Single nucleotide polymorphism (SNP)
DNA sequence variations that occur when a single nucleotide (A, T, C, or G) in the genome sequence is altered. See also : mutation, polymorphism, single-gene disorder

Single gene disorder
Hereditary disorder caused by a mutant allele of a single gene (e.g., Duchenne muscular dystrophy, retinoblastoma, sickle cell disease)

Single stranded binding protein
A protein that binds to the single-stranded DNA generated when a helicase opens the double helix; this stabilizes the molecule and prevents reannealing or formation of instrastrand hydrogen bonds.

Smith, John Maynard
An eminent evoluntionary biologist and author of many books on evolution, both for scientists and the general public. A professor emeritus at the University of Sussex, his research interests include evolution of human mitochondrial DNA sequences and investigation of evidence for extensive recombination.

Smith, Tom
An ornithologist and conservation biologist, Smith is executive director of the Center for Tropical Research at San Francisco State University. His work combines basic research in ecology and evolutionary science with applied research in conservation biology. Among other issues, Smith is interested in the role of ecological gradients in speciation and maintaining species diversity.

Social Darwinism
A doctrine that applies the principles of selection to the structure of society, asserting that social structure is determined by how well people are suited to living conditions.

Solenoid structure
A structure of eukaryotic chromatin structure resulting from supercoiling of nucleosomes.