Thursday, March 23, 2006
"Natural selection didn't come up with the best design; it just made the best of what was available.
The 'hand' of the Giant Panda (Ailuropoda melanoleuca) has six 'digits'. In processing its staple diet of bamboo, the Giant Panda drags the stalks between its sixth 'digit' and its paw to strip off the leaves. This sixth 'digit' or 'thumb' is a curious device. It is not, as one might expect, simply an additional finger of the type sometimes produced through congenital defect. In fact, the Panda's 'thumb' is not a real digit at all, but a greatly enlarged and specialised wrist bone called the radial sesamoid that lacks much of the flexibility of true digits. Why, one might wonder, did the Panda evolve a 'thumb' out of a wrist bone when it already had a 'real' thumb?
Evolution can be a fickle and opportunistic process. Often the end result can appear surprisingly imperfect, even 'sloppy'. The bottom line is that, at any juncture in its evolution, a species is constrained by accidents of history. When 'fashioning' new adaptations, natural selection can only work with what it's got. Because the real thumb (the first digit) of the Giant Panda was already modified and in use for another task (for walking on), evolution could only work with what was available, in this case, a radial sesamoid bone. It is this very fact of imperfection that underpins the reality of natural selection."
Wednesday, March 22, 2006
[A. Richard Palmer, PNAS, Dec '96]
From symmetry to asymmetry: Phylogenetic patterns of asymmetry variation in animals and their evolutionary significance
Phylogenetic analyses of asymmetry variation offer a powerful tool for exploring the interplay between ontogeny and evolution because (i) conspicuous asymmetries exist in many higher metazoans with widely varying modes of development, (ii) patterns of bilateral variation within species may identify genetically and environmentally triggered asymmetries, and (iii) asymmetries arising at different times during development may be more sensitive to internal cytoplasmic inhomogeneities compared to external environmental stimuli. Using four broadly comparable asymmetry states (symmetry, antisymmetry, dextral, and sinistral), and two stages at which asymmetry appears developmentally (larval and postlarval), I evaluated relations between ontogenetic and phylogenetic patterns of asymmetry variation. Among 140 inferred phylogenetic transitions between asymmetry states, recorded from 11 classes in five phyla, directional asymmetry (dextral or sinistral) evolved directly from symmetrical ancestors proportionally more frequently among larval asymmetries. In contrast, antisymmetry, either as an end state or as a transitional stage preceding directional asymmetry, was confined primarily to postlarval asymmetries. The ontogenetic origin of asymmetry thus significantly influences its subsequent evolution. Furthermore, because antisymmetry typically signals an environmentally triggered asymmetry, the phylogenetic transition from antisymmetry to directional asymmetry suggests that many cases of laterally fixed asymmetries evolved via genetic assimilation.
Sunday, March 19, 2006
Annual Review of Genetics
Vol. 39: 9-22 (Volume publication date December 2005)
The Genetics of Hearing and Balance in Zebrafish
Oregon Hearing Research Center and Vollum Institute, Oregon Health and Science University, Portland, Oregon 97239
The zebrafish is an excellent model system for studying the molecular basis of inner ear development and function. The eggs develop ex utero and the ear is transparent for the first few weeks of life. Forward genetic screens and antisense technology have helped to elucidate the signaling pathways and molecules required for inner ear development and function. This review addresses the most recent advances in our understanding of how the ear forms and discusses the molecules in hair cells that are essential for sensing sound and movement in the zebrafish.
[This item has been included because a post on the lateral line and the proposed internal evolutionary mechanism will eventually appear in the Personal Posts category of this weblog]