Por falar em "relógio molecular", alguém tem a hora certa?

quinta-feira, março 22, 2007

Já bloguei aqui sobre as dificuldades de corroboração dos "relógios moleculares". E a quantas anda o fato, Fato, FATO da teoria geral da evolução sob este campo científico de pesquisa? Na maior confusão, gente!

Os autores deste trabalho afirmam que a área ainda espera trabalhos "com algum nervosismo, considerando-se que nós suspeitamos [SIC ULTRA PLUS 1] que eles possam revelar que muitos estudos passados colocaram muita confiança em simples análises de relógio molecular [SIC ULTRA PLUS 2], e que suas conclusões devem por isso serem revisitadas [SIC ULTRA PLUS 3]."

Pulquério e Nichols são cientistas evolucionistas e não são simpatizantes do Design Inteligente.

Leiam e depois me digam o que o MEC/SEMTEC/PNLEM deve fazer com a abordagem dos "relógios moleculares" em nossos livros-texto de Biologia do ensino médio.


Dates from the molecular clock: how wrong can we be?
Mário J.F. Pulquério and Richard A. Nichols
Trends in Ecology & Evolution, Volume 22, Issue 4 , April 2007, Pages 180-184

Large discrepancies have been found in dates of evolutionary events obtained using the molecular clock. Twofold differences have been reported between the dates estimated from molecular data and those from the fossil record; furthermore, different molecular methods can give dates that differ 20-fold. New software attempts to incorporate appropriate allowances for this uncertainty into the calculation of the accuracy of date estimates. Here, we propose that these innovations represent welcome progress towards obtaining reliable dates from the molecular clock, but warn that they are currently unproven, given that the causes and pattern of the discrepancies are the subject of ongoing research. This research implies that many previous studies, even some of those using recently developed methods, might have placed too much confidence in their date estimates, and their conclusions might need to be revised.

Molecular clocks and substitution rates
This article was motivated by the experience of a colleague who estimated the time since the separation of two taxa from the number of substitutions that had accumulated between their DNA sequences; in other words, he was using the molecular clock. On submitting the work for publication, he was startled to be advised by a referee that his estimate was wrong by a factor of ten. The argument concerned the tick rate of the molecular clock; that is, the rate of accumulation of substitutions per million years. How could the scientific community hold two such contradictory opinions simultaneously?

Our colleague's original calculation was based on a rate estimated from inter-species comparisons, whereas the referee preferred a rate obtained from a pedigree study. Later, we address why such discrepancies exist between estimates of substitution rates. The central lesson for this article, however, is the realization that reasonable scientists working with the molecular clock can be using estimates that are so different. If neither the fast estimate nor the slow estimate were self-evidently wrong, it suggests that it is difficult to validate them using our knowledge of biogeography and the fossil record. Methods are currently being devised that deal with uncertainty about the variation in the rate and about the timing of the calibration points. Here, we consider the prospects of obtaining date estimates that take account of these issues when constructing their standard errors (or analogous measures of uncertainty): is there likely to be so much uncertainty about molecular dating that the estimates are no longer useful? We fear that, for many current studies, the answer is yes. However, it might be possible to gain extra precision using recently developed methods. The degree of improvement depends on the pattern of variation in the rate of molecular evolution and the availability of calibration points. We currently do not know enough to be confident in the prospects of these new methods, and some initial results are discouraging.

We await the more rigorous type of assessment with some nervousness, given that we suspect they might reveal that many past studies placed too much confidence in simple molecular clock analyses, and that their conclusions should thus be revisited.