Evolution of Swim Bladders
While the ID folks continue to wring their hands over the impossibility of explaining the origins of complex biological systems, real scientists are doing the hard work of unravelling their origin. Consider this article (only available by subscription, alas) from the March 18 issue of Science Magazine:
Scuba divers wear air-filled dive vests to move up and down in the water column. Researchers have now used the fish family tree to piece together how the piscine equivalent, an internal air sac called a swim bladder, evolved a complex capillary network and special hemoglobin molecule to inflate it with oxygen. Moreover, according to the proposal presented on page 1752 by Michael Berenbrink of the University of Liverpool, United Kingdom, and his colleagues, these innovations helped fish expand their species diversity. “The scenario developed presents a fascinating picture of the evolution and radiation of fish,” says Bernd Pelster, an animal physiologist at the University of Innsbruck, Austria.
Herring and other fish with primitive swim bladders must surface and gulp air to keep their bladders full and their bodies buoyant. The more sophisticated species use oxygen in the blood, an advance that freed them from their air tether and allowed for the expansion into the deep ocean. These species depend upon a network of blood vessels to concentrate oxygen in their swim bladder. However, high oxygen concentrations usually inhibit the release of oxygen from the blood. To get around this problem, these fish have a special Root-effect hemoglobin, a form of the protein that releases its oxygen cargo even when concentrations of the gas are high.
This new hemoglobin evolved before the swim bladder's capillary network, according to Berenbrink, a comparative animal physiologist. He and his Liverpool colleague Andrew Cossins reconstructed the history of the self-contained swim bladder by looking for its prerequisite components, such as the hemoglobin. The researchers studied species, ranging from sharks to dolphinfish, that represented the different stages of fish evolution.
According to the new study, the Root-effect hemoglobin evolved once in primitive fish. Although the molecules function at high oxygen concentrations in sharks, lungfishes, and even tetrapods, they are most efficient at releasing oxygen in those conditions in codfish and other modern fish. Next came a capillary network that supplied oxygen to fish eyes, allowing them to see better. This also evolved just once, about 250 million years ago, and depended upon the Root-effect hemoglobin. From that point, the hemoglobin was essential to fish.
About 100 million years later, a similar capillary network, this one supplying oxygen to the swim bladder, finally began showing up. This network arose four times in different fish groups, the researchers found.
“It's one of the few examples of our understanding of the evolution of a complex organ from simpler parts,” says Albert Bennett, an evolutionary physiologist at the University of California, Irvine. “They have done an excellent job of teasing apart what happened when.”
Of course, the accompanying technical article provides many of the details.
As a practical matter it is very difficult to unravel the precise evoutionary trajectory taken by the precursors to a modern, complex system. The individual steps of such a trajectory represent isolated events lost to deep time. We are constrained by our lack of evidence. Occasionally we have fossils to help guide us (as with the evolution of the mammalian inner ear from jaw bones in ancient reptiles), but most of the time we have only whatever inferences we can draw from modern species.
But that does not mean we must wallow in ignorance. As described in the excerpt above, what we can do is look for the individual parts of the system in closely related species. In other words, we mgiht try cataloging the parts of the system and investigating which modern species have which parts. If we find that the patterns of appearances and disappearances of the relevant parts are consistent with phylogenetic trees constructed from other data, we can assert with some confidence the sequence in which the parts of the system formed.
And that is what the researchers described in the excerpt did. As they write in their research article (subscription required):
By taking advantage of the wide divergence of fishes and by integrating data from all levels of organization, we reconstruct on a vertebrate phylogeny the likely sequence of evolutionary events leading to the ability to secrete O2. We support this analysis by identifying consistent patterns of secondary losses of several of these components in specific clades of advanced fishes, thereby characterizing factors that affect their maintenance.
In other words, they found that the patterns of appearances and disappearances of the parts of the swim bladder system was entirely consistent with the phylogenetic trees compiled from other data. If the swim bladder was not the product of evolution, there would be no reason to expect to find such patterns. Indeed, the only reason the researchers thought to look for the patterns they found was that they started from the hypothesis that the swim bladder had evolved gradually.
Of course, the ID folks will be quick to tear into this. They will blather about just-so stories and guess-work, they will claim that the parts of the sequence the researchers identified are themselves highly complex, and they will boast about their high evidentiary standards relative to those muddle-headed evolutionists. And they will be wrong on every count.
What this work shows is that yet another complex system reveals clear indications not of being a pristine creation from nothing, but of being the end result of a gradual evolutionary process. If the data had been slightly different from what it it is, the researcher's theory would not be tenable. Surely even the ID folks would have to concede that this data makes the hypothesis that the swim bladder evolved more likely than it was before this work.
One final thing. Creationists of all sorts have always been schizophrenic on this point. When they are trying to make the case for the direct action of an intelligent agent in natural history they claim that no one has the faintest idea how this or that complex system could have evolved naturally. But when a scientist comes along and says the clear evidence idicates the system went through phases X, Y and Z during its evolution, the creo's turn around and say it's easy to tell a story about how something evolved.
But the fact is, it's not easy at all. Hypotheses about how complex systems evolved are hard to test directly, but they still must be consistent with the large amount of data that's available from modern species. It seems that time after time the data comes out just the way it ought for evolution to be a viable hypothesis.
The ID folks will doubtless continue to fold their arms and shake their heads. With each passing day the sterility of their enterprise becomes a little more obvious. Meanwhile, evolution continues to be a reliable guide for scientists who actually care about understanding the natural world.