The Recapitulation Myth
Casey Luskin recently posted two blogs showing that textbooks still misuse Haeckel's long-discredited embryo drawings when attempting to provide evidence for Darwinian evolution (see here and here). Luskin provided ample documentation to demonstrate that these drawings are still printed in some recent textbooks.
Over at The Panda's Thumb blog, apologists for Darwinian theory have defended (see here and here) Ernst Haeckel from the charge of fraud and have argued, albeit unconvincingly, that, in principle, the concept of recapitulation is a valid one.
According to Nick Matzke:
Haeckel didn't ignore the differences in embryos in the earliest period just after fertilization (differences which are visually significant but mostly fairly trivial, due to the different amounts of yolk in different vertebrate eggs).
Apparently Matzke missed some of the authorities cited by Luskin, which contradict Matzke's claims. All the necessary refutations of Matzke can be found in Luskin's original posts:
I. Post 1: Contrary to Matzke's claims, authorities acknowledge that Haeckel did "ignore the differences in embryos":
1. Stephen Jay Gould: "Haeckel had exaggerated the similarities by idealizations and omissions. He also, in some cases -- in a procedure that can only be called fraudulent -- simply copied the same figure over and over again." [Stephen Jay Gould, "Abscheulich! (Atrocious!)," Natural History, Mar. 2000, at 42, 44--45.]
2. Michael Richardson: "It looks like it's turning out to be one of the most famous fakes in biology." [Elizabeth Pennisi, "Haeckel's Embryos: Fraud Rediscovered," 277 Science 1435, 1435 (1997).]
3. Richardson et al. 1997: "His drawings are also highly inaccurate, exaggerating the similarities among embryos, while failing to show the differences" [Michael K. Richardson et al., "There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development," 196 Anatomy and Embryology, 91, 92--104 (1997).]
II. Post 2: The differences between vertebrate embryos at their earliest stages are not said to be, as Matzke puts it, "fairly trivial, due to the different amounts of yolk in different vertebrate eggs":
1. Collazo (2000): "Recent workers have shown that early development can vary quite extensively, even within closely related species, such as sea urchins, amphibians, and vertebrates in general. By early development, I refer to those stages from fertilization through neurolation (gastrulation for such taxa as sea urchins, which do not undergo neurulation). Elinson (1987) has shown how such early stages as initial cleavages and gastrula can vary quite extensively across vertebrates." [Andres Collazo, "Developmental Variation, Homology, and the Pharyngula Stage," 49 Systematic Biology 3, 9 (2000)]
2. Richardson et al: "...it is preceded by variation at earlier stages, including gastrulation and neurulation." [Michael K. Richardson et al., "There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development," 196 Anatomy and Embryology, 91, 105 (1997)]
3. Later in embryo development, the differences at the supposedly conserved "pharyngular" stage are even greater. Richardson also writes, "We find that embryos at the tailbud stage -- thought to correspond to a conserved stage -- show variations in form due to allometry, heterochrony, and differences in body plan and somite number ... Contrary to recent claims that all vertebrate embryos pass through a stage when they are the same size, we find a greater than 10-fold variation in greatest length at the tailbud stage ... The wide variation in morphology among vertebrate embryos is difficult to reconcile with the idea of a phylogenetically conserved tailbud stage. ... Our survey, however, does not support the second claim, and instead reveals considerable variability -- and evolutionary lability -- of the tailbud stage, the purported phylotypic stage of vertebrates."
According to these authorities, it sure sounds like among early embryos there are more than "mostly fairly trivial" differences "due to the different amounts of yolk in different vertebrate eggs."
While Matzke had the decency not to defend recapitulation theory, blogger Matt Young also responded. His approach, quite incredibly, was not to challenge Luskin's discussion of textbooks, but rather to defend a modified version of Haeckel's long-discredited ideas about recapitulation!
My colleague Paul Strode wrote a very clear and concise explanation of Ernst Haeckel's "ontogeny recapitulates phylogeny" law for our book Why Evolution Works (and Creationism Fails). In Chapter 11, Strode explains that Haeckel was wrong in thinking that embryos resemble the ancestral adult forms; rather, early embryos resemble the embryos of ancestral forms. In other words, Haeckel was on to something, but he didn't get it quite right. Strode explains further, "Recapitulation nevertheless provides helpful insight into evolutionary relationships and ancestry," and argues that von Baer's law is closer to the truth.
The base principle behind any variation of "recapitulation" is that higher taxa evolved by the addition of developmental stages to the end of the morphogenesis of lower taxa. Karl Ernst von Baer demonstrated that development is a process of progressive specialization towards the adult form. While early-stage embryos may well superficially resemble one another, this is because of their unspecialized form. The embryos progressively diverge as they become specialized. For example, von Baer could recognize, in the development of a chick, a stage at which it could be identified as a vertebrate, a later stage at which it could be identified as a bird, etc. Only later could it be identified as a particular type of bird. While there is a marked similarity among early vertebrate embryos, they are still distinctively vertebrate. They do not pass through a form at which it resembles an invertebrate.
I would ask the bloggers at Panda's Thumb to point to a single case in which evolutionary modifications of ontogeny have taken the form of addition of a new terminal phase to the previously terminal phase during ontogeny. It is all of the stages of ontogeny that are modified during the process of evolution. Indeed, many structures arise early in the development of higher taxa that are missing from the embryos of lower taxa. One example would be the placenta in mammals.
The matter becomes still more problematic when one considers instances of species that have similarities of adult form but radically differ in early forms. In order to account for this, one needs to postulate that the forms evolved in a convergent fashion. Then there are the tissues that arise in the opposite order from the sequence in which they are presumed to have evolved, one example being the development of teeth prior to the tongue (whereas the tongue is presumed to have evolved first).
As Jonathan Wells observes in Icons of Evolution:
If the implications of Darwin's theory for early vertebrate development were true, we would expect these five classes [bony fish, amphibian, reptile, bird and mammal] to be most similar as fertilized eggs; slight differences would appear during cleavage, and the classes would diverge even more during gastrulation. What we actually observe, however, is that the eggs of the five classes start out noticeably different from each other; the cleavage patterns in four of the five classes show some general similarities, but the pattern in mammals is radically different. In the gastrulation stage, a fish is very different from an amphibian, and both are very different from reptiles, birds, and mammals, which are somewhat similar to each other. Whatever pattern can be discerned here, it is certainly not a pattern in which the earliest stages are the most similar and later stages are more different.