How many processed pseudogenes are accumulated in a gene family?

A simple kinetic model is developed that describes the accumulation of processed pseudogenes in a functional gene family. Insertion of new pseudogenes occurs at rate v per gene and is countered by spontaneous deletion (at rate delta per DNA segment) of segments containing processed pseudogenes. If there are k functional genes in a gene family, the equilibrium number of processed pseudogenes is k(v/delta), and the percentage of functional genes in the gene family at equilibrium is 1/[1 + (v/delta)]. v/delta values estimated for five gene families ranged from 1.7 to 15. This fairly narrow range suggests that the rates of formation and deletion of processed pseudogenes may be positively correlated for these families. If delta is sufficiently large relative to the per nucleotide mutation rate mu (delta greater than 20 mu), processed pseudogenes will show high homology with each other, even in the absence of gene conversion between pseudogenes. We argue that formation of processed pseudogenes may share common pathways with transposable elements and retroviruses, creating the potential for correlated responses in the evolution of processed pseudogenes due to direct selection for control of transposable elements and/or retroviruses. Finally, we discuss the nature of the selective forces that may act directly or indirectly to influence the evolution of processed pseudogenes.