MONOZYGOTIC TWINNING - AN EVOLUTIONARY HYPOTHESIS

Monozygotic twinning is rare within populations yet taxonomically wide spread. We explore the evolution of monozygotic twinning by modeling a n allele in a newly formed offspring that causes it to undergo mitosis and separation to form one or more clones (twins), potentially in con flict with the parents' best interest. The success of this twinning al lele in our haploid models depends on the balance of the benefit of in creased frequency in the clutch and the cost of reduced survival resul ting from limited parental resources. The trait reaches high frequency in a broad range of plausible conditions but also fails to spread or is kept at low frequency in others when the survival cost is high (e.g ., in small clutch sizes). Interestingly, there are two reasonable con ditions that predict high frequency of the trait but low visibility: r andom parental abortion and selection for low penetrance. Thus our mod els suggest reasons why monozygotic twinning might be rare, or alterna tively, be common yet appear rare. In addition, we discuss the implica tions for sex-linked twinning, dizygotic twinning, and twinning by gam etes.