Friday, February 6, 2009

Darwin Day

Thursday, Feb 12, is Darwin Day: the 200th anniversary of Charles Darwin's birth. This year is special because it also marks the sesquicentennial of Darwin's masterpiece, The Origin Of Species.

Although Darwin Day is primarily a time to celebrate biology, physics plays two important roles in Darwin's theory of evolution. First, physics constrains evolution. Natural selection has produced an amazing variety and diversity of organisms, but each and every one obeys the laws of physics. You can dream up all sorts of organisms in your imagination, but some just won't work. Readers of the 4th Edition of Intermediate Physics for Medicine and Biology will learn about several of these constraints. For instance, in Chapter 2 Russ Hobbie and I discuss scaling (see my blog entry from August 8, 2008 for an earlier discussion of scaling). One can imagine a giant spider a hundred feet high with thin spider legs, but physics won't allow this: the spider would be crushed under its own weight (weight scales as the volume, but the strength of the legs scale as the cross-sectional area, so the larger the spider the more difficult it would be to support the weight). In Chapter 4 we show that diffusion is an effective way to transport molecules over short distances, but is a poor method over long distances. One can envision a three-story high single cell--a giant amoeba--but if that cell depends on diffusion to obtain oxygen and get rid of carbon dioxide, it will not survive. So, physics limits biological evolution, and these limitations provide important insights into why animals are designed the way they are.

The second role of physics in the study of evolution comes from the interplay between evolution and astronomy. A famous example is the idea proposed by physicist Luis Alverez that an asteroid slammed into the earth 65 million years ago, leading to the death of the dinosaurs and many other species. I would like to highlight a different example, in part because it is new and less familiar, and in part because it has been developed by researchers in the Department of Physics at the University of Kansas, my undergraduate alma mater (go jayhawks!). Professor Adrian Melott and his colleagues have proposed that gamma-ray bursts may have caused other mass extinctions. In the January 2004 issue of the International Journal of Astrobiology (Volume 3, Pages 55-61), Melott et al. write
"Gamma-ray bursts (GRBs) produce a flux of radiation detectable across the observable universe. A GRB within our own galaxy could do considerable damage to the Earth's biosphere; rate estimates suggest that a dangerously near GRB should occur on average two or more times per billion years. At least five times in the history of life, the Earth has experienced mass extinctions that eliminated a large percentage of the biota. Many possible causes have been documented, and GRBs may also have contributed. The late Ordovician mass extinction approximately 440 million years ago may be at least partly the result of a GRB. A special feature of GRBs in terms of terrestrial effects is a nearly impulsive energy input of the order of 10 s. Due to expected severe depletion of the ozone layer, intense solar ultraviolet radiation would result from a nearby GRB, and some of the patterns of extinction and survivorship at this time may be attributable to elevated levels of UV radiation reaching the Earth. In addition, a GRB could trigger the global cooling which occurs at the end of the Ordovician period that follows an interval of relatively warm climate. Intense rapid cooling and glaciation at that time, previously identified as the probable cause of this mass extinction, may have resulted from a GRB."
On Darwin Day, as you celebrate Charles Darwin and his theory of evolution by natural selection, remember that a knowledge of physics as well as biology is crucial to understanding this important idea. The 4th Edition of Intermediate Physics in Medicine and Biology is a good place to obtain the necessary physics background.

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