>>CLARIFICATIONS FOR THE COMMON MAN:
>>>As George has pointed out, the likelihood of returning to exactly
the same state as an ancestor by random morphological chance is extremely
low to impossible.
As long as we are splitting hairs...
Firstly, I have to agree with George. Regardless of the difficulties of identifying the basal character state, a true "reversal" sensu-stricto would be nearly impossible. Given that an organism's phenotype is a result of an interaction between genotype and environment, an exact match is- even over significant geological time spans- VERY unlikely. John does (I assume) agree with this position. We are having a semantics debate. If a character cannot be a point in morphospace- if it is to be inclusive of a taxa/clade - it must at least be within a discrete range. This is how morphological analysis works.
One question this debate does raise... is there such a thing as genetic reversal? Does anyone remember a presentation at the 1996 Society of Ichthyology and Herpetolgy that involved a population of land-locked, deep-bodied sunfish that evolved a "new" body design within a demonstrated EXTREMELY small generation interval (6-8 possible breeding seasons due to higher water temps from the nearby powerplant). Due to the higher temps, the lake itself became much less inhabited by plants, altering "FISH STRATEGY" which led the sunfish to "evolve" a much-more streamlined body design (pike-like). This presentation raised several important , and relevant questions! (1) did the fish, due to selection pressure, "reverse" to an ancestral condition- nature selecting genotypic/phenotypic material that the animals possessed through genetic heritage, or (2) did the fish, via sexual recombination, form radically new genotypes that converged on an ancient body plan?
Also, please note that secondary flightless, or for that manner ANY morphological trait, would have it's origin in BEHAVIOR. Any Dino-Birds that later became morphologically flightless would have spent ever-increasing amounts of time on the ground FOR WHATEVER REASONS.
But the question still remains- was (if BCF is a valid theory) this new flightlessness based on convergence or reversal? While common sense dictates that selection pressures decreases the amount of genetic diversity of a population, others have shown that this is not necessarily true. Indeed, variance has been shown to increase following an episode of "gamete disequilibrium or other novel opportunity for recombinational episodes". QUANTIFIABLE variance, following a selectional pressure, has been shown to result not simply from loss of alleles at some loci via random drift. Rather, selectional stresses may temporarily favor the release of "cryptic" genetic variability. (see Speciation and The Recognition Concept- ch.Fitness and the Sexual Environment- editors Lambert and Spencer).
What this implies is that neither of the terms , "reversal" or "convergence", fully describe a return to an ancestral condition- since it would appear that via meiosis and recombination, new genotypes are created from old. This dino-bird link may prove enlightening for not just us paleonuts. As more species are discovered, and both the animals and stratigraphy are described, we should be able to discern between punctuated event(s) and gradual event(s). Weather these events are irregular or opportunistic (Caroll 1988) this information should be invaluable to the conservation biology and ecology.