ABSTRACT

This chapter describes mechanisms, extent, and function of horizontal gene transfer (HGT). A brief history of the discovery of HGT is followed by a discussion of the pan-genome concept to describe within-species variation of genome content. Different processes and types of genes that contribute to the accessory gene pool of a population are discussed, including the Black Queen and Red Queen hypotheses, selfish genetic elements, and selectively neutral processes. Consideration of the pan-genome as a shared genetic resource for the population leads to a discussion of the units of selection (gene, individual organism, population, and community). The chapter concludes with an overview of the different biological processes through which HGT occurs between bacteria, archaea, and eukaryotes.