Creationists often carp that evolutionary theory lacks predictive power. Ask a biologist they say how human evolution will proceed over the next 5 million years and there will be a shake of the head and a can't say for sure for an answer. If you cannot predict the future evolution of a species then evolutionary theory is not a true science. This line of argument is often persuasively sold by creationists to the layperson not familiar with the scientific method. It is true that it is not possible to forecast what specific evolutionary trajectory a particular lineage will take since there is contingency built into evolution. The process is very sensitive to starting conditions. Small initial variations between sister species may lead to large differences over time but on the other hand they may not. Unlike physical laws, biological "laws" are not universal in time and space. There are exceptions to just about every biological patterns and processes and this makes predicting long term evolutionary trends difficult. Moreover external events such as climatic shifts may push evolution this way or the other, disasters such as meteorite impacts may obliterate well adapted species and reorganize ecosystems. But in science prediction does not always mean foretelling future events. Especially in evolutionary theory a large part of prediction making deals with reconstructing events of the past, retro-dictions if you like. Which means predicting that certain biological patterns and properties will exist in nature given expectation of theory and complimentary data from another area of research. Based on anatomical comparisons between Dinosaurs and other vertebrates it is the consensus view today that birds are the closest living relatives of Dinosaurs. If gross morphology informs us such then so should individual molecules. A reasonable prediction would be that there would be greater DNA or protein sequence similarity between Dinosaurs and birds than between Dinosaurs and any other living vertebrate. And that is exactly the pattern that has been found in ancient proteins recovered from a 68 million year old T. rex bone. The analysis grouped T. rex closer to chickens and ostriches than to lizards and crocodiles. You can listen to the details in this NPR piece. This is how evolutionary theory gets strengthened. Predictions such as these are made every day in labs and tested. Lines of evidence from different fields converging on the same conclusion. Nothing earth shaking, no paradigm shifts or revolutions, just day to day science at work although in this case retrieving protein from a 68 million year old fossil was something special. But nearly 150 years of testing and validating predictions such as these is why evolution is such a strong theory.
Given the frequent coverage by media on dinosaurs and their relationship to birds I don't at all understand how Science Daily could have got it so wrong in another piece on dinosaurs. This one dealt with the absence of heat generating brown fat in lizards and birds. This is tissue that generates heat in mammals and a comparative molecular analysis suggests that the gene UCP1 responsible of this function arose in the common ancestor of mammals and birds. This is what the report said:
The ability to produce brown fat evolved in a common ancestor of birds and mammals, but the ability to generate heat was lost in the group that gave rise to birds and lizards after it separated from the mammalian lineage (the researchers found the lizard genome similarly lacks a UCP1 gene). This strongly implies that dinosaurs, which diverged from birds even later than lizards, also lacked brown fat.
Science Daily has got the evolution part all mixed up. The common ancestor of birds and mammals is the common ancestor of dinosaurs, lizards and mammals. See figure of vertebrate phylogeny below. Red indicates presence of UCP1 and black indicates its absence. Question mark indicate uncertainty on when exactly UCP1 was lost.
Source: BioMed Central
Birds are not ancestral to dinosaurs as the report implies but birds are highly derived dinosaurs. The best evidence suggests that they evolved from within the maniraptors, a group of small carnivorous dinosaurs. Birds lack UCP1 because dinosaurs lacked it and they (birds) inherited that condition and not the other way round.
Update: Just saw this in DNA. The Indian media is never far behind in copying the mistakes of others. DNA (April 28 08) pasting the same report came up with its own headline: Dinosaurs died as they failed to generate heat. The print edition went further and announced a new theory: New Theory on Dino's Extinction. The original research article did not approach the topic of extinction with a single condition answer and for good reason. End Cretaceous environmental changes must have affected organisms in various ways. Dinosaurs were varied in their size , shapes, diet and thermo regulation. It's unlikely the entire group would be affected in the same way by ecological changes. Absence of UCP1 and a lack of heat generating brown fat was a shared condition in lizards, non-avian dinosaurs and avian dinosaurs (birds). Only the non-avian dinosaurs went extinct. Any theory of dinosaur extinction has to explain this specificity since despite lacking UCP1 many ancient lizard and avian dinosaur (bird) species survived and proliferated. Many non-avian dinosaurs are thought to have had higher rates of metabolism than lizards and other reptiles so the answer to why dinosaurs went extinct has to account for that too.
No comments:
Post a Comment