BIOSYSTEMATICS AND EVOLUTIONARY NOISE

Summary The main pillars of plant evolution are the populations of normal species — diploid, sexual, and outbreeding. These basic stocks represent the diversity which has resulted from millions of years of phylogenetie change. Hybrids have occasionally appeared, but most have been sterile or ill‐adapted. Polyploids, apomicts, inbreeders, and hybrids arc common today and we have no reason to believe that they did not occur even among early land plants. The fact, however, that normal species with normal life cycles still prevail today strongly indicates that such plants have always been responsible for the primary thrust of evolution. A kind of evolutionary noise is produced through the repeated origin of populations with deleteriously modified life cycles in which the advantages of diploidy, sexuality, outbreeding, and species purity are counteracted or cancelled. Such deviations persist for longer or shorter periods while the normal parental stock continues to evolve.