We live in a species-rich world that is increasingly under human influence. As we have seen in earlier chapters, there is abundant evidence that many species are endangered, and this includes unknown organisms that have yet to be named and classified. If we are to appreciate the true extent of biodiversity, there is a formidable and urgent taxonomic task ahead (Mallet & Willmott, 2003; Bateman, 2011).

What are the prospects of the completion of a catalogue of life?

The challenges are discussed by Wheeler (2008), who comes to the conclusion that, at the very point in history where a major effort is required of taxonomists, there is a growing concern that taxonomy has suffered a ‘decline in prestige and support’.

As we have seen in earlier chapters, taxonomy has a very long history. Wheeler considers that two twentieth-century developments, in particular, have challenged the status, funding and prospects of taxonomic studies. We have seen in earlier chapters that, in the 1920s and 1930s, biologists became increasingly interested in experimental and genetic investigations of evolutionary patterns and processes. Many botanists, initially trained as taxonomists, turned to these investigatory studies to study intraspecific variation, speciation etc. These approaches became known as Experimental Taxonomy or Biosystematics. Huxley (1940) in his classic book New Systematics hoped for the rise of an integrated subject, but Wheeler points out that some biologists, enthusiastic for the emerging specialisms, came to believe that taxonomy, being ‘non-experimental’, was not really a science, but was involved with ‘subjectively naming and classifying species’. Thus, ‘taxonomy involved arbitrary bookkeeping and pigeonholing practices and legalistic wrangling over scientific names, while systematics was experimental and intellectually exciting and expansive’. In parenthesis, we may note that this perception of taxonomy, as ‘merely descriptive’ and ‘non-analytical’, has been very strongly challenged. For example, Lipscombe, Platnick & Wheeler (2003) and Sluys (2013) argue very effectively that taxonomic study is ‘hypothesis driven’.

Returning to Wheeler's analysis, he points to an even bigger revolution later in the century, when molecular and computational techniques were introduced to energise an already vigorous experimental tradition, providing the tools for major advances in all aspects of micro- and macro-evolution, and the emergence of molecular phylogenetics and modern biogeography.

We saw in Chapter 7 how different breeding systems can be expected to produce different patterns of variation. If we are to understand the variation patterns actually found in nature and the processes that give rise to these patterns, we must discover how the potential variation in seeds relates to the variation of reproductively mature plants. Historically, the first advances in this field were made by means of comparisons between plants belonging to the same species but from different populations. Taxonomists, biometricians and, later, geneticists became interested in genetic variation in the wild, and many of their studies converged at one point, namely, the controversy over the ‘reality’ of the intraspecific groups, which could be distinguished in nature, and whether they were the subspecies or varieties of the taxonomist or the ‘local races’ of the biometrician. A new look at this old question was provided by the famous researches of Turesson published in the early 1920s.

Turesson's pioneer studies and other experiments

At the time of Turesson's experiments, the question of the reality of ‘local races’ was combined with another controversial issue, namely how much of the observed variation in natural populations was the result of the direct modification of plants subjected to severe environmental stresses. By the end of the nineteenth century, many botanists reasoned that distinctive intraspecific variants were merely ‘habitat modifications’. Turesson, however, pointed out that in all previous cases known to him, only a partial test of the ‘habitat modification’ hypothesis had been carried out. For example, he considered the studies of Lathyrus japonicus (L. maritimus) undertaken by Schmidt (1899). Baltic populations of this plant have dorsiventral leaves, whilst on the North Sea coast of Denmark the plant has isolateral leaves. Schmidt showed, by experiment, that watering the Baltic variant with sodium chloride solutions induced a leaf structure typical of Danish plants. Given that the North Sea has a higher percentage content of salt than Baltic waters, Schmidt deduced that the leaf structure of the plants on the North Sea coast of Denmark was merely a habitat modification.

The logic of this type of deduction did not satisfy Turesson. His approach to the problem was to grow samples of several variants of a species in a standard garden, to see if ‘distinctiveness’ was retained or lost.

The ‘species concept’ in biology, as we saw in Chapter 2, has engaged the attention of most of the famous naturalists in earlier centuries, from Ray through Linnaeus to Darwin himself, and debate, argument and controversy continues to the present day (see Rieseberg, Wood & Baack, 2006; Wilkins, 2009; Mischler, 2010; Kunz, 2012).

Species as part of natural classifications

Kinds of organisms can be arranged in a hierarchy, each higher group containing one or more members of a lower group. A study of primitive biological classifications reveals that hierarchical classifications are a widespread feature of languages in general, and the particular hierarchy of genus and species, which modern biology uses, is really only a special case of a general linguistic phenomenon. The inescapable conclusion from such comparative studies is that hierarchical taxonomic classifications arise in human societies wherever they develop, and that the detail of the treatment found in folk taxonomies reflects the importance of the organisms concerned in the life of the particular tribe or culture. This view of taxonomy as the product of the human need to understand, describe and use the plants and animals applies to the Latin biological classification we use today.

Although we talk of associating species into genera and genera into families, that is not what happened in the early days of biological classification. It is indeed arguable that the idea of a kind of plant or animal envisaged by the ordinary person corresponds, in the history of classification, more closely to the modern genus than it does to species. It was these classical and medieval ideas of ‘kinds’ of plants that were available in the eighteenth century to Linnaeus, who stabilised the scientific names in the binomial form in which we still use them. So the Linnaean names (e.g. Quercus = Oak and Fraxinus = Ash) indicate the level of recognition of ‘kinds’ in the botany of medieval Europe. This is beautifully illustrated by the Carrot family (Apiaceae/Umbelliferae), many of which were familiar plants in classical times in Europe, mainly because they were cultivated for food or flavouring (e.g. Daucus = Carrot, Pastinaca = Parsnip) or because they were poisonous (e.g. Conium = Hemlock). All these familiar European plants were accurately described and given what later became their generic names, long before Linnaeus.

Populations do not exist in isolation: they owe their origin to pre-existing populations. Thus, populations establish, flourish, languish, decline and revive, and finally, for all manner of reasons, become extinct (Harper, 1977).

Metapopulations

A family of metapopulation models linking all these stages has proved important in the study of animal populations – from kangaroo rats to monk seals, from mountain sheep to butterflies. The concept has also influenced the study of plants.

Models take as their starting point that species occur in patches. At any one time some but not all suitable patches are likely to be occupied. Unoccupied patches are colonised by dispersal of seeds or fruits from an occupied patch. In time, while populations in newly occupied sites may expand, eventually numbers may decline and the population becomes extinct. Each population, therefore, has a finite life. Thus, the success of the species regionally will depend upon there being healthy colonies from which recolonisation can repopulate empty patches, where the species has become extinct. Brussard (1997) describes three different types. (i) ‘Classical metapopulations consist of several small extinction-prone local populations connected by a moderate amount of migration’. (ii) Mainland populations or large habitat blocks that supply colonists to small outlying island populations, including islands of land in the sea or lakes; ‘island’ water bodies scattered across a landscape; and isolated mountain tops in a lowland plain etc. (iii) Metapopulation systems may also consist of populations that ‘are declining to extinction because dispersal is too infrequent’ or does not occur to facilitate re-establishment.

The metapopulation concept has been employed in interpreting patterns of behaviour in plant populations. For example, Pedicularis furbishiae has been studied at sites along the banks of the St John River in Maine, USA. A total of 28 colonies were mapped over a four-year period, and the patterns of colonisation/extinction were consistent with the metapopulation model (Menges, 1990, 1991).

In another case study employing metapopulation models, populations of the short-lived perennial species Silene latifolia (S. alba) were examined over a number of years in roadside habitats in Virginia, USA (see Altizer, Thrall & Antonovics 1998; Thrall et al., 1998). Overall, 7500 habitat patches were detected, of which some 400–500 were occupied in any one year.

For forty years after the publication of the Origin, Darwin's ideas were a source of tremendous public controversy. For this reason he never received any awards from the state, although he was awarded honorary degrees and decorations in plenty. Despite thousands of critical sermons, many accepted Darwin's views on evolution, but, as seen in this chapter, the concept of natural selection was not generally accepted, until the rise of genetics in the mid-twentieth century. The biological literature of the period is full of papers speculating about the adaptive significance of various structures, the probable course of evolution in the plant and animal kingdoms, and so-called ‘evolutionary trees’ showing phylogenetic relationships (Fig. 5.1).

Some of this work is of lasting interest, but there was a depressing tendency in the later years of the period for armchair biologists to produce highly speculative theories, and there was a lack of critical experiment with living material. Towards the end of the century, however, there were signs of an increasing interest in the possibility of using experiments for the investigation of evolutionary problems.

Experimental investigation of evolution

A good example of this change in climate is provided by the controversy that enlivened the pages of Nature and the editorials of The Gardeners’ Chronicle in 1895. At a discussion meeting of the Royal Society, the Director of the Royal Botanic Gardens, Kew, W. T. Thiselton-Dyer, had shown specimens of the ‘feral’ type and cultivated variants of what he called Cineraria cruenta (now called Pericallis cruenta) – the gardener's Cineraria – and an extended account of his ideas was printed in Nature (1895). He suggested, as befitted an ardent and orthodox disciple of Darwin, that, as far as was known, the garden Cineraria was derived from Senecio (Cineraria) cruentus from the Canary Islands ‘by the accumulation of small differences’. Bateson (1895a, b) responded to this view in a lengthy letter to the editor, questioning the assertions of Thiselton-Dyer. Bateson concluded, after a study of the literature, that modern Cinerarias arose from hybridisation between several distinct species, that selection was practised on variable hybrid progeny, and that ‘sports’ may have been important, as well as subsequent improvements as a consequence of the selection of small-scale variation.

The processes of allopatric speciation discussed in Chapter 13 ‘may be responsible for many, if not most, speciation events’ (Soltis & Soltis, 2009). However, new species also arise abruptly in a sympatric context, often as a consequence of species hybridisation, the ‘evolutionary signature’ of which is ‘rampant’ in many groups of plants (Soltis & Soltis, 2009). In this chapter, our account will reflect the major change of outlook that has emerged in the study of abrupt speciation processes. For example, Stebbins (1971) considered that ‘polyploidy has contributed little to progressive evolution’. This view has been replaced by a radically different ‘consensus view that polyploidy is a prominent force in plant evolution’ (Wendel & Doyle, 2005).

Our present understanding of abrupt speciation is explored in three sections:

1. A. The emergence and testing of key concepts in the study of polypoidy

2. B. Recent insights into polyploidy from molecular studies

3. C. Other modes of abrupt speciation

A. The emergence and testing of key concepts in the study of polypoidy

For close on 100 years, botanists have been investigating abrupt speciation by polyploidy. Using the techniques available (and fashionable) in different periods, hypotheses about polyploid speciation have been tested. We consider the successes and limitations of the various approaches from cytogenetical and karyological methods (first developed in the nineteenth century), to chemical investigations of the 1960s. In presenting a number of case histories we review the emergence of key concepts and consider how early findings have been tested, confirmed, refined and extended by the use of an increasing range of molecular methods.

The concept of polyploidy: early cytogenetic studies

As we saw in Chapter 12, Winge's (1917) hypothesis that fertile derivatives could be derived from sterile hybrids by polyploidy was tested in a number of experimental studies.

A famous case of allopolyploidy was provided by Primula kewensis, which was discovered amongst seedlings of P. floribunda at Kew in 1899. The proposition that P. kewensis was a hybrid between P. floribunda and P. verticillata was put to the test by making the hybrid experimentally, using P. floribunda as female parent. P. kewensis was morphologically intermediate between the parental stocks and had the same chromosome number 2n = 18 (Digby, 1912; Newton & Pellew, 1929).

The study of a range of plants in the wild or on display in a Botanic Garden reveals a bewildering array of floral types. Many books describe in some detail a selection of pollination mechanisms, often discussing in highly technical botanical language the variety of floral structure. Special terms have, quite properly, been devised by botanists to enable them to write concise, accurate plant descriptions. Although the botanical literature reports extensively on the structures involved in reproduction, in our opinion, it does not pay sufficient attention to the variety of breeding systems in plants, systems of which complex structures and pollination mechanisms are only a part.

We have noted in earlier chapters the role of the internal sources of genetic variation, namely mutation and recombination, and have seen that a vast number of gametic types is theoretically possible as a consequence of these factors. Which gametes are actually brought together to form the zygotes, however, depends, to a great extent, upon the breeding system of the plant concerned.

In this chapter, as a prelude to our discussions of variation within and between species, we consider three areas:

1. A. The different breeding systems found in flowering plants.

2. B. Breeding behaviour discovered in studies of wild populations.

3. C. The evolution of breeding systems.

Our account discusses how studies of breeding behaviour have developed, and emphasises that a knowledge of the different breeding systems provides an indispensable framework for understanding the complexities of patterns and processes found in nature.

A. The different breeding systems found in flowering plants

There are three basic breeding mechanisms, which we examine in turn.

Outbreeding

In many animal groups outbreeding – crossing between different individuals – is rendered likely by sexual differentiation. In higher plants, however, separation of the sexes is the exception. According to Richards (1979), only about 4% of the flowering plants are dioecious. A slightly higher figure is reported by Renner & Ricklefs (1995), who note that c.6% of angiosperm species are dioecious, these species being distributed amongst 7% of genera. The incidence of dioecy varies in different floras. While only c.3% of British flowering plants are dioecious, much larger percentages are reported, for example, from groups of distant oceanic islands such as Hawaii (28%) and New Zealand (13%).

The endless variety of different organisms, in their beauty, complexity and diversity, gives to the biological world a special fascination. Some recognition of these different ‘kinds’ of organisms is a feature of all primitive societies, for the very good reason that humans had to know, and to distinguish, the edible from the poisonous plants, or the harmless from the dangerous animal. There is also a further dimension to the variation pattern. From folk taxonomies, the early product of human societies’ need to understand, describe and use the plants and animals around them, the modern scientific biological classifications we use today have been developed. Different ‘kinds’ of organism are arranged in a hierarchy, each higher group containing one or more members of a lower group. We now distinguish more than a quarter of a million flowering plants, and these are arranged in a hierarchical-nested classification of species in genera, genera in families, families in orders etc.

One of the main issues to be examined in this book concerns the nature of species in this hierarchical system. Anyone familiar with the vegetation of an area, or the plants in a Botanic Garden, has to face a number of questions that have puzzled biologists. Is there any objective way of delimiting species? How can one account for the different degrees of intraspecific variation found in species? Why are certain species clearly distinct, while in other cases we find a galaxy of closely similar species, often difficult to distinguish from each other? Why is it that hybridisation occurs frequently in certain groups of plants and not in others? What is the significance of the hierarchies in classification and what is the origin of these groupings?

Historically, an examination of these questions, as we shall see in Chapter 2, produced a static picture of variation, based on a belief in the fixity of species. The hierarchy of classification reflected the plan of Special Creation. However, since 1859, with the publication of On the Origin of Species, all such studies have been made in the light of Darwin's profound generalisation of evolution by natural selection: populations and species vary in time and space and are part of a continuing process of evolution.

In the second half of the nineteenth century, as Darwinism was making its impact upon biology, an interesting new approach to biological variation, especially intraspecific variation, was being examined. Instead of trying to describe variation patterns in words, the investigators, examining large samples of organisms, collected numerical data and subjected them to statistical analysis. In the following account we discuss selected themes: for comprehensive studies of the subject see Pearson & Kendall (1970), Kendall & Plackett (1977), Porter (1986) and Stigler (1986).

The first worker to study natural variation statistically, a science that became known as biometry, was probably the Belgian Quetelet (1796–1874). He wrote a famous series of letters on the subject to his pupil, the Grand Duke of Saxe-Coburg and Gotha. Later in the century, Darwin's cousin, Francis Galton (1822–1911), made notable contributions to the statistical investigation of variation and inheritance, ideas that were developed by many other investigators, especially in relation to social issues (see Bowler, 1989; Keynes, 1993; Gillham, 2001; Bulmer, 2003).

Biometrics and eugenics

While Darwin and others considered humankind to be a single species, others, for example the American biologist Louis Agassiz (1807–73), took the view that there were several species of humans. In the developing fields of anthropology and sociology, biometrical methods were increasingly employed to examine racial variation.

Darwinian concepts provoked many questions with social implications. In 1871, in his book The Descent of Man, Darwin wrote:

Darwin continues:

In 1660 Robert Sharrock, Fellow of New College, Oxford, wrote a book titled History of the Propagation and Improvement of Vegetables by the Concurrence of Art and Nature. He was concerned in its early pages to debate a live issue of the day (Bateson, 1913), namely:

The Bobarts were both professional botanists. Sharrock investigated their claim, and found ‘diverse bulbs growing as it were on the same stoole, close together, but no bulb half of the one kind, and the other half of the other’. In this age we find it hard to understand a belief in the possibility of transformation of Crocus into Gladiolus. Our reason for disbelief is partly concerned with the nature of evidence; we are not satisfied with the test for the alleged transmutation and would not have been content merely to examine the crowded underground parts. Another reason, however, relates to current ideas of the nature of species. We have a different notion of species from that of the seventeenth century.

Ray and the definition of species

It was the English naturalist John Ray (1628–1705) who was probably the first man to seek a scientific definition of species (Raven, 1950; Oswald & Preston, 2011). In his definition is an implied rejection of the sort of transmutation of species claimed by the Bobarts of Oxford, although in other passages in Ray's work he does not wholly dismiss the possibility of transmutation. For instance, he cites as reliable the case of cauliflower seed supplied by a London dealer, which on germination produced cabbage. Richard Baal, who sold the seed, was tried for fraud and ordered by the court at Westminster to refund the purchase money and pay compensation (De Beer, 1964).

The earth supports approximately five to ten million species of plants and animals (IUCN, 1980), which have been the result of three billion years of evolution, including mutation, recombination and natural selection. Humans have been in existence for some two million years. Our prehistoric ancestors were a hunter of animals and gatherer of plants. As compared to the history of man, agriculture is a very recent innovation. It was only about 12 000 years ago that women probably started collecting plants from the wild and began cultivating them, leading to the birth of agriculture. For its continued growth, however, we have to protect the basic life support systems consisting of soil, water, flora and fauna and the atmosphere.

Walter G. Rosen coined the term biodiversity (or biological diversity) in 1985, and it refers to the variety of life forms and habitats found in a defined area. The Convention on Biological Diversity (CBD) defines biodiversity as ‘variability among living organisms from all sources, including inter alia, terrestrial, marine and other aquatic ecosystems; this includes diversity within species (intraspecific) as well as among species (interspecific) and ecosystems’.

Plant genetic resources are the most precious but very vulnerable and irreplaceable natural assets of a nation. They have evolved through thousands of years of natural and human selection—both conscious and subconscious. However, certain developments of relatively recent times are posing great threats to the basic life support systems that protect rich plant diversity.

Threats to Biodiversity

The chief causes for the disappearance of plant species are as follows: