Homology

Biologists have long been fascinated by the diversity of life. The amazing variety of living things makes it natural to wonder how so many different life-forms came to be. Physical characteristics that could be easily observed, such as the shape of wing, the structure of a shell, or the size of a beak, provided the first means to search for an answer. Recognition of the variation within a species (imagine a Chihuahua and a Great Dane) led Charles Darwin to propose that new species emerge when selection favors certain traits within a population.

Today's biologists continue to study the effects of natural selection on the evolution of species, but they are no longer limited to beak size and wing shape. Now they can compare the positions of genes on chromosomes, the amino acid sequences of proteins, and the nucleotide sequences of genes. With DNA or protein sequences from over 133,000 species represented in the taxonomy database at the National Center for Biotechnology Information (NCBI) and over 800 genome sequences either published or in progress, researchers have an unprecedented opportunity to study evolution at a molecular level.

Sequence comparison of the globin gene for human, rabbit, and cow. Dashes represent bases that are not present in the gene. Note the large amount of homology, as well as the highlighted differences.