Molecular evolution of ribosomal DNA in Daphnia

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It has been observed that sequence similarity between copies of genes that are present many times within genomes (multigene families) is very high within species, even when divergence is very high between species. This phenomenon is known as "concerted evolution". Ribosomal DNA (rDNA) is a multigene family that is present in hundreds to thousands of copies in plants and animal cells, and encodes the RNA molecules (rRNA) that make up ribosomes: the site of cellular protein synthesis. One rDNA unit consists of 3 rRNA genes separated by "spacers". The spacers contain sequences that control the rate of rRNA production. Some regions of the rDNA unit diverge between species very slowly while others, such as the spacers, diverge very rapidly, even among closely related species. Despite this rapid divergence, the sequences still retain their function.