It is rather easy to determine that the genus Hosta is one of the younger plant genera among all of the liliaceous monocotyledons. No one has found fossils of plants that might even come close to resembling the morphological characters of the genus Hosta. We can only guess as to when they evolved.
It is difficult to find a simple way to answer the questions in the title heading. Complete and detailed answers involve scientific concepts and methods, including phylogeny, cytology, palynology, and other esoteric subjects. Nevertheless, in the following, I give some definitions and answers to questions about the evolution of hostas. Most of this presentation is based on evidence and data published in the scientific literature. Use of a good dictionary may be helpful to understand some of these concepts.
1) Fossil evidence: Although many plant species have been found in fossiliferous rocks and sedimentary rock layers, fossilized evidence of taxa belonging to the genus Hosta has not been found. This indicates that hostas have evolved in recent times.
2) Cytology (the study of cells and cell content): Cytologists found that Hosta species have a Karyotype similar to our desert plants like Yucca, Agave, and Camassia, in the family Agavaceae. The Karyotype of a plant species is the comparative character of the chromosomal complement, including number, form, and size of the chromosomes. It was determined that Yucca and Hosta have similar Karyotype: Yucca has 10 large and 50 small chromosomes (2n = 60) and Hosta has 12 large and 48 small (2n = 60). Thus, based on cytological grounds, it was proposed to include Hosta in the Agavaceae. These similarities notwithstanding, evolutionary ecology dictates otherwise. Species in the Agavaceae are mostly desert-adapted, xeromorphic succulent rosette plants, while Hosta in Hostaceae certainly has forest-adapted lineage. The evolutionary habitat of Agavaceae in Central and North America is in a xerophytic desert ecology, while Hosta species evolved in Asia in a non-xerophytic, montane-maritime ecology with high rainfall levels. The two evolutionary habitats are completely isolated and continents apart and there are no evidence of migration in either case. Thus, I have not accepted the inclusion of Hosta within the Agavaceae in the most recent molecular consensus classification of APG (Angiosperm Phylogeny Group - 1998).
3) Morphology and Morphometric Analysis: A study and analysis of the divergent morphologies of the original populations in Japan, Korea, and China reveals that Japan is not the evolutionary birthplace of the genus Hosta, since the native Japanese species are uniform in a broad sense. Morphometric separation is highest on the Korean peninsula and in China.
4) Palynology (pollen study): Pollen evidence indicates that the progenitor of Hosta may have been lily-type ancestors from which H. plantaginea evolved and an evolutionary trend can be seen in the developing pollen types from reticulate through rugulate or rugulate-baculate to rugulate-granulate type. I believe this trend is also seen in the variability of macro-morphological characters when correlated to geographic location and environment.
5) Evolutionary Geography: Predecessors of the genus probably migrated from the east-central Chinese mainland, where the most “primitive” hosta still exists (H. plantaginea), through southern Manchuria into the Korean peninsula and via this southern route to southern Japan. The northern route extended along the coast of the south-eastern USSR following a path on the southern side of the Sikhote-Alin mountain range and migrating to Sakhalin and from there south into Hokkaido and Honshu. The main Japanese islands providing a climatologically and ecologically very diverse habitat gave rise to increased speciation. Evidence shows that the taxa growing in northern Kyushu (H. tibai), on Tsushima Island (H. tsushimensis) and the southernmost islands of Korea (H. jonesii) may have originated with the northern branch of evolution, while all other, highly differentiated Korean taxa originated with the southern evolutionary branch after becoming geographically isolated in insular Korea. I propose it was only through geographic isolation that these species remained distinct. On the main Japanese islands natural, proximal populations probably have been hybridizing and intergrading for centuries. Hybridization is usually assumed to take place between completely divergent species but this may not be correct because it assumes that the two hybridizing stocks are composed of individuals, who have gone through past complete divergence, i.e. they evolved over centuries with the attendant production of a reproductive barrier. I have pointed out that such a barrier does not exist. The Chinese night-blooming H. plantaginea and day-blooming Japanese and Korean species are reproductively isolated from each other morphologically. Notwithstanding, this barrier can be easily overcome by human intervention.
6) Phylogeny: As pointed out under “Cytology,” a close affinity of Hosta with the species in Agavaceae have been suggested which has created rather numerous phylogenetic placements, each one having its proponents so there is no real agreement on the phylogeny of Hosta. The latest placements (APG 2003) involve placing Hostaceae (Hosta) in the Agavaceae and, alternatively in the Asparagaceae, while using the annotation “Hosta is geographically a little odd”! Rather than a “little odd,” I would state it is very odd, based on the evolutionary geography and conditions listed under 5), above.
Further Considerations: Which one is the youngest Hosta species? Some scientific studies have shown that H. venusta is less than 15,000 years old. The propagules of H. venusta moved from southeastern Korea to Cheju Island after the last ice age. The geological age of Cheju Island is estimated to be 13-14,000 years so H. venusta found only on this island must have speciated after the volcanic island became habitable after the ice retreated. During the process of adapting to a new, hostile, basaltic island habitat of recent origin, H. venusta underwent subsequent genetic changes. It was determined that 49 enzyme bands of H. venusta are a subset of 72 bands found in H. minor so establishing a very close relationship and making H. minor the starting point for H. venusta. Total DNA content has also shown this to be the case.
Hostas are not native in North America, yet many other Asian species crossed the Beringia land bridge between Siberia and Alaska some 12- to 15,000 years ago. For this reason, it can be assumed that hostas must have evolved after the land bridge disappeared or they were for some reason not able to migrate to North America like many other Asian species. Phylogeny has shown that the latter is the case, and that the genus Hosta evolved before the last ice age. What prevented it from migrating across the land bridge is unknown, but migrate it did from China into Korea and throughout Japan. We do know that from an evolutionary standpoint, Hosta is a very young genus occurring in a contiguous area of Asia and is found nowhere else on the globe. It has unique macro- and micromorphology and no other ornamental plant looks like it. So how old is it? The absence of fossil evidence shows the genus Hosta evolved during the latter part of the Pleistocene period from 1.8 million to 8,000 years ago and we can only guess as to whether it was near the beginning or the end of this period. Most likely, it was from the middle towards the end. The genus Hosta further developed through speciation, inter-specific hybridization, and adaptation during the Holocene Period, from 8,000 years ago to the present time. Some species out there are probably still speciating and adapting to our constantly changing environmental conditions.
W. George Schmid
Author, The genus Hosta – Giboshi Zoku (ギボウシ属)
Chair, AHS Classification and Nomenclature