As we have seen in earlier chapters, Darwin did not provide direct evidence for natural selection in his Origin of Species. In the post-Darwinian period, many patterns of variation in plants explicable in terms of selection were described, but the demonstration of natural selection in wild populations remained elusive. Then, in the 1950s, detailed studies of air pollution and sites contaminated with heavy metals began in earnest, providing a number of thoroughly investigated and convincing cases of natural selection in action in animals and plants (Antonovics, Bradshaw & Turner, 1971; Taylor, Pitelka & Clegg, 1991; Macnair, 1981, 1990, 1997; Shaw, 2001).

Bell and Treshow (2002) discuss the long history of concern about the effects of pollution. For example, the diarist John Evelyn published the celebrated account Fumifugium: Or the Inconvenience of the Aer and Smoake of London Dissipated in 1661. As the industrial revolution gathered pace in the nineteenth century, there was a marked deterioration in air quality in towns and cities. As urban growth accelerated and industrialisation developed in Europe and North America, trees and other vegetation were often killed or damaged, especially in towns and cities, and near smelters, factories and industrial installations. Lichens proved to be particularly sensitive indicators of the growing problem of air pollution (Bates, 2002).

The world has been transformed by human activities, and mankind is centre stage in the evolutionary drama now being played out in the world's ecosystems. In this chapter, different groups of plant species in the evolutionary play are introduced – native, wild, crop, weedy, introduced, invasive, feral and endangered species. On the face of it there would appear to be little difficulty in assigning plants to species, and deciding into which category particular species fall. But as we shall see, in confronting the following major questions, there are many complications. How are species to be defined? How many species of plants are there in the world? How are native and introduced species distinguished? What do conservationists mean by wildlife? How many species are cultivated, weeds or invasive? In response to different kinds of environmental change and disturbance, how many species are presently at risk of extinction, and are current extinction rates greater than those of the past? Is there any agreement on the numbers of species that are endangered? Is there any evidence that conservationists might have exaggerated the situation?

Species

The concept of the species is at the heart of any consideration of evolution, and both the theory and the practice of conservation are to a large extent based on these concerns. Different groups of plants – wild, cultivated etc. – are widely discussed in terms of the numbers of species they contain. In considering these estimates, however, a major concern must be faced.

While there are examples of remarkable natural long-range dispersal of plants (Ridley, 1930), human-mediated trans-global movement of a wide range of organisms has resulted in an extraordinary dispersal of alien species. A very large number of species have been, and are being, accidentally or deliberately transported vast distances to new regions across the world, well beyond the limits of natural dispersal. Some of these have proved to be winners: others have been less successful.

There are a number of important reviews of the introduced species, including invasive species (Gibbs & Meischke, 1985; Drake et al., 1989; Williamson, 1996; Mooney & Hobbs, 2000; Sandlund, Schei & Viken, 2001; Sax, Stachowicz & Gains, 2006). These sources provide information on a wide range of issues, including the economic damage wrought by introductions, and provide detailed information on the attempts to control invasions. The control of introduced plant species by pesticides or biological means will be discussed in some detail in Chapter 15.

Here, our main focus is on plants and considers microevolutionary issues relating to success or failure of introduced taxa. However, reference will also be made to introduced animals, fungi and micro-organisms, as these influence the native plants in many ecosystems. The following questions are considered: by what means have plant species been introduced to new territories, and how many of these have become established in functioning populations? What factors are important in determining whether or not species are successful as introductions, and what factors determine whether introductions become invasive?