Chapter 11 - Development

Abstract

The area of descriptive and comparative developmental biology has long been used as evidence of evolution—especially of common descent. Because early embryos of groups as different as fish and mammals resemble each other so closely in their early stages, a common line of descent seems to be indicated. Also, the presence of features in embryos that are absent in adults illustrated some more particular evolutionary histories—such as hind limbs in the embryos of dolphins and whales and the presence of gill slits in the embryos of terrestrial vertebrates. Today the fields of developmental biology and genomics have undergone a synthesis that has illustrated the very important role that early developmental changes can have in the creation of new forms and the divergence of species. The commonality of many genes acting as developmental “switches” plus the divergence in the number and role of these switches has gone a long way toward explaining the divergence of adult morphologies. The MADS-box genes in plants and the HOX genes in animals are two of the better known and understood sets of such developmental switching genes, also known as homeotic genes. Many developmental pathways have been simplified through the elimination or deletion of some developmental features. Examples here would the loss of teeth in birds, the loss of eyes in some cave animals, and even the loss of the “adult” stage in the axolotl salamander.