Ginkgo
Ginkgo is a genus of highly unusual non-flowering seed plants. The scientific name is also used as the English name. The order to which it belongs, Ginkgoales, first appeared in the Permian,[3] 270 million years ago, and 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 exception is the sole living species, Ginkgo biloba, which is only found in the wild in China, but is cultivated around the world. The relationships between ginkgos and other groups of plants are not fully resolved.
The ginkgo (Ginkgo biloba) is a living fossil, with fossils similar to the modern plant dating back to the Permian, 270 million years ago. The closest living relatives of the clade are the cycads,[4] which share with the extant G. biloba the characteristic of motile sperm. Fossils attributable to the genus Ginkgo with reproductive organs similar to the modern species first appeared in the Middle Jurassic,[5] and the genus diversified and spread throughout Laurasia during the Jurassic and Early Cretaceous. It declined in diversity as the Cretaceous progressed with the extinction of species such as Ginkgo huolinhensis, and by the Palaeocene, only a few Ginkgo species, Ginkgo cranei and Ginkgo adiantoides, remained in the Northern Hemisphere, while a markedly different (and poorly documented) form persisted in the Southern Hemisphere.[citation needed] 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.[6]
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. These are: extreme longevity; slow reproduction rate; (in Cenozoic and later times) a wide, apparently contiguous, but steadily contracting distribution coupled with, as far as can be demonstrated from the fossil record, extreme ecological conservatism (restriction to disturbed streamside environments).[7]
Modern-day G. biloba grows best in well-watered and drained environments,[8] and the extremely 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.[7] Ginkgo therefore presents an "ecological paradox" because, while it possesses some favourable traits for living in disturbed environments (clonal reproduction), many of its other life-history traits (slow growth, large seed size, late reproductive maturity) are the opposite of those exhibited by modern plants that thrive in disturbed settings.[9]
