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Some other major transitions that Kirschner and Gerhart concede remain unexplained are the origin of limbs in the first land vertebrates, the origin of neural crest cells that sculpt the heads and nervous systems of vertebrates, and the origin of the neocortex in vertebrate brains. "The origin of these processes and properties would seem to be the primary events of evolution, requiring high novelty," but the authors admit they cannot explain them. So, what does their theory explain?
Their theory purports to explain how organisms that share a given set of conserved core processes—say, land vertebrates—can diversify into a wide variety of forms. Kirschner and Gerhart argue that although the core processes themselves are conserved over evolutionary time, and invariant ("constrained") in the process of embryo development, they are interconnected by "exploratory behaviors" and "weak linkages" that are "compartmentalized" in the embryo. These three characteristics "deconstrain" development and facilitate the emergence of new variations. This "facilitated variation," they argue, makes it "easy" for organisms to evolve.
For example, when a limb develops in the embryo of a land vertebrate, nerve cells (core processes) in the spinal cord send out extensions in many directions (exploratory behavior), but only those that encounter signals from targets (more core processes) in the developing limb survive and establish functioning connections (weak linkages). If an extra embryonic limb is artificially grafted onto the side of a chick embryo, the added targets cause the stabilization and connection of more nerve extensions, and the chick develops five limbs instead of four. The core processes are the nerve cells and the targets; the exploratory behavior is the sending out of extensions; the weak linkages are the signals; and compartmentalization enables forelimbs and hindlimbs to develop differently.
Kirschner and Gerhart propose that slight changes in the regulation of these mechanisms would make it "easy" to rearrange core processes in the course of evolution. "Throughout biology," they write, "individual core processes are constructed so that new linkages can easily be forged and broken… . The implications for evolution are powerful, for if complex development is elicited by simple signals, then changes in complex development may be achieved by changing the amount or location of these simple signals, rather than by changing a highly integrated and complex process."
The authors compare exploratory behavior in an embryo to random variation in a species, and they compare stabilization by target signals to natural selection. But variation and selection, whether in the laboratory, on the farm, or in the wild, have never been observed to produce a new species. Variation and selection produce only minor changes within existing species.
Similarly, exploratory behavior and weak linkage have never been observed to change the species of an embryo. Any embryo that completes its development becomes the organism it was programmed to become. Exploratory behavior and weak linkage notwithstanding, the endpoint of embryo development appears to be predetermined—by some factor or factors missing from Kirschner and Gerhart's equation.
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