Re: "Evolutionary Rate" from Natural Selection

dkomo wrote:

> John Harshman wrote:
> 
> 
>>dkomo wrote:
>>
>>
>>
>>>John Harshman wrote:
>>>
>>>
>>>
>>>
>>>>dkomo wrote:
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>>Sam wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>This his something I can't recall every reading about, but I've been
>>>>>>recently considering.
>>>>>>
>>>>>>Going back to the beginnings of the evolutionary process and the forces of
>>>>>>natrual selection on Earth, does it not make sense that the rate at which
>>>>>>new species appear would have increased over time?  What I mean is, starting
>>>>>
>>>>>>from the simplest examples of a multi-cellular organism, doing it's thing,
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>reproducing and passing on traits, mutating and increasing genetic
>>>>>>variation, being acted upon by natual selection which choses the fittest,
>>>>>>etc... Over time in different enviroments populations follow different
>>>>>>enough paths that the generational products can be considered different
>>>>>>species.  Now say the same process repeats and you get 4 species, then 8,
>>>>>>then 16, then...  Down the road you end up with larger populations,
>>>>>>increasing the chances of mutations, and creating more "types of life" for
>>>>>>that natural selection to work with, you end up with more and more parallel
>>>>>>evolutionary processes all happening at the same time which increases the
>>>>>>rate of diversifcation.
>>>>>>
>>>>>>I know this is an oversimplification, that every species does not spawn two
>>>>>>more, you may get several or none.  I'm just thinking of how it took several
>>>>>>billion years to produce multi-cellular life but then only 500 million to
>>>>>>make it to the complex life of today.  It seems to me that the rate of
>>>>>>diversity as a result of evolution would in fact increase with the diversity
>>>>>>itself.
>>>>>>
>>>>>
>>>>>>Is this really the case or am I off base here?
>>>>>>
>>>>>>
>>>>>
>>>>>This is in fact the case.  I've seen graphs of the number of species 
>>>>>currently living plotted against geological time and it is a long term 
>>>>>rising curve which rises quite steeply toward the end like an 
>>>>>exponential curve.
>>>>
>>>>
>>>>I doubt that sincerely. Where have you seen such graphs? The most common
>>>>are derived from Sepkoski's compilation of marine families, and that's
>>>>not exponential at all. In fact what Sepkoski did was to fit three
>>>>summed logistic curves to the data.
>>>>
>>>
>>>
>>>See my reply to CMSchaum.
>>
>>
>>I saw it. All it does is establish that we're looking at the same graph.
>>But you are not seeing it well.
> 
> 
> That silly little graph?  How could anyone miss the obvious implication: 
> at the current time the number of families shows a peak that is much 
> higher than the number at, say, 500 MYA.  Ergo, evolution has progressed 
> in the diversity of organisms.  And some of the changes along that graph 
> are quite steep.

Yes, the graph shows continuous increase during the Mesozoic and
Cenozoic. But your interpretation of it as exponential growth is just
your imagination imposing an exponential curve on the data.

> But that's the graph.  If the graph is plotting something that may be 
> highly inaccurate for the various reasons discussed in this thread, then 
> that's another story...
> 
> In a similar vein, at 650 MYA there were only single-celled organisms 
> floating in the oceans (do I have that date right?), and now we have 
> funny looking primates orbiting the earth and sending spacecraft 
> throughout the solar system.  A casual observer might be tempted to 
> conclude that evolution has progressed there as well.

Certainly at least one lineage has become more complex. If you want to
draw from that lineage the conclusion that evolution has progressive,
then you have an odd view of what progress means. But we've been over
that before.

>>>>>It is, however, punctuated by a few steep drops due 
>>>>>to the great extinctions.  And the current tall peak of that curve may 
>>>>>be dropping sharply because of the species extinction caused by the 
>>>>>activities of mankind.
>>>>
>>>>
>>>>It may also be illusory. Is the phrase "pull of the Recent" familiar to you?
>>>
>>>Actually, haven't heard such a phrase before.
>>
>>
>>You need to become familiar. It's a key concept in paleontology. What it
>>means is that the fossil record becomes better and better the closer you
>>get to the present, and this biases any count of species towards
>>constant increase over time. There have been attempts to correct for
>>this bias. Some of them find an absolute diversity peak in the Miocene.
>>Others find that diversity has leveled off since the Eocene. You can't
>>take a raw count at face value.
> 
> Well, then that's the story hiding underneath the graph.

The more you know...

>>>>>We've had raging debates here on t.o. in the past as to whether global 
>>>>>evolution is progressive.  In terms of complexity of life, the debates 
>>>>>were indeterminate, IMHO.  In terms of diversity of life, however, it 
>>>>>clearly is progressive.
>>>>
>>>>
>>>>Nope. Sorry.
>>>
>>>A little editing to make it more clear:
>>>
>>>We've had raging debates here on t.o. in the past as to whether global
>>>evolution is progressive.  In terms of complexity of life, where the 
>>>question was whether the average complexity of life on earth has 
>>>increased over evolutionary time intervals, the debates were 
>>>inconclusive, IMHO.
>>
>>
>>That's a nice spin for you, IMHO.
>>
>>
>>
>>>In terms of diversity of life, where such life includes only what is 
>>>alive at the particular time being considered, however, evolution is 
>>>piecewise" progressive.  In terms of diversity of life, where such life 
>>>includes all organisms that have ever lived up to the particular time 
>>>being considered, however, evolution clearly is progressive.  The word 
>>>"piecewise" is a mathematical term which refers to the fact that the 
>>>diversity of life curve increased sharply during particular time 
>>>periods, decreased or remained static during other, shorter time periods.
>>
>>
>>Yes. And I'm saying that evolution is not clearly progressive even in
>>this sense.
> 
> Oh, you mean in the sense that evolution started with a primitive single 
> celled organism and through evolutionary time managed to generate tens 
> of billions of different types of organisms?  In that sense it's not 
> progressive, eh?

If you want to call that progressive, then it is. There is no clear,
general trend for the number of species to increase. If, as people
frequently suggest, most species are prokaryotes, then the number has
effectively not increased for billions of years. Feel free to define
progress in any way that makes you happy, though.

.

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