with their own DNA being captured organisms that have become permanently incorporated into eucaryotic cells. These are so ubiquitous that it is obvious that the capture event occurred back in the pre-cambrian when the capturing cell was not much more sophisticated than the cell being captured. However since then there is evidence that some of the plastid genes have been incorporated into the nucleus of the host cell. Where a parasite or symbiont is in continuous close contact with the host I can see how some of its genetic material could accidentally get incorporated into the genome of the host or vice versa, the question might be would enough get incorporated to have any effect or would small bits incorporated just add to the 'junk DNA'? Retro-viruses also copy bits of DNA from one organism and incorporate it into other's genomes; again only short sections; I have heard that is why domestic cats share a small sequence of DNA with savannah baboons. In theory if a large enough section of DNA was incorporated and was activated it could be a source of evolutionary novelty; but in practice I don't know if this actually occurs. Changes in the folding of DNA effect whether a gene can be successfully read or not; it is one of the ways that epigenetic factors turn genes on or off. So a whole gene would have to be incorporated in a position where it could be successfully read, and then when activated there would need to be the right cellular mechanisms available to act on whatever the gene produces and do something useful with it, and it would have to not interfere with any other vital cellular machinery. So it's possible given long enough periods of time but I am not sure how common. It is interesting however how a number of unrelated organisms produce either haemoglobin or haemocyanin as a respiratory chemical. Imagine if squids could produce haemoglobin instead or haemocyanin, they would really give fishes a run for their money.
Regarding the evolution of carnivorous plants, this seems at first a radically different way of sustaining themselves but the fact that it has separately evolved a number of times leads me to think that it is not such a difficult thing to do. Plants already absorb N, P and K through their roots, so one of the key features may be shifting this ability to cells in the leaves. Then various means of capturing prey may evolve secondarily to this. What I have found interesting is that related groups of carnivorous plants use totally different mechanisms to capture prey, with Nepenthes, Drosera and Dionaea being a good example; this suggests to me that once the underlying mechanisms to absorb prey are in place the actual trapping mechanism is secondary, and if a plant using one mechanism evolves a second better mechanism it rapidly loses the first mechanism. Thus an ancestral Dionaea with sticky hairs developed the ability to catch prey by folding the leaf halves together and rapidly lost the sticky hairs or transformed them into the marginal spikes, and a primitive Pinguicula like plant developed hollow structures that were better at capturing prey and lost the sticky leaves and evolved into Genlisea or Utricularia.
Another interesting thing is that once plants rely on carnivory or parasitism to supply nutrients, and photosynthesis becomes less important, then the constraints on growth that keep them looking plant-like seem to disappear and you can get things like Utricularia or Rafflesia evolving. I don't think any of this has much to do with evolutionary novelties caused by horizontal gene transfer from symbionts or parasites.
LeeB.
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Post by leilani on Jul 28, 2009 22:06:29 GMT -10
The simple stuff .... This is a fascinating and well constructed explanation. It only takes one, they can do without food for a good period of time and "floating tortoises" are more common than you would think. ;D I really love the last part .... Let me say that this is perfectly plausible. One of the few advantages we seem to have in trying to unravel evolution is "plenty of time". If an argument seems implausible in a local time frame it may still work if given enough time. If required then, it is well within the rules to call up a tsunami, hurricane or continental migration. Just adjust the metric. I am not suggesting that I doubt any of this but am just a bit amused by the convenience of it. Thanks for your response on the Margulis question. Its true, all the examples are restricted to simple micro-organisms but, then again, size and complexity follow. It would think that the vast majority of genetic material imported would be 'junk'. On the other hand, it seems to have worked to monumental effect when it has worked. Mitochondria and chloroplasts being the most outstanding examples. This does sound implausible. But if we conveniently adjust the metric ..... it seems quite plausible.
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leeb
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Post by leeb on Jul 29, 2009 11:13:00 GMT -10
Yes given enough time the incredible can become plausible. In the not too distant future our ability to study organisms DNA is going to make the answer to questions like this answerable and I think the answers are going to be very interesting. LeeB.
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Post by rsivertsen on Jul 29, 2009 11:19:50 GMT -10
Yes given enough time the incredible can become plausible. In the not too distant future our ability to study organisms DNA is going to make the answer to questions like this answerable and I think the answers are going to be very interesting. LeeB. I'm inclined to agree; this thread is becoming a dissertation! We might also take note of the fact that every life form in the Hawaiian archipelago has originally arrived from somewhere else, even tho, it may be endemic to these sites today! And these are isolated, sterile volcanic islands in the middle of the Pacific Ocean! Often, devastating storms, typhoons, tsunamis and other natural events can result in several large floating mats of debris that functions as rafts, or small floating islands rich in diversity of life in search of a new home. It's happened before, and it will undoubtedly happen many times again in the future regardless if we get to see it for ourselves or not. Just pointing out the obvious again! - Rich
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leeb
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Post by leeb on Jul 29, 2009 15:36:04 GMT -10
Yes Rich, and Hawaii was even richer in fauna before people arrived. Moa nalo's are fascinating; duck that evolved into flightless forms looking like small moa. And they spread among several islands while still able to fly, then became flightless before the main island of Hawaii emerged; instead there was a large Goose there.
LeeB.
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