Tuesday, October 16, 2012

More Evidence of Short-term Stasis

A group of paleontologists recently documented stasis in birds and mammals from 35,000 years ago to today (Prothero et al., 2012).

Intelligent Design Creationists such as Douglas Axe are surprised by this result [Tar Pit Study Shows Complete Absence of Evolutionary Change]. So, apparently, are the authors even though they are evolutionary biologists and they have published this observation before.

How many Sandwalk readers are surprised by this paper?
Abstract

Conventional neo-Darwinian theory views organisms as infinitely sensitive and responsive to their environments, and considers them able to readily change size or shape when they adapt to selective pressures. Yet since 1863 it has been well known that Pleistocene animals and plants do not show much morphological change or speciation in response to the glacial–interglacial climate cycles. We tested this hypothesis with all of the common birds (condors, golden and bald eagles, turkeys, caracaras) and mammals (dire wolves, saber-toothed cats, giant lions, horses, camels, bison, and ground sloths) from Rancho La Brea tar pits in Los Angeles, California, which preserves large samples of many bones from many well-dated pits spanning the 35,000 years of the Last Glacial–Interglacial cycle. Pollen evidence showed the climate changed from chaparral/oaks 35,000 years ago to snowy piñon-juniper forests at the peak glacial 20,000 years ago, then back to the modern chaparral since the glacial–interglacial transition. Based on Bergmann's rule, we would expect peak glacial specimens to have larger body sizes, and based on Allen's rule, peak glacial samples should have shorter and more robust limbs. Yet statistical analysis (ANOVA for parametric samples; Kruskal–Wallis test for non-parametric samples) showed that none of the Pleistocene pit samples is statistically distinct from the rest, indicating complete stasis from 35 ka to 9 ka. The sole exception was the Pit 13 sample of dire wolves (16 ka), which was significantly smaller than the rest, but this did not occur in response to climate change. We also performed a time series analysis of the pit samples. None showed directional change; all were either static or showed a random walk. Thus, the data show that birds and mammals at Rancho La Brea show complete stasis and were unresponsive to the major climate change that occurred at 20 ka, consistent with other studies of Pleistocene animals and plants. Most explanations for such stasis (stabilizing selection, canalization) fail in this setting where climate is changing. One possible explanation is that most large birds and mammals are very broadly adapted and relatively insensitive to changes in their environments, although even the small mammals of the Pleistocene show stasis during climate change, too.
This issue was thoroughly discussed by Stephen Jay Gould in his 2002 book "The Structure of Evolutionary Theory." He began by quoting a paper by Wake, Roth and Wake (1983) where they asked ...
"With natural selection operating in a changing environment as an agent of adaptation, we expect to see changes at the organismal, ultimately physiological and morphological, level. How, though, can we explain the paradoxical situation in which environments change, even dramatically, but organisms do not?"
Thus, the problem was well-known thirty years ago. Environmental change often doesn't cause evolutionary change.

Stabilizing selection is not an explanation for stasis as Gould notes on page 878.
... although we all acknowledge stabilizing selection as too important and pervasive a phenomenon to hold no relevance for this issue, a complete explanation of stasis in these conventional terms seems implausible both on empirical grounds, and also by the basic logic of proper scaling.

As often emphasized in this chapter, if stasis merely reflects excellent adaptation to environments, then why do we frequently observe such profound stasis during major climatic shifts like ice-age cycles (Cronin, 1985), or through the largest environmental change in a major interval of time (Prothero and Heaton, 1996)?
It looks like stasis in the face of environmental change has been a well-established fact for many decades. This should not be a surprise to anyone who studies evolution [The Paradox of Stasis?].

The premise—that evolution=adaptation and it is driven by environmental change—is obviously wrong as a generalization.


[Photo credit: I took this picture in February 2012 when I visited the tar pits; La Brea Tar Pits.]

Prothero, D.R., Syverson, V.J., Raymond, K.R., Madan, M., Molina, M., Fragomeni, A., DeSantis, S., Sutyagina, A., and Gage, G.L. (2012) Size and shape stasis in late Pleistocene mammals and birds from Rancho La Brea during the Last Glacial–Interglacial cycle. Quaternary Science Reviews 56:1–10. [doi: 10.1016/j.quascirev.2012.08.015]

34 comments :

  1. Not a single extinction mentioned. Many relevant ecological conditions (predators, prey, competitors) seemed to have remained amazingly constant. How severe was the climate change in coastal California? And how much change would be expected from current measures of latitudinal size differences (Bergmann's Rule). Were samples corrected for sex and age classes? The samples seem very variable, and it would be interesting to know what sample size would be necessary, for example, to detect a 5% difference. It is good to remember that good old Homo sapiens, despite a few thousand years in a very new cultural climate, also seems to not have evolved much morphologically, although our genes show that we have.

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    1. It seems to me that, absent other evidence not presented here, that the most that can be said is that morphological changes among specimens recovered seems minimal. That should in no way be equated (ala Adnan Oktar, aka Harun Yahya) that evolution mysteriously ceased in those instances-- whether for the Pleistocene pit samples that are the primary data point here (26 My period), or over time periods longer by an order of magnitude or more (think horseshoe crabs). Absent DNA sequence data obtained from the samples, any judgment regarding the pace of evolutionary change would seem hopelessly unsubstantiated.

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  2. Climate change decimated the populations making their sizes so small that genetic drift ruled.

    or

    During climate change the studied populations were able to move to where it was warmer (habitat-tracking). See Raia et al. (2012).

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  3. This yEC is not surprised because the tar pits only represent the 1700's BC or so.
    then diversity was greater in the past as biological systems were superior. The bible implies this.
    So our present creatures are just the survivors of those more diverse times or simply smaller versions.
    I know they say buffalos are just smaller versions of their ancestors back then and not a surviving type.

    Yet by evolutionary standards there should be more diversity up and down in these creatures. One can't have glorious evolution of bugs to buffalos and then say stasis is a option too.
    Otherwise the hypothesis just is flexible for anything and explains nothing.
    The researchers were right about being surprised.
    It should not be so fixed as to imply there is no evolution going on or on its way with these critters.
    Evolutionary biology is not true.

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    1. "The researchers were right about being surprised".

      The researchers WEREN'T surprised. They've been making this same observation since the 70's (Punctuated Equilibrium).

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  4. Did Gould provide an explanation for stasis, other than unspecified constraints?

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    1. When he died in 2002, Gould claimed that he no longer believed that "constraint" was the explanation for stasis (TSET p. 880).

      His preferred model at that time was that species with huge population sizes—that's mostly what they're looking at—are subdivided into many smaller demes. These demes might adapt slightly to local environments but the adaptations can't spread to other demes in slightly different environments. The net effect is that the species as a whole doesn't change very much even though there might be small localized changes in different parts of the range.

      You'll have to read Gould and others for a more detailed view of current thinking about stasis. The important point is that most species are doing quite fine in their current environment and they have enough buffering capacity to withstand fairly substantive changes in the environment. If the change is too drastic they move (habitat tracking).

      It's wrong to think that most species are so fine-tuned to their existing environment that any small environmental change requires adaptations in order to avoid extinction. On the other hand, it's clear that some catastrophic changes will cause mass extinctions.


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    2. Seems as if experiments in microorganisms could be relevant here. Some favorable mutations in microorganisms take place over such short time frames that some experimenters thought it appeared to be directed. But long term experiments by Lenski, for example, show bacteria taking tens of thousands of generations to hit the "right" combination of mutations to adapt well to certain environments. Seems as if even in microorganisms some mutations and combinations thereof are much more easily achieved and propagated through a population than others. So perhaps this might be one factor, along with lack of sufficient breeding isolation, etc.

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    3. LM writes
      The important point is that most species are doing quite fine in their current environment and they have enough buffering capacity to withstand fairly substantive changes in the environment. If the change is too drastic they move (habitat tracking).

      Somewhat confused. Do plants also have some form of habitat tracking? I'm trying to work out how the above statement is consistent with the vast variety and array of plant life.. It seems like plants would just go extinct, rather than adapt as prolifically as they have. Hundreds, if not thousands, of plants all inhabit the same plots of land, presumably all pretty much needing the same type of nutrients (water, sunshine, particular soil minerals), to survive. If they are all 'doing quite fine', why did they differentiate so much?

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    4. Plants "move" as seeds. The tracking would happen when a seed falls where it will grow right.

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    5. NE, thanks, I see that. But then, would they also move back again when the climate change restored itself? I'm still trying to figure out why so many species cohabit the same plot of land. It seems like the ones that had remained would be able to repel plants that tried to return.

      Still trying to make this fit with 'doing quite fine in their current environment' which seems to indicate diversification of life form as an exception rather than a rule.

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    6. sorry, the above should be 'suggest' rather than 'indicate' in the last sentence.

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    7. Andy wrote:
      "But then, would they also move back again when the climate change restored itself?"

      Presumably, yes. In fact, there are many indications that the Amazon forest is still undergoing ecological succession from a few loci that are richer in biodiversity and that map well to what is thought was the spots that managed to "survive" the last Ice Age. The same can be said about much of the taiga in the northern latitudes, which also seem to be moving "north" and undergoing ecological succession since the last Ice Age.

      Andy wrote:
      "It seems like the ones that had remained would be able to repel plants that tried to return"

      That, of course, is possible, but complicated to gauge in general terms. You have to take into account all the complexities of competition theory and look in practice at specific events to see what's happening. But there will certainly be some competition.

      Andy wrote:
      "Still trying to make this fit with 'doing quite fine in their current environment' which seems to suggest diversification of life form as an exception rather than a rule"

      That depends. You have to consider that both the separation of small populations from a bigger one and genetic drift will allow nicelly for diversification to happen without any conflict with "doing quite fine in their current environment". The apparent conflict seems more propense (is this a word?) to happen if you are thinking in adaptationist terms.

      I can be wrong, of course.



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    8. Pedro, thanks.
      I definitely see your last point i.e., "The apparent conflict seems more propense (is this a word?) to happen if you are thinking in adaptationist terms.", in which case it really does seem, to me at least, to actually BE a contradiction.
      In terms of other causes for species diversification, particularly genetic drift, I confess I am still trying to work out in my head exactly how it works.
      So, I'm still puzzling over it, but your answer does help me quite a bit.

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    9. andybeorger asks,

      Somewhat confused. Do plants also have some form of habitat tracking?

      Yes, of course. In fact the habitat tracking of plants is the paradigm for the entire phenomenon.

      We are seeing it right now as the treeline moves north in Canada and all kinds of plants are spreading into the valleys left by former glaciers. There's an entire subsection of ecology devoted to Ecological Succession.

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    10. Larry, thank you. Fascinating. Ecological Succession reminds me of an ongoing experiment taking place in an area of Colombia called Gaviotas. They are reforesting a deforested area, gradually restoring it to rainforest like conditions, by beginning with the groundlaying plants.
      The technique has turned out to be working more quickly there than even the scientists suspected it would. I think it's probably a fifty or sixty year old project at this point and various flora and fauna have already made their way back from the surviving rainforests in the region.

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  5. "Conventional neo-Darwinian theory views organisms as infinitely sensitive and responsive to their environments"

    No it doesn't. There is no expectation that any species should be "infinitely sensitive" to changing selective pressures. That's why countless species go extinct. The rest of their "paper" is thereby destroyed.

    What a fucking joke.

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    1. Going extinct in the face of a slight change in the environment strikes me as being quite sensitive and responsive.

      Don't most biologists believe we're facing a crisis of extinction just because the average temperature of our planet will increase by a few degrees?

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    2. The impacts section of the IPCC says "Protecting species that currently are vulnerable, endangered, or critically endangered (see Table 5-5) requires measures that, in general, reverse the trend toward rarity. Without management, there is high confidence that rapid climate change, in conjunction with other pressures, probably will cause many species that currently are classified as critically endangered to become extinct and several of those that are labeled endangered or vulnerable to become much rarer, and thereby closer to extinction, in the 21st century (Rabinowitz, 1981)."

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    3. Going extinct in the face of a slight change in the environment strikes me as being quite sensitive and responsive.
      Yeah but that's not what I meant, nor what I think was implied in their "paper". I took it to mean that Axe and co. thinks that evolution implies species will quickly adapt to even the most minute changes and so adaptation is infinitely minute, gradual, inevitable and smoothly follows the environment wherever it goes. It's essentially a strawman they erect, in order to point out how evolution must thereby be falsified when that doesn't happen.

      Don't most biologists believe we're facing a crisis of extinction just because the average temperature of our planet will increase by a few degrees?
      Yeah, because species are not infinitely sensitive in their adaptive ability, especially over so short timescales.

      When there's no more ice on the north-pole, noone but the ID strawman being erected is expecting polar-bears to become whales in 50 years or less. When polar bears aren't whales in 50 years, but extinct instead because they couldn't find land to stand on or food to eat(either because it migrated south or went extinct itself), the Discovery Institute will tout this as a falsification of evolution.

      Incidentially, a minor adjustment in temperature is not a complete or very adequate description of an environment to make predictions from. If the average temperature changes from -1 to +1, well, large landmasses can disappear under water. And the amount of CO2 in the atmosphere also has consequences for the acidity of the oceans etc.

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    4. I should add that the paper wasn't written by the ID guys of course, only that they chose to highlight this paper because of the implication they see.

      I still think it's weird to write that species are assumed to be "infinitely sensitive and responsive" to their environments. I'm not an evolutionary biologist, I'm a layman nobody, but I've read around quite a lot and I've never got the impression that this was a working assumption for anyone. It's definitely overstated.

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    5. I totally agree with Rumraket that it is weird to assume that populations are "infinitely sensitive and responsive" (and I am an evol. biol., fwiw). I don't think many evolutionary biologists think that adaptation is that simple.

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    6. I'll post a quotation from Richard Dawkins that you can discuss.

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  6. "How many Sandwalk readers are surprised by this paper?"

    I'm not, but then again I have a background in Geology and I was bombarded with the notion of Punctuated Equilibrium during my studies. I admit I always thought Stabilizing Selection was the answer to Stasis. Guess I need to catch up.

    @Helen above:
    "Did Gould provide an explanation for stasis, other than unspecified constraints?"

    I can't remember, as I refered above I thought it was stabilzing selection. I'd like to know that too. Anyone?

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    1. Stabilizing selection implies fine-tuning of a species to it's environment such that any deviation from the status quo will be eliminated because it reduces fitness. It suggests that all the observable phenotypic characteristics of a species are the best possible adaptations to the current habitat. In other words, most species sit atop an adaptive peak.

      This is inconsistent with punctuated equilibria because the observation is that evolutionary change is coupled to cladogenesis where a new daughter species is formed but the parental species continues to exist in the same environment.

      This gives rise to species sorting, according to Gould and others, because these species compete in the same environment. Cladogenesis wouldn't be possible if the changes in the new species were maladaptive as implied by stabilizing selection.

      It's much more likely that most species are only part way up an adaptive peak and for many phenotpyic characteristics there may not even be an adaptive peak. The reason you don't see more phenotypic adaptation is because it doesn't confer very much fitness advantage in a species that's doing well enough. Stasis then becomes simply a consequence of huge population sizes where change takes a very long time.

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    2. Thanks for your last two posts, they are quite elucidative. I didn't know about Gould's change of stance, but then again I didn't read his last book. Maybe I should.

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  7. "Conventional neo-Darwinian theory views organisms as infinitely sensitive and responsive to their environments"

    Wrong. In the Modern Synthesis models, response to selection depends primarily on genetic variability, which is not "infinite" or necessarily high. And when molecular studies showed lots of variability in populations, that was not what many "neo-darwinians" expected.

    "the data show that birds and mammals at Rancho La Brea show complete stasis and were unresponsive to the major climate change"

    Only in the skeleton morphology. Who knows how those animals could have been evolving at their life history, metabolism, fur coats, behavior...

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  8. I am not at all surprised by the results of this paper. Even some evolutionary ecologists (e. g. Douglas Futuyma and Michael L. Rosenzweig) recognize the existence of long-term ecological stasis as well as evolutionary stasis, which this paper is an excellent demosntration of both.

    John Kwok

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  9. I've got to say that I'm a little surprised, as I figured there would have been at least some variation among certain traits over that time. Then again, I'm not a biologist, so my surprise doesn't really count for much.

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    1. You'd be surprised how many professional biologists of "renown" think exactely the same, so no worries.

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  10. I'm not convinced that 'certain traits' didn't vary. There's more to variation than what can be determined with bones. As El PaleoFreak said:

    "Who knows how those animals could have been evolving at their life history, metabolism, fur coats, behavior..."

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  11. Not a biologist, but I would suspect that some, if not all, of these species were already adapted to fluctuations in seasons and thus would have changed very little over the duration.

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    1. There were some 50 glacial-interglacial cycles during the Pleistocene, weren't there? Adaptation to fluctuations sounds reasonable in such circumstances.

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  12. I think a large part of the concern about rising global temperatures is movement of organisms away from their usual habitat. They will become exposed to microorganisms which their immune systems are not familiar, and bring new microorganisms with which the endemic species are not familiar.

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