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Thread: Abrupt appearances.

  1. #1 Abrupt appearances. 
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    We are all familiar with the abrupt departures from the fossil record caused by extinction, my interest lies in the possibility of equally abrupt appearance of species/genera in the fossil record and what if any mechanism may account for it.


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    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    The rarity of fossilization accounts for it. Though the only truly abrupt appearances are from the Cambrian Explosion, this largely due to the evolution of hard parts that facilitated fossilization.


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    Well, the best example of rapid diversification is of course the Cambrian explosion. The abrupt appearance of new lifeforms in the fossil record doesn't need to be attributed to any sort of special mechanism, it's just that there may be different selective pressures on some genera of animals allowing them to evolve faster.

    An extinction event would provide a variety of newly opened niches to which a lifeform would be able to occupy. New variations of this organism would arise, and with little to no competition for these open niches, the organism would be able to flourish and diversify further.

    Of course, there are many other explanations that can explain the abrupt entry of a new organism into the fossil record.
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    As I should have mentioned in my above response, one of the possible contributing factors for the Cambrian explosion was the end-ediacaran extinction event. But our knowledge of the end-ediacaran extinction event is rather limited, due to the unavailability of fossils.
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    Quote Originally Posted by i_feel_tiredsleepy
    The rarity of fossilization accounts for it. Though the only truly abrupt appearances are from the Cambrian Explosion, this largely due to the evolution of hard parts that facilitated fossilization.
    If a species evolved somewhere with relatively poor conditions for fossil preservation, and then expanded in range to other areas this could easily result in an abrupt appearance in the fossil record.
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    Quote Originally Posted by i_feel_tiredsleepy
    The rarity of fossilization accounts for it. Though the only truly abrupt appearances are from the Cambrian Explosion, this largely due to the evolution of hard parts that facilitated fossilization.
    So, no vertebrates?
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    Quote Originally Posted by The Finger Prince
    Quote Originally Posted by i_feel_tiredsleepy
    The rarity of fossilization accounts for it. Though the only truly abrupt appearances are from the Cambrian Explosion, this largely due to the evolution of hard parts that facilitated fossilization.
    So, no vertebrates?
    Most groups, to one degree or another have some degree of abruptness to their first occurrence in the fossil record. The degree of that abruptness in general decreases as more and more fossils are found and described. There are few groups left that are considered enigmatic, and most, as noted in the previous posts, are from the oldest sediments.
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    Quote Originally Posted by Wikipedia
    "Eldredge and Gould proposed that the degree of gradualism commonly attributed to Charles Darwin is virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species."
    This quote is from the Wiki article on punctuated equilibrium. Is this seen as the rarity of fossilization acting as a mechanism causing these abrupt appearances, or is it actually an evolutionary phenomenon?
    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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    Quote Originally Posted by The Finger Prince
    So, no vertebrates?
    Lower Cambrian vertebrates from south China
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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    Quote Originally Posted by AlexP
    Quote Originally Posted by Wikipedia
    "Eldredge and Gould proposed that the degree of gradualism commonly attributed to Charles Darwin is virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species."
    This quote is from the Wiki article on punctuated equilibrium. Is this seen as the rarity of fossilization acting as a mechanism causing these abrupt appearances, or is it actually an evolutionary phenomenon?
    There is definitely a "jerkiness" in the fossil record, largely due to some of the factors described above by others. Gould's idea of PE has pretty much fallen out of favour these days. There may well be a few interesting examples, but generally it is seen as an artefact of the fossil record.
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    Moderator Moderator AlexP's Avatar
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    Yeah, I didn't think PE was widely accepted these days. Perhaps the idea came about before the rarity of fossils was well understood.
    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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    So why are we not seeing new species coming into existence to replace the extinct ones then? And how can we know fossils are related rather than products of convergent evolution or the like?

    I seem to remember vaguely a new species of mosquito which had evolved in the London Tube and was supposedly found there exclusively- struck me as rubbish, but I lost track of the story- anything to it?
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    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    Quote Originally Posted by The Finger Prince
    So why are we not seeing new species coming into existence to replace the extinct ones then? And how can we know fossils are related rather than products of convergent evolution or the like?

    I seem to remember vaguely a new species of mosquito which had evolved in the London Tube and was supposedly found there exclusively- struck me as rubbish, but I lost track of the story- anything to it?
    Genetic change is continual, human beings today are not genetically identical to human beings 50,000 years ago. New traits develop, blue eyes were not around then.

    New species are simply an inevitability, you have to get past the idea of species as fixed units and realize that the boundaries are blurry.
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    Quote Originally Posted by The Finger Prince
    So why are we not seeing new species coming into existence to replace the extinct ones then? And how can we know fossils are related rather than products of convergent evolution or the like?

    I seem to remember vaguely a new species of mosquito which had evolved in the London Tube and was supposedly found there exclusively- struck me as rubbish, but I lost track of the story- anything to it?
    Well I don't know about this mosquito you mean, but there are other incidents of new species evolving right under our nose. Take T. aceti, for example, a nematode that only lives in vinegar. (source: http://en.wikipedia.org/wiki/Turbatrix_aceti)

    One method paleontologists use to determine how something is related to another is phylogenetic analysis in combination with relativistic dating. i.e: we know Archeopteryx was very closely related to eumaniraptorids because of a number of physical similarities (bone structure etc). We know that Archeopteryx was more closely related to maniraptorids (such as oviraptor) than it was to ceolurosaurids. We can see this through the gradual evolution of large ceolurosaurids into maniraptors.

    I'm not sure if this is a sufficient answer. Hopefully so.
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    Quote Originally Posted by The Finger Prince
    And how can we know fossils are related rather than products of convergent evolution or the like?
    Lineages tend to diversify. That's the pattern observed in the fossil record (and reproduced using other things such as morphology, ecological and molecular data). If you wanted independent lineages converging then you'd need to turn the fossil record upside down. Specific features can converge rather than whole organisms from widely separated lineages.
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    Quote Originally Posted by finger
    So why are we not seeing new species coming into existence to replace the extinct ones then?
    We are.

    Takes a while - people don't live long enough to see the whole event very often. The beings that reproduce fast enough to speciate within a human lifespan are small and hard to observe with the necessary comprehensiveness. But we see the various stages or steps in evolutionary speciation all around us, routinely.
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