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Darwin on Dissimilarities
Explore Evolution focuses excessively on the details of what Darwin argued 150 years ago, rather than informing students about the dynamic field of evolutionary developmental biology. Whether or not Darwin argued that dissimilarities in early development do not cause a problem for evolution is less important than helping students understand how modern scientists view these issues. Explore Evolution fails to explain that the amount of yolk in an egg has adaptive value and is responsible for differences in embryogenesis. Instead of explaining a concrete example such as this, Explore Evolution makes a vague reference to Richard Goldschmidt's work from the 1930's and 1940's on macromutations – a hypothesis rejected by modern biologists.
From Explore Evolution:
Darwin was aware of these dissimilarities, but he argued that they do not disprove Common Descent. As Darwin explained, some groups of embryos have been "so greatly modified [by adaptations]* as no longer to be recognized."
Darwin did argue that adaptations to embryonic life would result in the dissimilarities of embryos, as Jerry Coyne observes:
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.
The early cleavage embryos of humans and chickens are quite different due to constraints imposed by the large yolk of chickens. The amount of yolk in an egg varies depending on how long the embryo will rely on the yolk for energy and nutrients. In amniotic mammals such as humans, relatively little yolk is needed because the developing embryo is able to get continual nutrition from maternal sources. Other organisms, such as chickens, have a large and yolk-rich egg as they lack constant maternal nutrition and lack feeding larval stages. Because yolk does not divide as the cells in the egg divide, the pattern of embryonic cell cleavage in chickens is necessarily different from the pattern in humans in order to accommodate their large amount of yolk.
Gastrulation is the process by which the embryo is converted from a single layer of cells to three different layers by a series of coordinated cellular movements. How these cell movements occur is dependent upon the earlier cleavage patterns. Therefore, embryos can differ significantly in their morphology at the gastrula stage. PZ Myers explains this relationship between yolk, cleavage patterns and gastrulation here..
Instead of explaining a concrete example of how the amount of yolk is an adaptation to embryonic life and how it affects the early development, students are referred to Richard Goldschmidt's work from the 1930's and 1940's.
*To explain what might cause these adaptations in embryos, some evolutionary biologists invoke "macromutations," large-scale changes in form that occur in one generation. One such biologist, the late University of California at Berkeley geneticist Richard Goldschmidt, believed that such macromutations could produce what he called "hopeful monsters."
Notably, Richard Goldschmidt's macromutation hypothesis is not included in modern evolutionary biology. As Michael Dietrich notes:
Richard Goldschmidt is remembered today as one of the most controversial biologists of the twentieth century. Although his work on sex determination and physiological genetics earned him accolades from his peers, his rejection of the classical gene and his unpopular theories about evolution significantly damaged his scientific reputation.
The mention of macromutations to explain adaptations to embryonic life by Explore Evolution is yet another failure to use "current evidence and arguments for and against the key ideas of modern Darwinian theory." The book is too focused on trying to attack Darwin to properly examine modern biology.