Re: Species diversity through time

Friar Broccoli wrote:

On Nov 8, 9:30 am, John Harshman <jharshman.diespam...@xxxxxxxxxxx>
wrote:

Friar Broccoli wrote:

On Nov 7, 7:55 pm, John Harshman <jharshman.diespam...@xxxxxxxxxxx>
wrote:

Bob Casanova wrote:

On Tue, 06 Nov 2007 23:33:17 GMT, the following appeared in
talk.origins, posted by John Harshman
<jharshman.diespam...@xxxxxxxxxxx> :

Bob Casanova wrote:

On Mon, 05 Nov 2007 17:03:29 -0800, the following appeared
in talk.origins, posted by John Harshman
<jharshman.diespam...@xxxxxxxxxxx> :

For those interested, this monograph came bundled with the latest issue
of Paleobiology:

Stanley, S. M. 2007. An analysis of the history of marine animal
diversity. Paleobiology 33(4s):1-55.

In it, Steven M. Stanley does a series of analyses based on Sepkoski's
compendium of marine invertebrate genera. He comes to a number of
conclusions that may interest some of the TO regulars. (These are not
necessarily the bits that Stanley considers most important.)

1. Background rates of speciation and extinction are strongly
correlated; that is, groups with high speciation rates also have high
extinction rates.

This seems to indicate that those groups are not as well
adapted to their environments. This being the case the
correlation would logically seem to follow. Or am I missing
something subtle?

Perhaps I am. I don't see how your conclusion follows.

Sorry; I'll try to be more clear. Perhaps incorrect, but
clear. I'm starting with the assumptions that populations
tend to adapt to their environments, and that a large number
of speciations within a group indicates that either the
group as a whole isn't well-adapted to the environment or
that the environment is in constant change. Or both.

Or neither, in my opinion. Why would poor adaptation to an environment
cause speciation? It's going to cause either better adaptation or
extinction. Nor should fluctuating environment cause speciation. If
would, if anything, cause adaption to fluctuation or, if on a longer
timescale, adaptation to new environments, none of which should result
in speciation. Speciation is the evolution of reproductive isolation
between populations. Generally this happens when two populations become
geographically isolated from each other.

I agree that "poor adaptation" in itself would not cause
speciation. (We are all always poorly adapted compared to some
imaginable ideal)

However, a fluctuating environment would cause speciation
precisely because it would lead to reproductive isolation.
Fluctuations would cause some members to stay where they are
physically after a change, while others would move to
environments more similar to the ones they knew before the
change. These separated populations would be under different
selective pressures leading to rapid speciation.

This is an interesting idea, but I don't see that it has any
biological support.


We know (or at least agree I hope/believe) that:

1 species evolve differently under differing environmental
pressures

2 members of species will tend to invade new territories that
are similar (for their purposes) to their existing
territories.

3 A fluctuating environment will see changes of many and varied
character, but after a fluctuation many existing environments
will have new adjacent environments that are similar to the
old one, but many of the new adjacent (or overlayed)
environments will be similar to the original in different ways.

Thus, in a fluctuating environment different subgroups of an
existing species will find themselves on opposite sides of
differing environments, under differential selective pressures.

Sorry, no. You have not presented any reason why these groups should be
separate. If two slightly different environments are adjacent to each
other, we expect a continuous population to spread out across them, if
they're both hospitable. The population will continue to exchange genes
across the boundary unless something prevents them, i.e. geographic
isolation. Now selection can produce some local divergence in a few
traits even in the face of genetic exchange. But it can't produce a
speciation except under very unusual conditions that may seldom (never?)
be found in nature.

I will complete this argument below.



Or
possibly that the group is expanding into new niches, with
the corresponding isolation and therefore the expected
multiple speciation that entails.

Why would expansion into new niches produce isolation? I think it's the
other way around, actually. Speciation, or even simple geographic
isolation, allows the two populations that were once a single population
to evolve along two trajectories rather than one, and thus increases the
probability that some population of the group will expand into a new
niche. The more speciation/isolation, the more chances.

There are examples of fruit feeding insects, of which I am
certain you are aware, that speciate when some individuals
switch to a new fruit. So the new fruit (niche) was the direct
cause of the isolation that lead to speciation.

You're talking about Rhagoletis host races, presumably. They're
generally not considered full species, and whether this sort of thing
ever leads directly to speciation is under dispute.

Thanks for the name. This is the one known from the US. There
is at least one other known from Northwest Europe. I can look
it up if you're interested, although I cannot imagine it having
any effect on this discussion.

I'm not going to waste time arguing about whether the diet
change in some Rhagoletis is by itself sufficient for
speciation (although I think this to be VERY VERY likely),
rather I will make the obvious point that fruit trees have
various and overlapping/separate ranges, governed by things
like soil conditions, temperature, parasites etc.

Consequently, by a simple process of hopping from host to host
over generations, some Rhagoletis will eventually find
themselves reproductively isolated, like those often discussed
seagulls (whose name I cannot -as usual- remember).

Don't bother. It turns out that the seagulls don't fit the story anyway.
Why would reproductive isolation arise? If there's gene flow, it almost
certainly will not.

So going back to the argument above, constant changes to, and
instability in, the environment would result in a similar
pattern of range changes as species moved about to find more
convivial areas.

I can see a scrap of meaning in all this. If you are suggesting that
fluctuating environments would be more likely to create geographic
barriers to gene flow -- i.e. areas in which there is no suitable
habitat, too wide to cross, separating populations in suitable habitats,
then you will start to have something. Geographic isolation fuels
speciation.

As an aside, it occurs to me that there might be something to
this (generally poorly adapted species = faster speciation
model) because if you are relatively more poorly adapted there
are more changes that would turn out to be improvements. Thus
adaptive changes would (on average) be more frequent.

Until the species became well adapted, after which point they would
stop. So you need a mechanism here whereby a particular group is
continually becoming poorly adapted, at a rate greater than its ability
to adapt. Aside from being a recipe for extinction, why should a
particular group retain this characteristic of being poorly adapted over
time?

Sometimes these changes would make new territory/niches
available to the subgroup with the new modification and thus
cause reproductive isolation of that group.

Sorry, but it wouldn't. Reproductive isolation almost always requires
prior geographic isolation.

And I recall reading that
there's an "average lifetime" for species as there is for
individuals, although I don't know the numerical data (mean
*or* standard deviation) regarding that lifetime. On the
assumption that the species lifetime is similar for
similar-sized (number of species) groups (although probably
wildly variable *within* any particular group), a group
which produces more than the average number of species in a
given time would also produce more than the average number
of extinctions in that same time.

"Species lifetime" doesn't require any mechanism other than chance.
Don't assume that species get old and die. There would be an average
lifetime if a species had a constant probability of extinction at any
time, and that's all it is. Now, if every species has the same
probability of extinction per unit time, the more speciose group would
have more extinctions simply because it has more species, just as the
more speciose group would also have more speciations.

Of course, if my
assumptions are incorrect the logic chain breaks down.

I'm sorry to say that I think all your assumptions are incorrect.

I'm sorry to say that none of your arguments support my views.

Perfectly fair. After all, none of your views support my arguments.

Finally an acknowledgement of perfection!!

I know of other ways to use "perfect" in connection with you, if you
would like. The form would be "Friar Broccoli is a perfect -----", and
there are a number of possibilities to fit into the blank.

.

Relevant Pages

  • Re: POTM: Re: The Ways New Species Originate
    ... Speciation, in sexual organisms anyway, occurs when two genetic groups ... In the case of duplication of one species' chromosomes, ... Secondary selection eliminates genes that are less fit. ... "reinforcing selection" to maintain isolation. ...
    (talk.origins)
  • POTM: Re: The Ways New Species Originate
    ... Speciation, in sexual organisms anyway, occurs when two genetic groups ... In the case of duplication of one species' chromosomes, ... Secondary selection eliminates genes that are less fit. ... "reinforcing selection" to maintain isolation. ...
    (talk.origins)
  • Re: Species diversity through time
    ... groups with high speciation rates also have high ... tend to adapt to their environments, ... precisely because it would lead to reproductive isolation. ... species evolve differently under differing environmental ...
    (talk.origins)
  • Re: Species diversity through time
    ... groups with high speciation rates also have high ... tend to adapt to their environments, ... precisely because it would lead to reproductive isolation. ... generally not considered full species, and whether this sort of thing ...
    (talk.origins)
  • Re: Species diversity through time
    ... groups with high speciation rates also have high ... tend to adapt to their environments, ... precisely because it would lead to reproductive isolation. ... generally not considered full species, and whether this sort of thing ...
    (talk.origins)