Done Quickly

I’ve never seen anyone talk about it much, but when you think about mass extinctions, you also have to think about rates of change

You can think of a species occupying a point in a many-dimensional space, where each dimension represents some parameter that influences survival and/or  reproduction: temperature, insolation, nutrient concentrations, oxygen partial pressure, toxin levels, yada yada yada. That point lies within a zone of habitability – the set of environmental conditions that the species can  survive.  Mass extinction occurs when environmental changes are so large that many species are outside their comfort zone.

The key point is that, with gradual change, species adapt.  In just a few generations, you can see significant heritable responses to a new environment.  Frogs have evolved much greater tolerance of acidification in 40 years (about 15 generations). Some plants in California have evolved much greater tolerance of copper in just 70 years.

As this happens, the boundaries of the comfort zone move.  Extinctions occur when the rate of environmental change is greater than the rate of adaptation, or when the amount of environmental change exceeds the limit of feasible adaptation.  There are such limits: bar-headed geese  fly over Mt. Everest,  where the oxygen partial pressure is about a third of that at sea level, but I’m pretty sure that no bird could survive on the Moon.

What this means is that, when considering environmental changes that are within the limits of adaptation, rapid changes are much more likely to cause a mass extinction than slow changes.  An asteroid strike (of sufficient megatonnage) is far more likely to cause a mass extinction than changes in sea level. I’m not saying that an asteroid is the only way to cause a mass extinction: I’m saying that an abrupt event is far more effective than a slow change of the same magnitude. For creatures with short generations, like insects, even 1000 years looks slow.

Several orders of insects disappeared in the Permo-Triassic extinction, the biggest of all.    Whatever caused that extinction must have been a humdinger.

Paleontologists prefer gradualist explanations for mass extinctions, but they must be wrong, for the most part.

 

 

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4 Responses to Done Quickly

  1. Gorbachev says:

    Palaeontologists prefer gradualist explanations partly because catastrophism got a bad rap in the 19thn century.

    But there’s no reason extinctions can be often quite gradual, and often very sudden.

    The problem is being two-dimensional about how you think about it.

  2. ” Extinctions occur when the rate of environmental change is greater than the rate of adaptation”

    This makes me think of modern societies and birth control. A hundred years from now will scientists look back and talk about the “mass extinction” caused by the introduction of reliable birth control? With the West’s birth rates below replacement I would think *some* kind of shift is happening and relatively quickly since it’s happening fast enough for people to notice it within their own lifetimes.

  3. erica says:

    gcochran,

    OT, but noticed the Dec. 8, 2011 article in “Science Daily” entitled “Why Aren’t We Smarter Already? Evolutionary Limits on Cognition” and in it, this paragraph:

    “Even increasing general intelligence can cause problems. Hills and Hertwig cite a study of Ashkenazi Jews, who have an average IQ much higher than the general European population. This is apparently because of evolutionary selection for intelligence in the last 2,000 years. But,
    at the same time, Ashkenazi Jews have been plagued by inherited diseases like Tay-Sachs disease that affect the nervous system. It may be that the increase in brain power has caused an increase in disease.”

    http://www.sciencedaily.com/releases/2011/12/111207133053.htm

    Heretical ideas have finally reached the mass audience? About time.

  4. MikeP says:

    Yes, you are correct. That was an especially big one when you consider that even the K-T extinction had no consequence for insects at the ordinal level. We even have many modern insect genera or close relatives in late Cretaceous ambers. Unfortunately, paleontologists seem to naturally gravitate toward more “nuanced” (I.e. wrong) explanations, such as Bakker’s absurd pandemic hypothesis.

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