Homology

The number and types of differences that accumulate between genes or proteins of two different species can be used to assess their evolutionary relatedness and the amount of time since they diverged from a common ancestor. Such studies, termed "molecular systematics," can be used to show that humans are more closely related to chimps than to gorillas, for instance, and how long ago the split in these lineages occurred.

Homologous proteins that perform the same function in different species are called orthologs. For example, hemoglobin, a protein that transports oxygen, has a similar amino acid sequence in both horses and dogs. If the predicted amino acid sequence of a newly discovered protein is similar to a known protein in another species, researchers can make guesses about the function of the newly discovered gene. If the sequence of a newly discovered protein was similar to hemoglobin, one might guess that the new protein is able to bind to oxygen and function in transporting oxygen. In the way, orthologs help researchers about the functions of newly discovered genes.

Natural selection acts against harmful mutations in critical genes. Gene duplication, however, makes extra copies of less critical genes, which are more free to acquire mutations. Members of these gene families are known as paralogs. Researchers look for paralogs in order to find proteins with new abilities. Cytokine genes, for example, are all derived from the same ancestral gene and share common sequence motifs, yet they fill a variety of roles in the immune system. New members of the cytokine family might be valuable tools for fighting disease. Just as species diverge and fill new biological niches, genes become duplicated and acquire new functions. On a molecular scale, the evolution of the genome reflects the evolution of all living things.