rther evolution of SINEs
After the emergence, SINE families can further change. Minor changes in their structure (point mutations and indels) give rise to SINE subfamilies. More substantial changes (module exchange and duplication of modules or whole SINEs) give rise to new SINE families. SINE families and subfamilies can coexist or replace each other. Some of them (or even all) can lose their activity with time and extinct, while their gradually degrading copies remain in the genome.
Emergence of SINE subfamilies
In all likelihood, only a minor fraction of SINE genomic copies is capable for retroposition (Roy-Engel et al., 2002b). Active copies with beneficial (or neutral) modifications can give rise to new SINE subfamilies. One can propose that these changes correspond to the fine-tuning of SINEs to the critical factors of their amplification. For instance, the changes in Alu sequence modulating the Alu RNA capacity to bind the SRP9/14 complex gave rise to subfamilies with different amplification rate (Sarrowa et al., 1997). LINE RT is another factor of SINE amplification. Considering that LINE subfamilies also replace each other in time, the structure of SINEs mobilized by them can also change accordingly (Human Genome Sequencing Consortium, 2001).