Evolution: A Theory in Crisis Revisited (Part Three of Three)
Michael Denton
THIS IS the third, and final, part of a three-part essay in which I defend Evolution: A Theory in Crisis (Evolution), in the light of scientific advances and discoveries of the past thirty years. 1 In Evolution, I argued that the major, taxa-defining innovations in the history of life were not derived from ancestral forms by functional intermediates.
In Part One of this essay, I considered in detail the origin of the enucleate red cell; in Part Two, the tetrapod limb, the feather, and flowering plants. In Part Three, I continue an argument that proceeds by the accretion of examples.
The Wings of the Bat
APPEARING IN the fossil record fifty million years ago, the first bats had modern wings and were as completely adapted to flight as any modern bat. 2 It is by no means clear how this took place. The issue is an old one: of what use is a partially developed wing? Witness Glenn Jepson:
It has long been debated whether the processes and mechanisms responsible for phenotypic variation within a population or between closely related populations can be extrapolated to explain … the generation of novel structures. 3
Push and pull, the staples of political science, are of little use in biology. Jepson again:
No one has successfully proposed any kind of selection pressure that would be effective in the change from one niche to the other; whether the bridging group would be pulled by advantages in the new milieu or pushed by disadvantages in the old. 4
Whether by pull or push, the evolution of the bat “required many molecular changes to dramatically alter morphology from a limb to a wing.” 5
Rewiring the gene circuits required for bat flight, recent studies show, is immensely complex:
Overall, comparisons of gene expression profiles between digit morphologies and limbs identified hundreds of differentially expressed genes. Several interesting patterns have emerged from this data. Specifically, we highlight 21 genes likely related to wing formation or to morphological and functional similarities between thumb and hindlimb digits … First, we found 14 genes that are likely associated with digit elongation in bats—two Tbx genes (Tbx3 and Tbx15), five genes from the BMP pathway (Bmp3, Rgmb, Smad1, Smad4 and Nog), four Homeobox genes (Hoxd8, Hoxd9, Satb1 and Hoxa1) and three other genes (Twist1, Tmeff2 and Enpp2) related to either digit malformation or cell proliferation. Next, we identified seven genes (Tbx4, Pitx2, Acta1, Tnnc2, Atp2a1, Hrc and Myoz1) that are likely associated with the morphological and functional similarities between the thumb and hindlimb digits. 6
Karen Sears has argued that “a simple change in a single developmental pathway” might lead to dramatically different morphologies in the bat. 7 Other researchers have also speculated that major morphological transitions may be achieved by minor genetic changes. 8
In light of the facts, this is a view best described as primitive. Morphological change requires extensive genetic rewriting.
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Read more here/Leia mais aqui: Inference
Read more here/Leia mais aqui: Inference
