'Selfish genetic elements', such as transposons, homing endonucleases, meio
tic drive chromosomes and heritable microorganisms, are common features of
eukaryotes. However, their importance in the evolution of eukaryotic genome
s is still controversial. In this review, we discuss these diverse elements
and their potential importance in the evolution of genetic systems, adapta
tion, and the extinction and birth of species.