You are not currently logged in.
Access JSTOR through your library or other institution:
If You Use a Screen ReaderThis content is available through Read Online (Free) program, which relies on page scans. Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Extreme Selection in Humans against Homeotic Transformations of Cervical Vertebrae
Frietson Galis, Tom J. M. Van Dooren, Johan D. Feuth, Johan A. J. Metz, Andrea Witkam, Sebastiaan Ruinard, Marc J. Steigenga and Liliane C. D. Wijnaendts
Vol. 60, No. 12 (Dec., 2006), pp. 2643-2654
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/4134823
Page Count: 12
Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Preview not available
Why do all mammals, except for sloths and manatees, have exactly seven cervical vertebrae? In other vertebrates and other regions, the vertebral number varies considerably. We investigated whether natural selection constrains the number of cervical vertebrae in humans. To this end, we determined the incidence of cervical ribs and other homeotic vertebral changes in radiographs of deceased human fetuses and infants, and analyzed several existing datasets on the incidence in infants and adults. Our data show that homeotic transformations that change the number of cervical vertebrae are extremely common in humans, but are strongly selected against: almost all individuals die before reproduction. Selection is most probably indirect, caused by a strong coupling of such changes with major congenital abnormalities. Changes in the number of thoracic vertebrae appear to be subject to weaker selection, in good correspondence with the weaker evolutionary constraint on these numbers. Our analysis highlights the role of prenatal selection in the conservation of our common body plan.
Evolution © 2006 Society for the Study of Evolution