Ever notice that traits of organisms travel in flocks? That certain things are almost always found together? Of course you have, because we classify things in everyday life by their commonalities.
When we see feathers, we expect to see a beak, wings, and scaly legs. If we're a biologist, we also expect to see a lower jaw of several bones and a middle ear with but one. And when we see hair, we expect pregnancy rather than eggs and young fed with milk. The biologist also expects the lower jaw to have only one bone per side and three bones in the middle ear.
But why not a bird with hair or a mammal with only one middle ear bone?
In the 19th century, it slowly became clear—birds have a particular set of characters
because they've inherited them from a single ancestral bird, and mammals have a
different set because they're descended from a different ancestor with different
characters. Thus evolution accounts not only for diversity, but also for the common
themes of life in our Chihuahuan Desert.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
Even though several of the bird jaw bones (bottom) fuse at an early age, a vertebrate biologist would have no difficulty in correctly identifying them since no bird has a single bone making up each side of the lower jaw and all mammals do. Mandibles of Gray Fox (Urocyon cinereoargenteus), top, and Great Black-backed Gull (Larus marinus). Photograph by A.H. Harris; edges enhanced for clarity.
Strickberger, M. W. 2000. Evolution. 3rd ed. Jones and Bartlett
Publishers, Sudbury, MA, 722 pp.