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