Friday, August 25, 2017

Niles Eldredge explains punctuated equilibria

Lots of people misunderstand punctuated equilibria. It's a theory about small changes leading to speciation. In many cases the changes are so slight that you and I might not notice the difference. These are not leaps or saltations and there are no intermediates or missing links. The changes may be due to changes in the frequency of one or two alleles.

Punctuated equilibria are when these speciation events take place relatively quickly and are followed by much longer periods of stasis (no change). Niles Eldredge explains how the theory is derived from his studies of thousands of trilobite fossils.



9 comments :

  1. I don't think you ought to call it speciation. Really, it's just morphological change in a population. There's no evidence that it's coincident with speciation, i.e. what we mean by the term in all the rest of the literature on speciation.

    And of course there are intermediates or missing links. That's what made the Phacops rana case so interesting, because the intermediates, or at least the intermediate populations, were found.

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    1. How else do you expect speciation to be determined through paleontological evidence though? Maybe it just can't be done.

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    2. I hate to say "can't", but I think it's pretty close.

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    3. One has to distinguish between punctuated patterns of change and Punctuated Equilibrium as a theory of change by species selection rather than individual selection. PE advocates tend to document the former and then say therefore the latter is the dominant mode of evolutionary change. But within-species selection can also produce change that, on a geological scale, has a punctuated pattern. So PE mechanisms are not necessarily implied by PE patterns.

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    4. Joe, I don't agree. Species selection is not a part of PE, it's just built on top of PE. PE itself is a claim, based on Ernst Mayr's notions, that most morphological change happens during speciation events, with stasis prevailing at other times. If that's true, then species selection might be a more widespread phenomenon than if populations were capable of responding to selection at any time.

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    5. I do not, of course, argue that all named paleospecies are true biospecies, or that I can even estimate the percentage properly so defined (any more than we know the relative frequency of modern taxa that represent true biospecies). But I do not see why the probability that well-defined paleospecies, based on good collections from many times and places, might represent proper biospecies should be any lower than the corresponding figure for equally well documented, but entirely morphologically defined, modern taxa. (In fact, one might argue that well-documented paleopsecies probably represent a higher probability for representing biospecies because we know their phenotypes, and have measured their stability, across long periods of time and wide ranges of environment—whereas modern "morphospecies" may arise as ecophenotypic expressions of a single time and place, therefore ranking only as local populations, rather than true species.

      Stephen Jay Gould
      The Structure of Evolutionary Theory pp. 785-786

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    6. As a central proposition, punctuated equilibrium holds that the great majority of species, as evidenced by their anatomical and geographical histories in the fossil record, originate in geological moments (punctuations) and then persist in stasis throughout their long duration.

      Stephen Jay Gould
      The Structure of Evolutionary Theory p. 766

      The consequences of Punctuated Equilibrium Theory give rise to speculations about species sorting and/or species selection. That's because speciation by cladogenesis gives rise to two different species that live together in similar environments. This continuous increase in the number of species (birth) can't continue unless some species go extinct (death). Extinction and survival could be random (sorting) or due to selection at the species level (species sorting).

      Species sorting and species selection are not part of PE Theory. They are concerned with the consequences of what happens AFTER punctuations and stasis.

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    7. The increasing number of cryptic species and the huge number of species that would be cryptic if we relied only on characters that are likely to be preserved in fossils argues against Gould. Also, in the overwhelming number of cases of observed punctuation, one "species" is replaced by a similar one rather than two similar ones. Sometimes the ancestral morphotype remains, but even then how can it be determined that the speciation didn't occur long before the punctuation that made it visible? If speciation is recognized solely by morphological change, there is no way to study whether morphological change coincides with speciation, except tautologically.

      It also appears that most species arise in allopatry, which argues against Larry's point in another comment.

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  2. First it is based on fossils which means it based on the deposition of dead creatures and then the unique mechanism for fossilizing them.
    I see this change between the two based on fossils found in a eastern/western geology succession as very unlikely. Instead it shows they lived together at the same time, though in segregated populations, and were all deposited in this segregation by a common event of movement.
    They lived together at the same time. One did not evolve from another.
    the differences are as little as modern fox populations etc.

    If its a different population after evolution has happened then it is what the world calls a speciation event.
    the mechanism for the morphological change is the mechanism called evolution.!

    What would be a intermediate in this case? what would its eyes look like?
    PE is trying to explain these changes as so quickly happening as unlikely fossilization would catch it. just a geographically segregated area would allow some breeding pairs to eVOLVE and then expand in numbers.
    Anyways its speculation based on geology anatomy. No biological processes are fossilized. Its all after the fact.
    Which puts into question conclusions. PE changed conclusions and I think actually changed the whole concept of using fossils for biology origin evidences.
    PE was a disator for evolutionism. It highlighted the problem of using fossils/rocks as evidence for living mechanisms.

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