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Ontogeny & Phylogeny
There is no question that the study of development played an important role in Charles Darwin's thinking on evolution, and that it continues to play an important role in modern evolutionary biology. Ernst Haeckel's early suggestion that ontogeny (development) recapitulates phylogeny (evolution) was far less influential and is rejected by modern biologists. Alternative views on the relationship between development and evolution include Baer's Laws, which simply state that general characteristics of the group to which an organism belongs often develop earlier than special characters of a species, and that organisms tend to become more divergent through their development. This formulation remains valid today, and (unlike Haeckel's proposal) has a straightforward relationship with evolutionary mechanisms. Despite these simple facts, Explore Evolution invests substantial effort trying to tie Darwin to Haeckel's views, rather than exploring the modern state of evolutionary developmental biology. In doing so, the book mangles both history and biology.
Intelligent design proponents have a tradition of flogging embryology in order to attack common descent (Wells 2000, 2003, 2005). Explore Evolution continues this tradition of misrepresenting embryology and evolution. The first misrepresentation is the claim that Darwin accepted Haeckel's Biogenetic Law. In this case, Explore Evolution presents a minority position among scholars who have studied this question. This claim is the first move by Explore Evolution to link Darwin and Haeckel as closely as possible, so as to tarnish Darwin's arguments for common descent with the controversy about Haeckel's embryos. It is notable that the only embryological data Explore Evolution offers students in support of common descent is a modified diagram of Haeckel's embryos from 1894. In protecting their anti-evolution viewpoint, the authors omit recent research showing evolutionary conservation of the genetic pathways regulating animal development (e.g., Carroll et al. 2005, Davidson, 2005).
Intelligent design proponents have attacked the idea of common descent through promoting a misunderstanding of embryology and the new field of evolutionary developmental biology (evo-devo):
There is a whole stable of intelligent design creationist writers associated with the Discovery Institute, and we will see more slick books of bogus science produced to influence the teaching of biology, and even federal funding of research. Evo-devo data have become a part of the creationist rhetorical weaponry, and as evo-devo grows in prominence, the problem will grow in severity.
From Explore Evolution:
Darwin noticed certain similarities in the embryos of vertebrate animals, similarities he thought were especially great during the embryo's earliest stages of development.
This short statement in Explore Evolution makes two significant mistakes, one of omission and one of commission. Darwin considered that similarity of early embryos provided very strong support for common descent. However, he never personally examined vertebrate embryos. Instead he relied upon Karl von Baer’s observation in 1828 that early vertebrate embryos were more similar to each other than when fully developed. Failing to distinguish von Baer from Haeckel later allows Explore Evolution to present Adam Sedgwick's erroneous criticism of Baer's Laws as if they were criticism of Haeckel.
In addition, Explore Evolution falsely asserts Darwin thought the similarities between embryos were greater at the earliest stages of development. This begins a relentless pattern of suggesting that common descent and Haeckel's Biogenetic Law require that the earliest stages of animal development are most similar. However, as Jerry Coyne notes, that is not what Darwin actually thought, embryos at the earliest stages of development can vary signficantly from one another depending upon the amount of yolk in their eggs.
Darwin himself noted that embryos must adapt to the conditions of their existence, and the earliest stages of vertebrate embryos show adaptation to widely varying amounts of yolk in their eggs.
While Darwin accepted von Baer's Law, it is much less clear whether Darwin accepted Haeckel's Biogenetic Law, proposed in 1866, which claimed that embryonic development recapitulates the adult stages of their ancestors ("ontogeny recapitulates phylogeny"). Nonetheless, Explore Evolution claims:
Darwin thought that the observable similarities in different embryos revealed what the ancestors to these organisms would have looked like.
This claim by Explore Evolution contradicts the majority view of prominent Darwin scholars (including Ernst Mayr, Stephen Jay Gould, David Hull, and Peter Bowler) who have argued that Darwin did not accept the Biogenetic Law.
most authors (including Darwin) rejected the claim that ontogeny is the recapitulation of the adult stages of the ancestors.
Darwin saw that ancestral groups in an established community of descent would differ least in their adult form from the embryonic state common to all members of the community. The gill slits of the human fetus represent no ancestral adult fish; we see no repetition of adult stages, no recapitulation.
belief that ontogeny (individual growth) recapitulates phylogeny (the history of the type) thus owes little to Darwinism and is more characteristic of the non-Darwinian, or developmental, view of evolution.
Not all historians of science who have studied embryology and evolution are in agreement that Darwin was not a recapitulationist (Roberts, 1990). However, this fails to warrant the claim by Explore Evolution that Darwin accepted Haeckel's Biogenetic Law.