Biology does not have laws. Natural Selection is a Principle. Evolution is a Theory. But the closest biology comes to having a law is scaling, yet there are so many exceptions to the rule, how lawful really is it?
Now, a paper came out claiming a new law: Unified Physics Theory Explains Animals' Running, Flying And Swimming, and it is mighty interesting:
The researchers show that so-called "constructal theory" can explain basic characteristics of locomotion for every creature -- how fast they get from one place to another and how rapidly and forcefully they step, flap or paddle in relation to their mass. Constructal theory is a powerful analytical approach to describing movement, or flows, in nature.
They said their findings have important implications for understanding factors that guide evolution by suggesting that many important functional characteristics of animal shape and locomotion are predictable from physics.
This is reminiscent of D'Arcy Thompson, except that he applied physics to the problem of development of morphology and not to biomechanics of behavior.
“From simple physics, based only on gravity, density and mass, you can explain within an order of magnitude many features of flying, swimming and running,” added James Marden, professor of biology at Penn State. “It doesn’t matter whether the animal has eight legs, four legs, two, even if it swims with no legs.”But this is the coolest part! Fish make waves! When I was reading this part I immediatelly thought of a silly ditty back from Yugoslavia - "More se giba, to je od riba" (The sea undulates, that is because of fish"):
The researchers, funded by the National Science Foundation, report that the constructal law predicts universal relationships between animals’ body mass and speed, as well as the frequency and force of the strides, beats or undulations that propel their bodies forward.
“Running, swimming and flying occur in vastly different physical environments and, likewise, involve quite different body mechanics,” Bejan said of the new application of constructal law. “Nonetheless, there are strong convergences in certain functional characteristics of runners, swimmers and fliers.”
For example, the stride frequency of running vertebrates bears the same relationship to the animals’ mass as does the rate at which fish swim. Similarly, the velocity of runners conforms to the same principles as the speed of birds in flight.
Marden said he first stumbled across the problem in the 1980s when studying the variability in flight performance of insects and other flying animals. He attached weights to them and got a “strange universal result.” All the organisms he tested -- birds, bats, insects -- could all lift approximately the same amount of weight in relation to the size of their flight muscles regardless of their many other biological differences.
“The size of the wings didn’t matter; nothing else seemed to matter.” Marden said. “It was fascinating, but there was no explanation for this commonality when so much about the animals seemed to be different.”
Years later, a student of Marden’s suggested they analyze the function of jet engines, to determine whether they, too, followed the same principle. Although Marden said he at first dismissed the idea as ridiculous, a 2002 report by the two in Proceedings of the National Academy of Sciences showed that flying insects, birds and bats, running and swimming animals, piston engines, electric motors and jets all showed the very same pattern.
“We found that all of the motors used by humans and animals for transportation have a common upper limit of mass-specific net force output that is independent of materials and mechanisms,” Marden said.
So far, really interesting. Click on the link to read the details of the theory. Where I frowned was at the very end:
“It was swimming that stumped us,” Bejan said. “Everyone knows that, in water, fish are weightless.”
In other words, they explained, fish are neutrally buoyant, or nearly so, meaning that their tendency to float counteracts the force of gravity and they do not sink or rise. In essence, then, scientists have considered fish to move as though unaffected by gravity.
Based on the data, swimmers exhibit the same body-mass scaling as runners and fliers. “The question was: How could a theory including gravity apply to swimming fish?” Marden said.
Bejan finally realized the answer. Although fish are neutrally buoyant, they still have to push water out of the way to move forward, he said. That water raises the surface – a phenomenon that is often imperceptible as it may be spread across an entire lake, stream or ocean.
“The water can only go up because the bottom and sides of the channel are rigid,” Bejan said. “That bulge, however undetectable, is the fish’s footprint.”
Fish must, therefore, work against gravity to lift an amount of water equal to their own mass for each body length they move forward.
“It puts fish in the same physical realm as runners and fliers,” Marden said.
The findings may have implications for understanding animal evolution, Marden said. One view of evolution holds that it is not a purely deterministic process; that history is full of chance and historical contingency. It is the idea purported by Steven Jay Gould and others that if you were to “rewind the tape” and run it again, evolution would proceed down a different path, he said.This is a misunderstanding of the "rewind-the-tape" thought-experiment and of Gould's notion of contingency. Of course physical laws apply every time and all the time. If this new law is really as universal as is claimed, it is yet another piece of evidence supporting Gould.
"Our finding that animal locomotion adheres to constructal theory tells us that -- even though you couldn't predict exactly what animals would look like if you started evolution over on earth, or it happened on another planet -- with a given gravity and density of their tissues, the same basic patterns of their design would evolve again," Marden said.
If every moving organism follows the same laws of physics, yet so many millions of species have evolved over time, as different as grasshopper, tuna, chameleon, hawk and horse, this is a great demonstration of the power of natural selection to work WITH physics to produce enormous diversity.
Physics laws do not make it harder for evolution to produce new morphologies, it makes it easier to do so, as natural selection can tinker with shapes and sizes without worrying too much to make everything optimal - physics takes care of that.
Thus, if the tape of the history of life on Earth was rewound over and over again, we would always see completely different organisms - no grasshoppers, tuna, chameleons, hawks or horses, yet all of them would be expected to obey this, as well as all other laws of physics.