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Author | Topic: Is convergent evolution evidence against common descent? | |||||||||||||||||||||||||||||||||||||
randman  Suspended Member (Idle past 4889 days) Posts: 6367 Joined: |
This came up on another thread, but was closed down, presumably for being off-topic. I am sure this has been studied and debated somewhere, but I have not seen it so I ask this question here.
Convergent evolution, of course, is the view that some data suggests that traits and similarities, including behaviour of species, can develop independently of each other, and not from both inheriting those traits from a common ancestor. To my mind, this is strong evidence against common descent because it shows similarities, including morphology, traits, behaviour, etc,...can develop not from mutual common descent, but independently. It also seems to me that convergent evolution, which can occur due to similarities in environmental pressure, same physical laws, and other commonalities besides mutual ancestry poses a lot of problems with using fossils and DNA to try to link organisms and determine "relatedness." If the bone structures, behaviour, diet, etc,...developed in 2 different lines of creatures evolving, different branches if you would, independently of one another, and we discovered their fossils, we may incorrectly assume they shared a common ancestor that passed these traits on when that was not the case. This could especially happen if the fossils were found in the near vicinity. Take Marsupials. If they migrated over here to thousands of years ago, and we found them all mixed up with placental mammals, how might we think they had evolved? Let me use an illustration of something I think could happen as well. Keep in mind that I am going to try to express things in everyday language, or as best I can, to get at the process involved. I am not interested in debating terms and words since the process is the same regardless of how one describes it. If someone misunderstands what I am saying because I do not use or lack technical terminology in a post, please just ask me, and I will explain the process itself and try to get that across. Back to the illustration. Ford and Chevy SUVs are very similar and with some differences, and both the similarities and differences are driven by competition. Let's call them species for this illustration. If one species (or model) does not adapt to the other improvements, it may well lost marketshare and go extinct. Likewise, some differences must develop, due to environmental pressures (the consumer's tastes), or they will not gain an edge, and something else can come along and knock both out of the water. The key here is that changes are driven by marketplace "natural selection". So you could have 2 independent models in the same environment, not necessarily separated and isolated geographically, that nevertheless through convergent evolution due to a commonality in environmental pressures selecting for similar traits. If that occured, would we not come to the wrong conclusion starting out with the assumption that these traits preexisted in a common ancestor when in reality, they did not. For sake of brevity, I will hold off on some additional thoughts, and wait to see if this topic is adopted. This message has been edited by randman, 06-04-2005 03:35 AM This message has been edited by randman, 06-04-2005 03:35 AM
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AdminAsgara Administrator (Idle past 2293 days) Posts: 2073 From: The Universe Joined: |
Thread moved here from the Proposed New Topics forum.
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crashfrog Member (Idle past 1457 days) Posts: 19762 From: Silver Spring, MD Joined: |
Convergent evolution, of course, is the view that some data suggests that traits and similarities, including behaviour of species, can develop independently of each other, and not from both inheriting those traits from a common ancestor. To my mind, this is strong evidence against common descent because it shows similarities, including morphology, traits, behaviour, etc,...can develop not from mutual common descent, but independently. What leads us to conclude that convergent evolution is responsible for some similarities and common ancestry is responsible for others is a kind of "inside-outside" problem. In situations of convergence we find that, while the exterior function and purpose of some system may be the same as another, when you look below the surface, you find two totally different ways of doing the same thing. This would be like finding two cars of identical function and discovering that one was electric and the other gasoline. That's not what it looks like when organisms share ancestry; the insides match, not just the outsides. In the case of human-chimp similarity, the similarity extends to stretches of DNA that have nothing to do with morphology and therefore, cannot be explained by convergent evolution. Common ancestry is the only scientific explanation for these similarities.
If that occured, would we not come to the wrong conclusion starting out with the assumption that these traits preexisted in a common ancestor when in reality, they did not. First off, Chevy and Ford SUV's do share a common ancestor: the Jeep. Now, if we wanted to ascertain which features of these two SUV's were a result of their common ancestry (wide wheelbase, powerful engine, high driver position) and which were the result of convergent evolution (Chevy sound systems vs. Ford sound systems) we would need to examine both their exterior function and their interior structure. Convergence isn't inconsistent with common ancestry. When you get right down to it it really isn't that hard to tell the difference between them.
edit: AdminTL added a "not" to the were in the comment about sound systems edit: Crashfrog rewrote the phrase in question to what he originally meant to say. Thanks to AdminTL. This message has been edited by AdminTL, 06-04-2005 11:37 AM This message has been edited by crashfrog, 06-04-2005 01:08 PM
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NosyNed Member Posts: 8996 From: Canada Joined: |
As crash noted the similarites in cases of convergent evolution are not all the way through.
Excellent examples are both the convergence of sharks, dolphin and ichthysosaur and the convergence of mammals and marsupials. In both cases there are strong superficial similarities but a closer examination makes it clear that they are separate lineages. When the DNA is examined independently from the morphology we find that the separation is confirmed. It is, of course, harder to separate animals when all we have are fossils but, again, the bone structures are clearly different even when the external shape is similar. The shark, dolphin, ichthyosaur example supports this. Do you have examples where it is less clear?
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Hrun Inactive Member |
Randman, since this sort of follows up a discussion we started elsewhere, I would like to start of with my post from the previous thread. I think it will provide the first step in answering your questions. Again, my intent is to figure out exactly on what we can agree and disagree and then find a way to tie it back into looking at DNA evidence and relatedness of species:
quote:
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EZscience Member (Idle past 5144 days) Posts: 961 From: A wheatfield in Kansas Joined: |
I think Crash has explained the basics very well. Convergent evolution is really a question of similar biological problems being solved with similar morphological solutions. When similar solutions are arrived at in separate lineages by virtue of independent evolutionary events, there are invariably some detectable differences in those solutions, because the animals don't always play with the same deck, so to speak (they are subject to different evolutionary constraints).
Take wings, for example. We KNOW that bird wings and bat wings were produced by completely separate evolutionary events because a bird's wing is contructed exclusively from bones analogous to our arm bones (only derived from their reptilian ancestors). Bats took flight much later and tarsal bones were also modified to support this wing frame structure - bones analogous to our fingers. Two superficially similar, but architecturally different, solutions to the same problem. BAT WING:
BIRD WING:
Consider now that insects also fly, and their wings have various airfoil shapes reminiscent sometimes of vertebrate wings - and yet they have no bones at all! So convergent evolution is in no way inconsistent with common descent.On the contrary, it is evidence that we are able to detect when similarities are NOT a function of common descent, i.e. it is evidence that the hypothesis of common descent is falsifiable with respect to particular traits. This message has been edited by EZscience, 06-04-2005 01:47 PM This message has been edited by EZscience, 06-04-2005 01:48 PM
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randman  Suspended Member (Idle past 4889 days) Posts: 6367 Joined: |
I cannot comment in-depth at the moment, but thanks for the responses thus far.
It is interesting though that if there appears to be internal differences that produce similar function and morphology, that the assumption is convergent evolution, but that nonetheless shows some similarities and traits can develop independently. What if other aspests did develop independently, but since there is more agreement and less differences, such as internal differences, the assumption is that there must be a common ancestor? It still seems to me that the general principle is that we are accepting the possibility of evolution producing similar traits, function, etc,...without a common ancestor passing those traits on. It may seem most logical to assume convergent evolution in comparing some species, and mutual ancestry passing traits along comparing for others, but that is more or less a guess, an inference. It's possible that we are assuming common descent as an explanation when other commonalities can explain shared traits and similarity, right?
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
If you are claiming that birds don't have digits, which is suggested by saying the wing is constructed from exclusively etc..., then you are completely wrong.
The following figure shows the digit morphology in a wild type chick wing and a reduplicted pattern in a wing with a transplanted ZPA.
Maybe that wasn't what you were suggesting, in which case your figure was just rather misleading. TTFN, WK
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
The real hot topic is the extent to which we see convergent evolution in DNA sequences.
Vowles and Amos (2004)document convergent evolution in sequences close to microsatellite regions. This is partially atributable to mutation biase such as those for C->T transversion but there seem to be a number of other effects yet to beaccounted for. Perhaps the most fruitful are of study would be studying the underlying sequences of DNA key to the development of analogous/ convergent structures for signs of convergence. TTFN, WK
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RAZD Member (Idle past 1395 days) Posts: 20714 From: the other end of the sidewalk Joined: |
I think you're missing the difference between common ancestor at some distant point, as derived through classic cladistics or genetics or whatever, with apparent similarity at a later date.
this is like saying that two leaves on a tree are closely {related\positioned} because they are near each other, when two leaves can come from the same twig OR be from branches as basic as the first divide in the tree trunk birds and bats share a common ancestor, just not one with wings, so the characteristics that they show in common from that ancestor have nothing to do with flight. convergent evolution does not refute common ancestry, rather it is just another example of evolution able to select organisms that adapt to similar situations, and when situations repeat, solutions are also likely to repeat. there are many examples of convergent evolution and they all fit the overall pattern of common ancestry at a basic level. hope that helps. we are limited in our ability to understand by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}
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NosyNed Member Posts: 8996 From: Canada Joined: |
It is interesting though that if there appears to be internal differences that produce similar function and morphology, that the assumption is convergent evolution, but that nonetheless shows some similarities and traits can develop independently. What if other aspests did develop independently, but since there is more agreement and less differences, such as internal differences, the assumption is that there must be a common ancestor? It still seems to me that the general principle is that we are accepting the possibility of evolution producing similar traits, function, etc,...without a common ancestor passing those traits on. I find this a bit confusingly worded. I hope you will try to reword it later. You are though suggesting that we might mistake a case of convergence as a case of common ancestry. Which is where this started. So far the examples put forward of convergence have all exhibited strikingly different ways of producing similar function. In other cases that do not show these differences we conclude common ancestry. You disagree with that conclusion. However, there are clearly two different kinds of situations here. They are in stark contrast. Perhaps you could develop this kind of sharp contrast into a way of, finally, defining "kinds" (as used by creationists) in some sort of specific and useful way. That is when there are these sharp contrasts of underlying support for a particular function then you might conclude that each of them was the result of a specially created "kind" and then subsequent hyper evolution from that kind (as many creationists suggest). Would you suggest that as an alternate explanation for the patterns shown? This message has been edited by NosyNed, 06-04-2005 06:59 PM
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crashfrog Member (Idle past 1457 days) Posts: 19762 From: Silver Spring, MD Joined: |
It still seems to me that the general principle is that we are accepting the possibility of evolution producing similar traits, function, etc,...without a common ancestor passing those traits on. What we're not accepting the possibility of is of convergence resulting in similar non-expressed, non-functional pseudogenetic sequences, because these sequences are not exposed to selection by environment. These are the sequences that we use to infer common ancestry, since because they have no function, they cannot be a result of convergent evolution.
It's possible that we are assuming common descent as an explanation when other commonalities can explain shared traits and similarity, right? Not in regards to the sequences we're using to infer ancestry. They do not interact with the environment, thus, they cannot be a result of evolutionary convergence.
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randman  Suspended Member (Idle past 4889 days) Posts: 6367 Joined: |
I've got a few minutes, and will try to get some of these.
My point is more on the reliability of how we view the evidence, namely the veracity of underlying principles in which we examine data, and by "we", I really mean the scientific community. I do not work or, nor have a degree in life sciences. Maybe that was clear... But be that as it may, if similarities, and I read WK as suggesting even the possibility of DNA sequences arising through convergent evolution, then we have very strong evidence that commonalities can arise in different species independently, right? That means that commonality is not necessarily evidence for mutual ancestry. Now, I realize the assumption is they all eventually had a common ancestor, but that's getting ahead of ourselves in the thinking. Tne belief in common descent is based in large part on how we view the data, and specifically in light of this principle. If we see some fossils, for example, in one strata and consider they are of a certain age, and then another set of fossils not so far with just some differences, and then a third with more differences at a later date, but they share similarities, we put them together in the tree of evolution, as you pointed out, but convergent evolution suggests that we could well be wrong. These traits could have emerged all independently, and so we are making false assumptions. That would seem, to me, to cast doubt on the entire process of examining data and assigning relatedness. Maybe all we are seeing are similarities that arose from something besides common descent.
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NosyNed Member Posts: 8996 From: Canada Joined: |
If we see some fossils, for example, in one strata and consider they are of a certain age, and then another set of fossils not so far with just some differences, and then a third with more differences at a later date, but they share similarities, we put them together in the tree of evolution, as you pointed out, but convergent evolution suggests that we could well be wrong. These traits could have emerged all independently, and so we are making false assumptions. This is the pattern we see, of course. Initially, a couple of centuries ago, those who first understood this but accepted a single act of creation had to try to construct an alternative explanation. From that they devised a series of creation events. Is that your alternative explanation?
That would seem, to me, to cast doubt on the entire process of examining data and assigning relatedness. Maybe all we are seeing are similarities that arose from something besides common descent. What is that something then? Would you describe what you believe happened to produce the patterns we see in the fossil record and the forms of life alive today?
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randman  Suspended Member (Idle past 4889 days) Posts: 6367 Joined: |
What leads us to conclude that convergent evolution is responsible for some similarities and common ancestry is responsible for others is a kind of "inside-outside" problem. But the inside has to had according to common descent have evolved as well, from creatures with a vastly different "inside" if you go all the way back, even to arguably no "inside" or it's all the same. So just because we see that convergent evolution had to have occurred for common descent to be true, that does not rule out the possibility of convergent evolution occurring for both the inside and outside to a degree, especially since it is not like we have full details between species all along the evolutionary chain. In fact, we have a great dearth of evidence often, and just have fossils, many of which do not show everything. So just because there are obvious cases pf convergent evolution does not rule out the chance that we are assigning mutual ancestry passing a trait along when that is incorrect, and more importantly, the principle of assuming similarities are the result of common descent is thus highly questionable. Mutual ancestry is just one possibility, not the only possibility for acquired traits, including the inside, outside, etc,.. in species to be similar. What if there was multiple origins for descent? We don't understand how or even if abiogenesis really occurred, and to consider that if it is a naturally occuring phenomenon, then who is to say it would not produce a somewhat similar life form each time? Or, let's take the idea of special creation at different times, or ID, and then those "kinds" evolve. Please suspend all the arguments on whether it's proper for science, and let's just think as human beings if the data we have said rules that out, in this one area, is correct. I don't want to divert this topic so maybe we should stay away from ID for now, but if there is an Intelligent Force introducing new species into the environment, it might well look just like the data we do have.
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