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Author | Topic: All species are transitional | |||||||||||||||||||||||||||||||
Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 4.9 |
robinrohan writes: Gene pool isolation occurs when there is no more and cannot be any more gene flow between two groups. In the example, we ended up with 3 groups, unspeckled, lightly speckled, and heavily speckled. The genetic connection between unspeckled and heavily speckled was the lightly speckled. When the last lightly speckled Eutherian died, there was isolation between the other two groups. Okay, I recall this example from earlier in the thread, but it doesn't deserve much consideration. Yes, under a total gene pool isolation criteria that there could be speciation events, but this example is highly contrived since it has a species being produced through the extinction of an intermediate population. Such speciation events are only artifacts of your classification system and have nothing to do with species change. This is one reason why total gene pool isolation isn't a useful basis for classification anyway. There are other reasons, and we can discuss them if you're interested. Species classificaton with multiple criteria, gene flow being only one of them, would make much more sense.
If we had a different definition, having to do with morphological change, then we might call the two groups variants or we might call them separate species. It wouldn't matter. The designation is arbitrary. This is more in keeping with the real process of evolution. Morphological difference is another important species differentiator, as is relative fertility. Range and habits are others. Why do you feel the need for a species classification system based on limited criteria. As I've said a couple times now, and I think others have said the same, it still feels as if you're trying to eliminate the ambiguity in species classification systems, and this isn't possible. We can improve and refine our systems and reduce ambiguity, but eliminating them isn't possible. --Percy
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robinrohan Inactive Member |
Why do you feel the need for a species classification system based on limited criteria. One might call my reason pedagogical. If you are trying to explain TOE to someone who thought in terms of speciation events (and ultimately of "kinds"), it's much easier to understand the seamlessness of the process if one uses morphological change as the criteria for speciation. It's easy to see that the labelling is arbitrary. It's not at all easy to see if we define it the other way. If you were trying to explain it to someone who didn't know much about it, you would be speaking on a basic level.
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 4.9 |
Yeah, explaining discrete classification into species when change is seamless isn't easy. Use Randman as your guinea pig. If you come up with an explanation he understands then you've got a winner.
--Percy
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RAZD Member (Idle past 1435 days) Posts: 20714 From: the other end of the sidewalk Joined: |
or the camel\llama hybrid ... or the "liger" ...
I was talking about the galapagos finches during the drought where some interbreeding was observed between {otherwise classified as different species} individuals.
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KCdgw Inactive Member |
quote: Interbreeding has been reported in about half of the species in the whole archipelago, and not just during the drought. However, only two actual breeding studies have been done confirming this-- one on Daphne Major and the other on Genovesa. Many of the interbreeding reports were based on observations of individuals that appeared to be intermediate in morphology between breeding populations. Nevertheless, the rate of interbreeding is extremely low. I think the general feeling is that many (possibly all) of the species identified in the Galapagos are more incipient species, populations that are at various stages of divergence, but without complete reproductive isolation. The situation is extremely complex there, because of immigration between certain islands, and the geology, where some islands are relatively new and have arisen since the birds arrived. We won't really know the complete picture until full breeding studies have been done on all the breeding populations on all of the islands-- a LOT of painstaking work. KC Those who know the truth are not equal to those who love it-- Confucius
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NosyNed Member Posts: 9004 From: Canada Joined: |
Reading E.O. Wilson's "The Diversity of Life" (around page 70 + or -) I think that we can say there are speciation events. This doesn't mean that speciation isn't also fuzzy it just means it is wrong to say an "event" never happens.
He speaks of polyploidy producing a new species in a single generation. This he says accounts for something like half of living plant species and a "smaller number of animal species".
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 4.9 |
Good point! In these discussions we keep forgetting about the plant kingdom.
I just looked up polyploidy (answers.com), and in plants it isn't clear that the process produces a new species, unless your criteria is soley genetic and you stipulate that different numbers of chromosome copies equates to different species. We also often forget about asexual reproduction, and in animals it says the result of polyploidy is usually sterile and must reproduce asexually. We keep mentioning the reproduction problem when we describe species change as continuous, because an animal with too large a change will likely find no mutually fertile mates and die without reproducing, but of course this isn't a problem with asexual reproduction. --Percy
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NosyNed Member Posts: 9004 From: Canada Joined: |
I just looked up polyploidy (answers.com), and in plants it isn't clear that the process produces a new species, unless your criteria is soley genetic and you stipulate that different numbers of chromosome copies equates to different species. Wilson says:
quote: It seems it follows the biological species concept. They can not breed with their parents species. This message has been edited by NosyNed, 11-01-2005 12:20 PM
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 4.9 |
I suppose that if a population all at once produces a subgroup whose members all have the same or compatible polyploid change, then you can get a new species in a single generation. But this is just the kind of unlikely event that we rule out for creating new animal species. How does polyploidy produced a mutually fertile population in a single generation?
--Percy
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Mammuthus Member (Idle past 6505 days) Posts: 3085 From: Munich, Germany Joined: |
Hi Percy,
I had avoided plants because I can barely keep track of the definitions of xylem and floem In any case, here is an example from the UC Berkley site on rapid speciation by ploidy changes in plants. Plants are much better and rapidly becoming reproductively isolated by changing ploidy than most other groups. Causes of speciation - Understanding Evolution
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Mammuthus Member (Idle past 6505 days) Posts: 3085 From: Munich, Germany Joined: |
Just another point, given that plants can often self fertilize, one would not necessarily need the sudden appearance of a reproductively isolated population. The hybridization of two species generating an individual (or several) with novel ploidy that cannot breed with either parent species could propagate by self fertilization. During the founder event when all individuals of the new species are genetically very similar to near identical, it would be very difficult to distinguish progeny resulting from self fertilization or sexual reproduction...either way, it would not require the instantaneous generation of a new population. This however is not a viable method of speciation among mammals...even if John Salty Davidson thinks it is...
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
There is a wide range of ploidy observed in different species of Xenopus. For example X. tropicalis is diploid, X. laevis tetraploid, X. amieti octoploid, and X. ruwenzoriensis dodecaploid.
Allopolyploidization, via hybridization of species, is considered a source of new species in Xenopus (Evans, et al., 2004) TTFN, WK
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 4.9 |
Mammuthus and Wounded King,
Okay, that explains what I thought was unlikely. I didn't see how simultaneous identical polyploidy producing a mutually fertile population was likely enough to ever happen, but with asexual reproduction a population can be produced from a single individual. Thanks for the information! --Percy
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Omnivorous Member Posts: 3991 From: Adirondackia Joined: Member Rating: 6.9 |
Percy writes: How does polyploidy produced a mutually fertile population in a single generation? Another possibility is extrinsic causation: for example, the drug colchicine can create polyploidal plants. If an external stimulus (chemical, viral, etc.) creates polyploidy, and a significant fraction of a population (rather than just an individual) is subjected to the stimulus, then finding a polyploidal mate might be fairly likely.
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NosyNed Member Posts: 9004 From: Canada Joined: |
Wilson brushes over this.
I recall we discussed mice on an island somewhere. Any details? Any other examples. How do sexually reproducing mammals get around the not-having-a mate-available problem?
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