SPECIES VARIATION IN PROTEIN STRUCTURE
One of the major questions to be answered in arriving at a clear understanding of the phylogenetic relationships between different forms of life is whether there exist identical, or closely homologous, genes in widely separated species, or whether similarities in phenotype are due to analogous genes which determine equivalent appearance or function by different pathways. The techniques of experimental genetics permit us to compare the genetic makeup of only those organisms that can be successfully crossed. We know, for example, that the eye pigments of a wide variety of species contain the same light-sensitive compound. However, we have no genetic way of testing whether the synthesis of this compound in different organisms is under the control of the same set of genes, structurally modified perhaps in some slight manner but still essentially identical, or whether completely different genes are involved which act in concert to achieve the same end result.
All genetic analyses depend on the availability of some recognizable phenotypic character, be it morphological, functional, or meta