A key source of genetic variation is exchange of genetic material among individuals. Genetic exchange among closely related individuals is common in nature, and occurs in prokaryotes via conjugation and other processes, and in eukaryotes during sexual reproduction. When these same processes occur among individuals that are more distantly related and would not typically exchange genetic material, they are referred to as hybridization. But occasionally genetic material is exchanged among organisms that are very distantly related. This phenomenon, termed horizontal (or lateral) gene transfer, occurs relatively rarely, and was long presumed not to be important in evolution. However, as increasing amounts of information from deoxyribonucleic acid (DNA) sequences have become available, it has become clear that horizontal gene transfer is a significant part of evolution in nature.