Re: Species Selection Redux

From: William Morse (wdmorse_at_twcny.rr.com)
Date: 03/03/05 

Date: Wed, 2 Mar 2005 23:01:23 -0500 (EST)

Wirt Atmar <atmar@aics-research.com> wrote in
news:d00q50$1s19$1@darwin.ediacara.org:

>> The reason this struck me is that the ability of an individual to
>> adapt to its environment is limited by gene flow. Barlow in "The
>> Cichlid Fishes" gives the example of deep-bodied reef cichlids
>> adapted for feeding by scraping that disperse to a reef where the
>> major food source is free swimming plankton. They will do better by
>> switching to suction feeding and more streamlined bodies. However,
>> if they are still part of the gene pool of the scraping feeders, they
>> will be unable to change in this new direction.
 
>> In such a situation, a species that has the ability to speciate
>> parapatrically, perhaps by assortative mating favoring divergent
>> selection, would be favored. So there might be a tendency to evolve
>> evolvability at the species level, even though there is no benefit at
>> the individual level.
 
> Species such as the cichlids have attracted so much attention that
> their stories are being repeated second- and third-hand by now, and in
> the process the story of their speciations is becoming more muddled
> and more hyperbolic with each retelling. But there doesn't appear to
> be anything magic about the lineages or their proclivities to
> speciate. Rather the mechanism underlying their speciations appears to
> be wholly mechanical: episodic allopatric speciations abetted by
> rising and falling water levels.
>
> Let me recommend two excellent talks that we've recorded on the
> subject, the first by Axel Meyer (Univ. Konstanz), on cichlids, and
> the second by Doug Futyuma (SUNY Stony Brook), on host-insect
> associations. They were given during the centenary celebration of
> Mayr's 100th birthday at Harvard last year and appear at:
>
> http://aics-research.com/lectures/ernstmayr/
>
> Both Meyer and Futyuma argue that the speciation events that they have
> measured are the result of the simplest form of speciation:
> geographical isolation, and in that regard, both even "out-Mayr" Mayr
> in their beliefs that sympatric speciation does not occur.

Thanks for the references. I haven't had time yet to reviiew the Futuyma
piece, but I enjoyed the talk by Meyer.

To set the record straight, Barlow's "The Cichlid Fishes" is definitively
_not_ a second or third-hand repeat of the cichlid story. Barlow (professor
emeritus at Berkeley) has been studying cichlids for over 50 years. Much of
the work Meyer recounts in the talk is discussed in detail in the book. And
much of the book has to do with other aspects of the cichlids that shed
much light on evolution, in particular the variety of social arrangements
seen in the cichlids, which include essentially the same range seen in the
primates, including monogamy and assistance in rearing kin. If you haven't
read the book, you should.

And to further clarify, Barlow does not advocate parapatric speciation, but
notes the influence of changing lake levels in leading to allopatric
speciation.

BUT

Reading both the book and Meyers' talk, one is still left with 500 cichlid
species in Lake Victoria in 500,000 years, with a significant bottleneck at
only 14,000 years ago. (Barlow, in his book, indicated that the bottleneck
might be complete, probably based on the article that Meyer refers to in
his talk. Meyer notes - which I find more believable - that based on
current variety in an adjacent shallow lake, it is likely that considerable
variety survived in pockets of shallow habitat in Lake Victoria.) Still -
500 species of cichlids, vs. a handful of species in each of the other fish
families that inhabit the lakes. I will grant you allopatric speciation -
you still need an explanation for why the cichlids speciate more rapidly
than other fishes.

And Meyers in his talk does refer to assortive mating and sexual selection.
Barlow in his book tends to downplay this, preferring the explanation that
the special jaw structure of the cichlids (the pharyngeal arches) as well
as their use of mouth brooding and other particulars of their behavior
leads them to differentiate more easily than other fish. However, Meyers
noted in his talk that work on South American cichlids showed divergence in
polymorphism based on color was more positive than that based on jaw
structure.Again, assuming strictly allopatric speciation, a mechanism that
causes divergent selection could still be one of the factors that accounts
for the high speciation rate of the cichlids.

Coelacanths have been around for several hundred million years, with
slightly over a hundred documented species. Cichlids in Lake Victoria have
speciated at a thousand times that rate. I do not think it is unreasonable
to speculate on a mechanism that can explain this difference at a different
level than that of individual characteristics, even though careful study
may eventually show that it is the characteristics of the individuals that
explains the discrepancy.

In summary,while we can discuss the nuances in parapatric vs. allopatric
speciation, I think Meyers work still supports my conclusion that assortive
mating with divergent selection is one mechanism to favor "evolvability",
and that some lineages seem to be more evolvable than others.

Yours,

Bill Morse

Relevant Pages

  • Re: The MEANING of "gene flow"
    ... > Both Meyer and Futyuma argue that the speciation events that they ... but I enjoyed the talk by Meyer. ... emeritus at Berkeley) has been studying cichlids for over 50 years. ... variety survived in pockets of shallow habitat in Lake Victoria.) ...
    (sci.bio.evolution)
  • Re: Species Selection Redux
    ... > species in Lake Victoria in 500,000 years, with a significant bottleneck at ... I will grant you allopatric speciation - ... you listen and look again to what Meyer has to say in his slides 5 and 6. ...
    (sci.bio.evolution)
  • Re: Defining&testing "macroevolution" (was: Testing the Laws of Intelligence)
    ... evolution of populations below the level of speciation. ... It means that changes within a population or within one species is ... called "microevolution", as opposed to the origin of new species and ... Bacteria don't reproduce sexually, ...
    (talk.origins)
  • Re: Oregon: Teacher Fired Over Bible References
    ... This makes the speciation ... then according to the BSC they are the same species. ... On the other hand the paper does cite, skeptically, a report of a hybrid between the main group and a whistling duck. ... The three tetraploid species Brassica napus, Brassica juncea and Brassica carinata have been experimentally resynthesised ...
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  • Re: How does a new species emerge?
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