<P> In evolutionary biology, conserved sequences are similar or identical sequences in nucleic acids (DNA and RNA) or proteins across species (orthologous sequences) or within a genome (paralogous sequences). Conservation indicates that a sequence has been maintained by natural selection . </P> <P> A highly conserved sequence is one that has remained relatively unchanged far back up the phylogenetic tree, and hence far back in geological time . Examples of highly conserved sequences include the RNA components of ribosomes present in all domains of life, the homeobox sequences widespread amongst Eukaryotes, and the tmRNA in Bacteria . The study of sequence conservation overlaps with the fields of genomics, proteomics, evolutionary biology, phylogenetics, bioinformatics and mathematics . </P> <P> The discovery of the role of DNA in inheritance, and observations by Frederick Sanger of variation between animal insulins in 1949, prompted early molecular biologists to study taxonomy from a molecular perspective . Studies in the 1960s used DNA hybridization and protein cross-reactivity techniques to measure similarity between known orthologous proteins, such as hemoglobin and Cytochrome C. In 1965, Émile Zuckerkandl and Linus Pauling introduced the concept of the molecular clock, proposing that steady rates of mutation could be used to estimate the time since two organisms diverged . While initial phylogenies closely matched the fossil record, observations that some genes appeared to evolve at different rates led to the development of theories of molecular evolution . Margaret Dayhoff's 1966 comparison of ferrodoxin sequences showed that natural selection would act to conserve and optimise protein sequences essential to life . </P>

What does it mean that the genetic code is highly conserved