The new field of "evo-devo"–an integration of evolutionary biology with our growing understanding of embryonic development–is an exciting a fruitful area of intense scientific research. A book purporting to "explore evolution" would do well to address this exciting field, yet the discussion in Explore Evolution is mired in disputes about what Darwin thought about embryos 150 years ago, and the legitimacy of illustrations by Ernst Haeckel 100 years ago. Current work that shows how the developmental process can evolve, and how that understanding deepens our understanding of the common ancestry of modern species, goes unmentioned.
Instead, Explore Evolution obsesses over whether certain illustrations of embryos from textbooks are or are not legitimate, and whether actual photographs of embryos confirm or disconfirm comments made by Darwin or by Haeckel. Modern evolutionary biology does not stand or fall on the views of people a century ago, but on the current evidence and research, research and evidence the book omits.
p. 66: "Darwin thought… similarities in … embryos revealed what the[ir] ancestors would have looked like."
This view that "ontogeny recapitulates phylogeny" was stated by Haeckel, not Darwin, and historians generally agree that Darwin did not accept that view. Darwin, like modern evolutionary biologists, preferred a much broader view advanced by von Baer: that characters acquired earlier in a species' evolution tend to develop earlier in an embryo.
Yolked into it: Removing yolk from photographs of embryos makes them look much more like Haeckel's drawings. p. 66: Haeckel … show[ed] that [vertebrate] embryos … were very similar during their earliest stages.
Historians have shown that Haeckel meant something different by "earliest stages" of an embryo than modern authors, but the book uses the confusing translation to create a straw man. Haeckel was aware of divergence in early stages of embryos, and like modern embryologists, was aware that these divergences are driven in part by factors like the amount of yolk, not by fundamentally different developmental processes.
p. 69: "[Haeckel's] pictures were faked and the facts were distorted."
Historians investigating Haeckel's drawings have shown that he used the best illustrations available at the time, and continued to update the illustrations in his books as better ones became available. Comparisons to Richardson's photographs are flawed because Richardson, unlike Haeckel, did not remove the yolk from all of his embryos. This distorts the shape and outline of the embryos and makes it difficult to compare lineages with the yolk removed and those without. As one prominent Haeckel biographer concludes: "fraud not proven."
p. 68: "In 1894, Adam Sedgwick…challenged Darwin's two claims…"
Sedgwick's essay is not a reply to Darwin, as shown by the title of the cited essay: "On the Law of Development commonly known as von Baer's Law." Darwin, who died 12 years before Sedgwick's essay was published, did accept von Baer's results and, like modern biologists, found them bourn out by his own experience. To attribute those laws to Darwin, or to claim Sedwick was responding to Darwin, is simply false.
p. 68: "chickens and ducks[] display specific differences very early in development."
Researchers find that, other than a small difference in the rate of development, early chicken and duck embryos are "nearly identical."
p. 68: Sedwick claims: "There is no stage of development in which the unaided eye would fail to distinguish [chicken and shark]."
Sedwick acknowledges that these embryos are "superficially" not similar, but notes "striking similarities" of embryos, shared traits "which the adults do not exhibit." Rather than relying on a single paper from over a century ago, a truly inquiry-based book would lay out those structures and encourage students to explore their evolutionary implications.
p. 69: "This error [in Haeckel's drawings] remains in many modern high school and college biology textbooks."
Most textbooks in use today do not show Haeckel's drawings. They either use photographs, or redraw the embryos to correct errors. Evolutionary biologists' agreement about evolution's impact on development is hardly driven by a reliance on century-old illustrations, but on their experience with actual embryos in the lab. Explore Evolution gives students no understanding of that research.
p. 71: "To explain … adaptations in embryos, … evolutionary biologists invoke 'macromutations'"
This is simply false. Richard Goldschmidt's ideas were never widely accepted when he proposed them in the 1940s, and play no role whatsoever in modern evolutionary developmental biology. Explore Evolution later acknowledges this rejection in a passage plagiarized from a creationist website.
p. 70: "Darwinists have effectively made it impossible to challenge the theory with counterevidence"
Embryological evidence can and does challenge specific hypotheses of common ancestry. They also know that explanations of embryonic morphology must incorporate our knowledge of common ancestry and the species' immediate adaptive history. Finding that embryos differ because of adaptions specific to different environments is a prediction of evolution, not a falsification of it.
Major Flaws:
History of embryology: Rather than focusing on modern evolutionary developmental biology, this chapter rehashes arguments from the 19th century. Darwin's own views are serially misrepresented. A single essay from 1894 is transformed into a scientific attack on Darwin and evolution, despite the original author's narrower focus and the subsequent research which has invalidated his claims. Errors in a century-old illustration are dolled up as a evidence that the illustrations, and all of evolutionary developmental biology since, are frauds. This despite extensive research by biologists and historians showing that the illustrations were the best available at the time, and that the evolution of development is well-documented.
Philosophy: The authors misrepresent science by comparing the scientific process to how a jury operates. Students cannot be a jury if they are not given adequate background in the modern state of the field. This book does not give that background, and neither do many high school biology texts. Students are thus not qualified to sit in judgment on evolution, and this book does not give them the investigative tools they would need to be qualified. The authors also misrepresent the way evolutionary developmental biology works by pretending that the only force operating on embryos ought to be common ancestry. A falsification of evolutionary biology must address its totality, including both the constraints of common ancestry and divergences driven by other evolutionary mechanisms.
Embryology: Remarkably, this chapter simply does not discuss modern evolutionary developmental biology. While this means it is devoid of factual errors on that point, it also means that the chapter's title is inaccurate, and the content uninformative for students.