Re: Fossil Records Show Biodiversity Comes and Goes

From: John Harshman (jharshman.diespamdie_at_pacbell.net)
Date: 03/17/05 

Date: Thu, 17 Mar 2005 01:11:37 GMT

George wrote:

> "John Harshman" <jharshman.diespamdie@pacbell.net> wrote in message
> news:4V2_d.11214$C47.8877@newssvr14.news.prodigy.com...
>
>>George wrote:
>>
>>
>>><jtreat@cox.net> wrote in message
>>>news:1111004999.087065.229890@f14g2000cwb.googlegroups.com...
>>>
>>>
>>>>John Harshman wrote:
>>>>
>>>>
>>>>>Some observations on the published diversity curve.
>>>>>
>>>>>1. It doesn't have the shape we would expect from "mass extinction
>>>>>events". The curve is roughly a cycloid, i.e. the shape created by a
>>>>>point on a rolling circle. From a trough, diversity rises sharply at
>>>>>first, then slows as it comes to a peak, then decreases gradually,
>>>>>accelerating into the trough, and suddenly reverses. What we would
>>>>>expect from a mass extinction is a more saw-toothed shape: diversity
>>>>>increases quickly from a trough, perhaps slowing as it reaches a peak
>>>>
>>>>or
>>>>
>>>>
>>>>>perhaps not, then plunges instantly into another trough. This
>>>>
>>>>observed
>>>>
>>>>
>>>>>gradual decline is not consistent with either periodic impacts or
>>>>>episodes of flood vulcanism, which are geologically instantaneous
>>>>>events. Instead it calls for an explanation in which some kind of
>>>>>environmental stress begins slowly, increasing over millions of years
>>>>
>>>>>(starting some time before a peak and attaining a maximum at the
>>>>
>>>>trough,
>>>>
>>>>
>>>>>so covering 30ma or more), and then suddenly vanishing. I have no
>>>>
>>>>idea
>>>>
>>>>
>>>>>what that would be. It might be argued that this curve shape is an
>>>>>artifact caused by Signor-Lipps effect, but I don't think that's
>>>>>possible on this timescale, with marine invertebrates.
>>>>
>>>>I wondered why this should be an astronomical or geologic event. It
>>>>looks very much like a predator/prey or epidimic cycle. It is not
>>>>unreasonable to be looking at a ecological cause which would tend to
>>>>have a wave form like this.
>>>
>>>
>>>Indeed. If you look at the evolution of many species through the fossil
>>>record,
>>>one thing stands out like a sore thumb to me. As an example, look at
>>>crinoids.
>>>Crinoids most likely evolved from very simple animals found in the Burgess
>>>shale. By their heyday, they had branched out into many ecological niches,
>>>and
>>>had diversified into many very highly specialized species. The crinoids
>>>demonstrate a record of diversity that at their heyday in the late
>>>Mississippian
>>>can only be described as bizarre. Similarly, dinoaurs evolved from very
>>>simple
>>>animals, and in their heyday in the Cretaceous had evolved into complex, and
>>>equally bizarre, and very specialized forms. Such overspecialization tends
>>>to
>>>make species vulnerable to factors such as environmental change. If a
>>>species
>>>evolves to become a specialist in eating Eucalyptus trees, and all the
>>>Eucalytpus trees die out in some environmental catastrophe (severe drought,
>>>for
>>>instance), so do the Eucalytpus eaters.
>>
>>Is there any evidence that specialization really is a one-way ratchet?
>
>
> Well, there is an amusing, if tragic anecdote. consider the Dodo bird.

Hardly specialized. It was flightless and it lived on a little island.
That was its downfall. You need a weird definition of specialization to
make that an example of anything relevant.

>>I don't think so. Evolution wanders all over the place. In retrospect, we
>>consider the earliest members of a group to be "simple" or
>>"non-specialized" only because we see them sitting in the middle (in
>>morphospace) of a diverse cloud of descendants.
>
> Or we could be seeing the earliest members of a species as being opportunists,
> ready and genetically able to fill voids in many ecological niches when they
> become available, as other species die out. Once those voids are filled,
> specialization takes hold, and the critters become more vulnerable to extinction
> because of their specialization.

You could see that, but is there any evidence for it? Not that I can
see. Of course this is a great point where phylogeny can step in and
address the question. One problem is that "specialization" isn't very
well defined. One man's specialization is another man's key innovation.

>>Any one of those
>>descendants could probably be, at some future time, be considered as
>>"simple" if it happened to spawn a large radiation too. Look for example
>>at birds -- just one group of theropods. I suppose you would consider
>>the earliest birds "unspecialized" too, compared to the variety of
>>extant birds; but if you're talking about theropods, those same birds
>>would be very strange, "specialized" theropods.
>>
>>At any rate, you would need some form of coordinated specialization
>>across all groups to produce the pattern you are trying to explain here.
>>It looks to me rather that groups appear and diversify at different
>>times with no real coordination.
>
> Coordinated? Why would they need to be coordinated? Species live and die,
> alternately filling and then vacating niches. When the niches become available,
> other species fill in the gap. We see it time and time again in the fossil
> record.

But that wouldn't produce a pattern of rising and falling diversity
across all marine invertebrates, would it? It would only if all the
different groups filled niches at the same time, and then vacated them
at the same time.

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