Ginkgo is a genus of non-flowering seed plants, assigned to the gymnosperms. The scientific name is also used as the English common name. The order to which the genus belongs, Ginkgoales, first appeared in the Permian, 270 million years ago, and Ginkgo is now the only living genus within the order. The rate of evolution within the genus has been slow, and almost all its species had become extinct by the end of the Pliocene. The sole surviving species, Ginkgo biloba, is found in the wild only in China, but is cultivated around the world. The relationships between ginkgos and other groups of plants are not fully resolved. Although commonly used as a traditional medicine or dietary supplement, there is no good clinical evidence that ginkgo has any anti-disease effect or health benefit.
Evolution
Fossil history
Trichopitys heteromorpha from the earliest Permian period in France is one of the earliest fossils ascribed to the Ginkgophyta. It had multiple-forked non-laminar leaves with cylindrical, thread-like ultimate divisions. Sphenobaiera (early Permian–Cretaceous) had wedge-shaped leaves divided into narrow dichotomously-veined lobes, lacking distinct petioles (leaf stalks). Baiera (Triassic–Jurassic) had similar multiple-lobed leaves but with petioles. The extant ginkgo (Ginkgo biloba) is a living fossil, with fossils similar to the modern plant dating back to the Permian, 270 million years ago. The ancestor of the genus is estimated to have branched off from other gymnosperms about 325 million years ago, while the last common ancestor of today's only remaining species lived not earlier than 390,000 years ago. The closest living relatives of the clade are the cycads. The time of this divergence is estimated to be extremely ancient, dating to the early Carboniferous. Fossils attributable to the genus Ginkgo with reproductive organs similar to the modern species first appeared in the Middle Jurassic, and the genus diversified and spread throughout Laurasia during the Jurassic and Early Cretaceous. At the end of the Pliocene, Ginkgo fossils disappeared from the fossil record everywhere except in a small area of central China, where the modern species survived. It is doubtful whether the Northern Hemisphere fossil species of Ginkgo can be reliably distinguished. Given the slow pace of evolution and morphological similarity between members of the genus, there may have been only one or two species existing in the Northern Hemisphere through the entirety of the Cenozoic: present-day G. biloba (including G. adiantoides) and G. gardneri from the Palaeocene of Scotland.
Evolutionary ecology At least morphologically, G. gardneri and the Southern Hemisphere species are the only known post-Jurassic taxa that can be unequivocally recognised. The remainder may have been ecotypes or subspecies. The implications would be that G. biloba had occurred over an extremely wide range, had remarkable genetic flexibility and, though evolving genetically, never showed much speciation. While it may seem improbable that a species may exist as a contiguous entity for many millions of years, many of the ginkgo's life-history parameters fit. It displays extreme longevity and a slow reproduction rate. Additionally, in Cenozoic and later times, the ginkgo's distribution is wide and apparently contiguous, although steadily contracting. The fossil record shows extreme ecological conservatism as the niche of the ginkgo is restricted to disturbed streamside environments. Modern-day Ginkgo biloba grows best in well-watered and well-drained soils, and the very similar fossil Ginkgo favoured similar environments. The sediment records at the majority of fossil Ginkgo localities indicate it grew primarily in disturbed environments along streams and levees. Ginkgo is therefore paradoxical in ecological terms because, while it possesses some favourable traits for living in disturbed environments (such as clonal reproduction), many of its other life-history traits (like slow growth, large seed size, late reproductive maturity) are the opposite of those exhibited by "younger", more-recently emerged plant species that thrive in disturbed settings. Given the slow rate of evolution of the genus, it is possible that Ginkgo represents a pre-angiosperm strategy for survival in disturbed streamside environments. Ginkgo evolved in an era before angiosperms (flowering plants), when ferns, cycads, and cycadeoids dominated disturbed streamside environments, forming a low, open, shrubby canopy. The large seeds of Ginkgo and its habit of "bolting"—growing to a height of approx. 10 metres (30 feet) before elongating its side branches—may be adaptations to such an environment. Diversity in the genus Ginkgo dropped through the Cretaceous (along with that of ferns, cycads, and cycadeoids) at the same time the flowering plants were on the rise, which supports the notion that flowering plants, with their better adaptations to disturbance, displaced Ginkgo and its associates over time.
Phylogeny As of 2013, molecular phylogenetic studies have produced at least six different placements of Ginkgo relative to cycads, conifers, gnetophytes and angiosperms. The two most common are that Ginkgo is a sister to a clade composed of conifers and gnetophytes, and that Ginkgo and cycads form a clade within the gymnosperms. A 2013 study found that the best support was for the monophyly of Ginkgo and cycads:
… excerpt ends here. Continue reading the full article.
