Two major issues:
I. Origin of developmental trajectories
Irreducible complexity?The origin of developmental trajectories poses a challenge to neo-Darwinism. It would appear that developmental trajectories cannot be built step-by-step, because a complete trajectory must be in place in order to result in a viable reproducing organism. In other words, natural selection cannot act unless oganisms are sufficiently developed that they are able to reproduce. For complex multicellular animals, a very complex trajectory is required to acheive a reproducing adult. Where do such complex programs come from? In developmental time, the information is present at the start, put in by the mother. In evolutionary time, such an abrupt front-loading of information would amount to intelligent design.
II. Variation in developmental trajectories
To change body plans, mutations must occur at the point in development where body plans are put into place. But this often occurs right at the beginning. Changes in body plan cannot be accomplished through the accumulation of late-acting mutations. Arthur points out that neo-Darwinism does not take into account the structure of development. Microevolutionary changes, which involve mutations of genes that act rather late in developmental time, cannot be extrapolated to account for the origin of new phyla.
from Wallace Arthur, "The Origin of Animal Body Plans"
from Wallace Arthur, "The Origin of Animal Body Plans"
"This appoach has important consequences for the neo-Darwinian view that major morphological differences between organisms from different higher taxa have been produced through the gradual accumulation of very small changes. Specifically, while in a non-developmental approach it seems plausible that many small changes can indeed accumulate to give a larger one, in a developmentally explicit approach it is clear that many late changes can not accumulate to give an early one. Thus, if taxonomically distant organisms differ right back to their early embyogenesis, as is often the case, the mutations involved in their evolutionary divergence did not involve the same genes as those involved in the typical speciation event, where usually the early embryogeneses of the daughter species concerned are virtually identical."Yet mutations that act early in development are almost always lethal. The only counter example cited by Arthur is a class of mutations that changes the direction of coiling in snails (chirality in gastropods).Wallace Arthur, The Origin of Animal Body Plans, pg 22.
"The variations we see are not the ones Darwin's theory needs, and the ones it needs we do not see."Paul Nelson, lecture at the "Nature of nature" conference, Baylor University, 1999.
Those genes "that are obviously variable within natural populations do not seem to lie at the base of the many major adaptive changes", while those that "seemingly do constitute the foundation of many, if not most, major adaptive changes apparently are not variable within natural populations".John F. McDonald, "The Molecular Basis of Adaptation: A Critical Review of Relevant Ideas and Observations," Annual Review of Ecology and Systematics 14 (1983):
77-102; pp. 92-93.
from Wallace Arthur, "The Origin of Animal Body Plans"
Three views of the role of development in evolutionary
change
from Wallace Arthur, "The Origin of Animal Body Plans"
Arthur's view:The more complex the organism is the harder for individual mutational changes to produce a coordinated effect that stands a chance of being beneficial. This is consistent with the notion of big experiments early and then progressively more restricted modifications.
For more on this subject see the following paper by Paul Nelson:
http://www.iscid.org/nelsonchat.pdf
and discussion surrounding it: