A gene signature is a group of genes in a cell which signal a specific phenotype. In population genetics, signature presence can indicate a population or location specific allele. A gene signature can signify genetic isolation or unique populations. Contrarily, overlapping genetic signatures would indicate gene flow or population blending. It's also commonly used in medicine to as an indicator of a person's potential to respond to a drug.
Gene signature. (n.d.). Retrieved March 18, 2015, from http://en.wikipedia.org/wiki/Gene_signature
Itadani, H., Mizuarai, S., & Kotani, H. (2008, January 1). Can systems biology understand pathway activation? Gene expression signatures as surrogate markers for understanding the complexity of pathway activation. Retrieved March 18, 2015, from http://www.ncbi.nlm.nih.gov/pubmed/19517027?dopt=Abstract