Genetics & Molecular Biology

The fields of genetics and molecular biology are able to provide strong evidence for evolution. All organisms on Earth are composed of the same building blocks: DNA, RNA, and proteins. DNA and RNA are composed of specific sequences of nucleic acids that are inherited through generations. Protein is composed of specific sequences of amino acids encoded by the sequence of nucleic acids in RNA.

If two species are closely related, they have very similar molecular sequences. The longer that they have been evolving as separate species, the more likely that they will have undergone genetic mutations that make their DNA sequences less similar.

Isolating and sequencing DNA and RNA from living organisms, such as a bivalve, is relatively simple chemistry in today’s molecular laboratories. Click here to learn the basic procedure.

Unfortunately, it is very difficult to isolate DNA or protein from fossils. Soft tissues have been recovered (most famously in the dinosaur Tyrannosaurus rex ^[1] — click here for more information), but only from fossils of “exceptional preservation” (preservation under very special circumstances that do not happen often). Therefore, molecular biology is mainly used to study the relationships among living species. Once the relationships between living species are understood, the characteristics of ancestral species can be inferred and confirmed through the fossil record.

These are short aligned molecular sequences (part of the 18S ribosomal RNA nuclear gene) from four common species of bivalves — the Eastern Oyster, Blue Mussel, King Scallop, and Hard-Shelled Clam. The letters in the sequences represent the four bases that make up DNA and RNA: A = adenine, C = cytosine, G = guanine, T = thymine (replaced by U = uracil in RNA; these sequences are DNA copies of the RNA gene). This part of the 18S gene sequence illustrates differences — notice that the highlighted part of each sequence includes differences from one species to another, and that the unhighlighted parts of the sequences on each side are exactly alike. The number of differences reflects how closely related two species are. There are only three differences between Crassostrea and Mytilus, and three between Mytilus and Pecten, but there are nine differences between Pecten and Mercenaria. In actuality, only a very small percentage of a typical sequence shows meaningful differences at all between closely related species (some additional differences reflect variationVariation:
The differences among individuals in a population. among individuals, so are not "meaningful" from an evolutionary point of view). (Sequences provided by Gonzalo Giribet. Original graphic by Sarah Chicone, for Science on the Half-Shell exhibition, Paleontological Research Institution, 2010)

Footnotes

Schweitzer, M. H., J. L. Wittmeyer, J. R. Horner, and J. B. Toporski. 2005. Soft tissue vessels and cellular preservation in Tyrannosaurus rex. Science, 307: 1952-1955.