Moorad
-----Original Message-----
From: Biochmborg@aol.com <Biochmborg@aol.com>
To: asa@calvin.edu <asa@calvin.edu>
Cc: evolution@calvin.edu <evolution@calvin.edu>
Date: Friday, August 13, 1999 5:00 PM
Subject: Re: Behe on Kansas in Today's NYT
>Greetings to One and All:
>
>Once again Behe is leaving certain crucial facts out his essay.
>
>>
>> For example, let's look at three claims of
>> evidence for Darwinian evolution
>> often cited by high school textbooks.
>>
>
>[snip]
>
>>
>> Third, the embryos
>> of fish, amphibians, birds and
>> mammals look virtually identical in an early
>> stage of development, becoming
>> different only at later stages.
>>
>
>[snip]
>
>>
>> The story of the embryos is an object lesson
>> in seeing what you want to see. Sketches of
>> vertebrate embryos were first made in
>> the late 19th century by Ernst Haeckel, an
>> admirer of Darwin. In the intervening years,
>> apparently nobody verified the accuracy of
>> Haeckel's drawings.
>>
>
>This is untrue, because in fact a large number of professional scientists
>used their own microscopes to compare Haeckel's drawings with their own
fetal
>preparations. Only a very few challenged the accuracy of Haeckel's
drawings;
>nearly everyone accepted that Haeckel was being accurate, based on the
>standards at the time. What most of them opposed was Haeckel's theory
>explaining this similarity. Haeckel proposed that because ontogeny
>progressed by recaptulating adult phylogeny, early embryos looked alike
>because they were in an early stage of ontological development, which
>represented an early stage of phylogenetic development. In other words,
>Haeckel was saying that embryos evolved their way from a single cell to a
>fully developed modern form. von Baer represented an early group of
>objectors. He stated that the phylogenetic structures that were being
>recapitulated were fetal structures, not adult structures, and that their
>recapitulation was coincidental to ontological development, not the cause
of
>that development. However, he and his colleagues had no viable theory of
>ontological development to replace Haeckel's. His (Wilhelm His)
represented
>a later group of objectors. His rejected recapitulation altogether and
>claimed that ontological development was a physiological phenomenon caused
by
>changes in cell growth rates. Because His rejected recapitulation, he also
>challenged the accuracy of Haeckel's drawings. His' concept of ontological
>development eventually proved correct, but only after the rediscovery of
>Mendelian genetics provided the mechanism that his theory lacked. Until
>then, Haeckel's theory remained dominant. In point of fact, it was never
>refuted; it was simply abandoned as the growing science of genetics made it
>irrelevant. Even so, von Baer's recognition of the recapitulation of
>embryonic features was readopted and applied to Haeckel's famous statement
>that ontogeny recapitulates phylogeny. Only now, instead of being the
>driving force behind embryology, it is simply a consequence of embryonic
>development. For more details see _Ontogeny and Phylogeny_ by Stephen Jay
>Gould.
>
>>
>> Prominent scientists
>> declared in textbooks that the theory of
>> evolution predicted, explained and was
>> supported by the striking similarity of
>> vertebrate embryos. And that is what
>> generations of American students have learned.
>>
>> Recently, however, an international team of
>> scientists decided to check the drawings'
>> reliability. They found that Haeckel had, well,
>> taken liberties: the embryos are significantly
>> different from each other. In Nature, the head
>> of the research team observed that "it looks like
>> it's turning out to be one of the most famous
>> fakes in biology."
>>
>
>What Behe doesn't state is that this team wasn't challenging the idea that
>embryonic development supports evolution; what they were challenging were
>specific hypotheses based on the concept of the common body plan, which in
>chordates is called the "pharyngula". This is stated in a letter they
wrote
>to _Science_ (280:5366, 983-986) after creationists started using their
>report to criticize evolution: "Hackel's drawings are used in many modern
>textbooks, but not always as primary evidence for evolution. In _Molecular
>Biology of the Cell_, the drawings are used mainly to support hypotheses
>about the stages of development acted on by natural selection. It is only
in
>this limited context that we have reservations about the implications of
the
>drawings." The only evidence they chose to examine were Haeckel's
drawings,
>despite the fact that -- contrary to their claim -- they are no longer the
>most significant evidence in support of these hypotheses. The tone of the
>article is due largely to the lead author, Michael Richardson, who
apparently
>has an axe to grind against Haeckel (based on personal communications and
his
>subsequent comments in Nature).
>
>>
>> What's more, the embryonic
>> stages shown in the drawings are actually not
>> the earliest ones. The earliest stages show
>> much greater variation.
>>
>
>What Behe fails to mention is that this is old news to embryologists; Behe
>also fails to mention the actual relationship these various embryonic
stages
>have with each other. Embryologist have known for a long time that the
stage
>of the common body plan, known as the phylotypic stage, is preceeded by a
>fair number of earlier stages. This is necessary, however, because the
>elements of the body plan arise during these earlier stages. Embryologists
>have also known for some time that many aspects of development in these
early
>stages differ between classes and even between orders and families. There
>are two types of differences: differences in egg size, shape, shells and
>accessory food sources, which are provided by the mother and the germline
>cells before fertilization; and differences in the extra-embryonic tissues
>developed by the embryo itself. Yet despite all these differences, some
key
>aspects of development are conserved, and it is these that produce the
>elements of the common body plan, which for chordates is chiefly the
>notocord, the dorsal hollow nerve cord, the pharyngeal gill pouches and the
>post-anal tail. This body plan is possessed by all chordates, including
>ascidians, even though the the exact form of the pharyngula differs from
>class to class, and even between orders and families. These differences
are
>created by the different developmental stages leading up to the chordate
>phylotypic stage. Then of course, further development after the pharyngula
>has formed causes the embryos to diverge further as they develop the adult
>form. However, some aspects of post-phylotypic development are also
>conserved, thus constraining this divergence. See _Cells, Embryos, and
>Evolution_ by John Gerhart and Marc Kirschner for more details.
>
>In any event, the point is this. Behe implies that development between
>chordate classes, orders and families are now seen to be virtually
unrelated,
>but this is untrue. All known chordates have different developmental
>pathways for oogenesis, primary and secondary axis specification,
>endo-mesoderm induction, organizer inductions, gastrulation and
neurulation,
>but they all end up at the same intermediate developmental stage with the
>same basic body plan. This is the stage that Haeckel drew, and most modern
>embryologists agree that all chordates go through this stage. Even the
>authors of the report Behe mentioned agree with this: "On a fundamental
>level, Haeckel was correct: All vertebrates develop a similar body plan
>(consisting of a notochord, body segments, pharyngeal pouches, and so
forth).
> This shared developmental program reflects shared evolutionary history.It
>also fits with overwhelming recent evidence that development in different
>animals is controlled by common genetic mechanisms." They later state:
"We
>suggest that Haeckel was right to show increasing difference between
species
>as they develop. He was also right to show strong similarities between his
>earliest embryos of humans and other eutherian mammals (for example the cat
>and the bat)." Their only major complaint was that by drawing the embryos
as
>having virtually no differences at this stage, Haeckel implies that there
was
>virtually no evolutionary change in early embryonic stages of development.
>
>[snip]
>
>>
>> But I would also want them to learn to make
>> distinctions and ask tough questions. Questions
>> we might discuss include these:
>>
>
>[snip]
>
>>
>> If supposedly identical embryos were touted
>> as strong evidence for evolution, does the recent
>> demonstration of variation in embryos now count
>> as evidence against evolution?
>>
>
>According to the authors of that report, the answer is no. Again from the
>letter to _Science_: "Our work has been used in a nationally televised
>debate to attack evolutionary theory, and to suggest that evolution cannot
>explain embryology. We strongly disagree with this viewpoint. Data from
>embryology are fully consistent with Darwinian evolution." "Unfortunately,
>Haeckel was overzealous. When we compared his drawings with real embryos,
we
>found that he showed many details incorrectly. He did not show significant
>differences between species, even though his theories allowed for embryonic
>variation." "This does not negate Darwinian evolution. On the contrary,
the
>mixtures of similarities and differences among vertebrate embryos reflects
>evolutionary change in developmental mechanisms inhereted from a common
>ancestor." "Thus, certain 'phylotypic' embryonic stages, which Haeckel
>showed as identical, may in fact be significant targets for natural
>selection." "Haeckel's inaccuracies damaged his credibility, but they do
not
>invalidate the mass of published evidence for Darwinian evolution.
>Ironically, had Haeckel drawn the embryos accurately, his first two valid
>points in favor of evolution would have been better demonstrated."
>
>In other words, embryonic variation (which has been known for sme time and
>was not just recently demonstrated as Behe claims) actually strengthens
>evolution rather than weakens it.
>
>>
>> If some
>> scientists relied for a century on an old, mistaken
>> piece of data because they thought it supported
>> the accepted theory, is it possible they might
>> even now give short shrift to legitimate contrary
>> data or interpretations?
>>
>
>As long as analyses like the one Behe mentioned are published, I doubt that
>will become a problem.
>
>What I find more troubling is Behe's insistence upon mischaracterizing
>research in order to make his case. This is the third instance I have
>discovered of this trend in as many months. This is a trait of political
>activists, not scientists.
>
>Kevin L. O'Brien