Homology refers to the underlying sameness of distinct body parts or other organic features. The concept became crucial to the understanding of relationships among organisms during the early nineteenth century. Its importance has vacillated during the years between the publication of Darwin's Origin of Species and recent times. For much of the twentieth century, homology was regarded as nothing more than evidence of common descent. In recent times, however, it has regained its former importance. It is regarded by advocates of evolutionary developmental biology as a source of evidence for developmental constraint, and evidence for biases in phenotypic variation and the commonalities in organic form. The recent growth in understanding of developmental genetics has made possible a new theory about the mechanical causes of homology, which was impossible under the naïve views regarding genetic causation that were available during most of the twentieth century. This new theory, proposed by Günter Wagner (2007), states that homologies of character are due to the development of a certain kind of genetic regulatory network called a character identity network.

Key Concepts:

• For much of the twentieth century, the concept of homology was regarded as secondary in importance to adaptation, with homology regarded as merely the evidence for common ancestry.
  
• Recent advances in molecular biology and developmental genetics have enhanced the importance of homology, and it has regained the importance that it had during the nineteenth century.
  
• One might expect homologous characters to develop from identical embryological precursors, but this is not correct; paradoxically, homologous traits can have distinct embryological origins.
  
• This ‘paradoxical’ fact about homology has led to great frustration among those who have attempted to explain the developmental basis of homology.
  
• Recent developments in developmental genetics may have given us a way to explain the ‘paradoxical’ nature of homology.