1.1 Botanical classification and geographical distribution
The basic unit of biological classification is the species. According to the âbiological species concept,â a species is defined as a community of individualsâthat is, a population or group of populations whose members can interbreed freely with one another under natural conditions but not with members of other populations (Mayr, 2001; Soltis and Soltis, 2009). In other words, such communities are reproductively isolated. Although each individual of a sexual population is genetically unique, each species is a closed gene pool, an assemblage of organisms that do not normally exchange genes with other species. Their genes compel the individuals belonging to a species to perpetuate themselves over many generations. Yet all life forms on Earth are interrelated; they all ultimately descended from a common ancestor and âdanceâ to the same genetic code, whereby different combinations of three consecutive nucleotides of each organismâs deoxyribonucleic acid (DNA) specify different amino acids that can be assembled into proteins. Because they are thus interrelated, organisms can be grouped according to the degree of their genetic similarity, external appearance, and behavior. In the classification hierarchy, closely related species are grouped into a genus, related genera into a family, allied families into an order, associated orders into a class, similar classes into a division (plants) or a phylum (animals), related divisions or phyla into a kingdom, and, finally, allied kingdoms into a domain. The âevolutionary species conceptâ recognizes this ancestorâoffspring connection among populations that may follow distinct evolutionary paths to occupy separate ecological niches but may continue to interbreed for some time (Soltis and Soltis, 2009). For example, although they have been geographically isolated for over 20 million years, Eurasian and North American Vitis species are still able to interbreed readily.
As is the case with many plants, the species of the genus Vitis are not very well defined because of the extreme morphological variation among and within populations of wild vines (Currle et al., 1983; Hardie, 2000; Mullins et al., 1992). This implies the following: (i) all Vitis species are close relatives that share a relatively recent common ancestor and (ii) evolution is still at work, throwing up new variants all the time (see Section 2.3). Many vine species are actually semispeciesâthat is, populations that partially interbreed and form hybrids under natural conditions, which is in fact common among plants and may be an important avenue for the evolution of new species (Aradhya et al., 2013; Soltis and Soltis, 2009). Despite some hybridization where their natural habitats overlap, however, the various Vitis gene pools usually stay apart so that the populations remain recognizably different. Nonetheless, species that occur in close proximity are more similar than distant species in similar habitats. Grapevines are a good example of the limits of taxonomic systems. They demonstrate that there is a continuum of differentiation rather than a set of discrete, sexually incompatible units.
As early as 1822, the Rev. William Herbert asserted that âbotanical species are only a higher and more permanent class of varieties,â and in 1825 the geologist Leopold von Buch postulated that âvarieties slowly become changed into permanent species, which are no longer capable of intercrossingâ (both cited in Darwin, 1859). A few decades later, Charles Darwin expressed it clearly: âWherever many closely allied yet distinct species occur, many doubtful forms and varieties of the same species likewise occurâ and, furthermore, âthere is no fundamental distinction between species and varieties,â and, finally, âvarieties are species in the process of formationâ (Darwin, 1859). Indeed, > 150 years after the first publication of Darwinâs revolutionary insights, modern genetic research has confirmed that the various Vitis species evolved within the last 18 million years from a common ancestor (Aradhya et al., 2013; PĂ©ros et al., 2011; Wan et al., 2013; Zecca et al., 2012). They have not yet developed the complete reproductive isolation that normally characterizes biological species. Thus, Vitis species are defined as populations of vines that can be easily distinguished by morphological traits, such as the shape and size of their leaves, flowers, and berries, and that are isolated from one another by geographical, ecological, or phenological barriers; such species are termed ecospecies (Hardie, 2000; Levadoux, 1956; Mullins et al., 1992). The following is a brief overview of the botanical classification of grapevines, starting with the domain and finishing with a selection of species.
Domain Eukaryota
All living beings, making up the Earthâs biological diversity or biodiversity, are currently divided into three great domains of life: the Bacteria, the Archaea, and the Eukaryota. The Eukaryota (eukaryotes; Greek eu = true, karyon = nucleus) include all terrestrial, sexually reproducing âhigherâ organisms with relatively large cells (10â100 ÎŒm) containing a true cell nucleus, in which t...