Homology and homoplasy: the retention of genetic programmes.

Homology describes the inevitable evolutionary phenomenon that the similarity of structures among different organisms is due to the commonality of their descent. This continuity of information is maintained in evolutionary lineages in terms of genes and developmental mechanisms and will retain 'sameness' and retard, funnel and direct evolutionary diversification. Analogous 'sameness' is said to be due to independent, convergent evolution, and also involves similarity of function; the latter is not a necessary condition for structures to be identified as homologous. Here, I suggest that the biological basis for these seemingly disparate kinds of 'sameness' in evolution may in some, or even most, instances not be all that different and may be based on the same principle-the long evolutionary retention of genes, gene interactions and developmental mechanisms. Evolution might recycle and re-recruit similar mechanisms repeatedly during its course, and it often makes do with what is already available to it rather than to newly evolve or reinvent many gene interactions and developmental mechanisms repeatedly. Apparently there is no, or only a negligible, 'genomic cost' or even a selective advantage to maintaining genes and developmental mechanisms for long evolutionary periods of time, even if they are not continuously used in all members along an evolutionary line. Therefore, the biological basis of both homologous traits (those that are evolutionarily always expressed) and homoplasious traits (those that are not always 'on', but are 're-awakened' during evolution) might not be so different, and the distinction between homology and some forms of homoplasy may be somewhat artificial.