Tuesday, February 2, 2010

Atavism

Vestigial organs and features are well known evidence for common ancestry and macroevolution.Similar in concept to anatomic vestiges, another quirk of common ancestry is that DNA preserves the blueprints for many “lost” features but are not expressed during normal development. Occasionally though, an organism will be born with what’s called an atavistic structure. Much like its literary definition, biological atavism is some phenotypic reversion to an ancestral form.

Atavistic features may commonly be observed during early life stages since many organisms share similar embryonic forms. For example, human fetuses having tails then eventually losing them during development. The amazing strength of evolutionary theory is that we can use it to make predictions. For instance, we understand how whales evolved supported by a long list of transitional forms. So we can reasonably make the prediction that we would sometimes expect there to be a mutation in living whales to express some atavistic feature. And evolution is once again confirmed by observations in nature. Whales and dolphins have occasionally been caught possessing rudimentary hind limbs, a testament to their terrestrial past.

Hind limbs on cetaceans are nonfunctional though, often very small in size – likely to the ancestral species’ gradual reduction in size and loss over time. They also vary in their complexity. Some may be present only beneath the skin and attached to the pelvic bone (a vestigial structure), while others as buds protruding on the skin to full leg-like structures. The skeletal remains also confirm that these limbs are also not just random malformations, but actual primitive limbs. On the rare specimens that have been analyzed, individual bones can be identified including femur, tibia and metatarsus. Cetacean hind limbs are also much like human tails in embryonic stages. Whales too form hind limb buds during development but the cells degenerate during later stages.

From vestigial bones, atavistic features and an abundance of transitional forms, all the lines of evidence points to cetaceans having a common quadrupedal terrestrial ancestor and the power of evolution. There is one more Atavism of note though, the Slough Feg album of the same name with the song below coincidentally being about natural selection and evolution. Anyone else agree we need more metal songs about science?


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