Besides these variation processes there are many external selection criteria determined by the environment (adaptation), by the internal stability of the organism (spontaneous abortions can be considered as variations eliminated by internal stability criteria), by the reproductive capacity of the organism ("fitness" is defined as the expected number of offspring).
(many external selection criteria; determined; by the environment) (spontaneous abortions; can be considered; as variations; by the reproductive capacity of the organism) (variations; eliminated; by internal stability criteria) (fitness; is defined; as the expected number of offspring)
many external selection criteria spontaneous abortions variations fitness
determined can be considered eliminated is defined
by the environment
as variations by the reproductive capacity of the organism
by internal stability criteria
as the expected number of offspring