Cold Spring Harbor Symp. Quant. Biol. 24. 141-152. (1959)
Differentiation-hybridization cycles and polyploidy in Achillea.
Friedrich Ehrendorfer

The concept of differentiation-hybridization cycles was first elaborated in a group of diploid Galium species from the Eastern Mediterranean (Ehrendorfer 1958). In this group crossing barriers between the very distinct Galium graecum and G. canum remain practically intact on some Aegean Islands, e.g., Rhodes, where strong competition tolerates only a small amount of hybrid introgression in spite of extensive sympatric occurrence. On the Anatolian mainland, however, crossing barriers between G. graecum and G. canum have broken down under less rigorous competition and more possibilities for ecological and geographical expansion. Hybridization has led to the origin of the highly polymorphous G. dumosum, a hybrid complex which has widely replaced the parental species and is in full course of new differentiation, forming a center of variability. The data concerning this and other groups of Galium have been incorporated into a generalized diagram, conveying the concept of differentiation-hybridization in pictorial manner (fig. 1) ...

King: California Wild Radish (2007)
The California Wild Radish originated when common radishes (Raphanus sativa) and the related Jointed Charlock (R. Raphanistrum) hybridized in the wild. Some of the progeny took off in all directions, varying madly, as they established themselves as a novel weed. Apparently this began in the 19th century, but the descendants continue to vary in suitably "hybridized" environments. These pictures were taken along a drainage "creek" in Belmont, CA.

It is also worth noticing the wild variations of the Rainlilies (Zephyranthes) that Fadjar Marta bred from just two original parents: Zephyranthes citrina and Z. Fadjar's Pink, which resembles Z. grandiflora but sets seed.

Pfitzer (1911) Also raised an impressive range of varieties derived from hybrids of Begonia Veitchi and B. boliviensis.

Viehmeyer (1944) discussed hybrid Penstemons, including the Flathead Lake complex, that contained rebloomers even though neither parent rebloomed. He offered advice that applies to many other genera.

  1. If two species, which occupy different environments in nature, are crossed, the first hybrid generation will require an intermediate environment for optimum performance.
  2. Advanced generations, if large enough, will contain individuals that exceed the tolerance of the parental species to limiting factors of the new environment.
  3. The breeder, working with a segregating population in any intermediate habitat, may expect to select (if the population is large enough) lines fairly well adapted to the particular habitat in which he is growing his advanced generation population.
  4. The "raw hybrids" that are the result of interspecific hybridization offer the amateur breeder excellent material for penstemon improvement. By selecting and interbreeding those segregates best adapted to his garden he can isolate strains that perform well and consistently. To do this he needs only the most rudimentary instruction in plant breeding. The process is almost automatic if he chooses his breeding material from that part of the segregating population best adapted to his breeding plots.