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Bayesian Modeling of the Clovis and Folsom Radiocarbon Records Indicates a 200-Year Multigenerational Transition
- Briggs Buchanan, J. David Kilby, Jason M. LaBelle, Todd A. Surovell, Jacob Holland-Lulewicz, Marcus J. Hamilton
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- American Antiquity / Volume 87 / Issue 3 / July 2022
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- 28 February 2022, pp. 567-580
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- July 2022
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An enduring problem in North American archaeology concerns the nature of the transition between the Clovis and Folsom Paleoindian complexes in the West. Traditional models indicate a temporal hiatus between the two complexes implying that Folsom was a population replacement for Clovis. Alternatively, if Folsom was an innovation that occurred within Clovis populations and subsequently spread, we would expect to see a temporal overlap. Here, we test these hypotheses using high-quality radiocarbon dates and Bayesian statistics to infer the temporal boundaries of the complexes. We show that the Folsom complex initially appears between 12,900 and 12,740 cal BP, whereas Clovis disappears between 12,720 and12,490 cal BP. Therefore, Folsom may have appeared about 200 years before Clovis disappeared, and so the two complexes likely co-occurred in the West for nearly eight generations. This finding suggests that Folsom was a successful adaptive innovation that diffused through the western Clovis population, eventually going to fixation over multiple generations.
Sleep: the neglected life factor in adults with intellectual disabilities
- Laura Korb, David O'Regan, Jane Conley, Emma Dillon, Rachel Briggs, Ken Courtenay, Bhathika Perera
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- BJPsych Bulletin / Volume 47 / Issue 3 / June 2023
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- 23 December 2021, pp. 139-145
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- June 2023
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Sleep is vital for our physical and mental health. Studies have shown that there is a high prevalence of sleep disorders and sleep difficulties amongst adults with intellectual disabilities. Despite this, sleep is often overlooked or its disorders are considered to be difficult to treat in adults with intellectual disabilities. There is a significant amount of research and guidance on management of sleep disorders in the general population. However, the evidence base for sleep disorders in adults with intellectual disabilities is limited. In this review paper, we look at the current evidence base for sleep disorders in adults with an intellectual disability, discuss collaborative working between intellectual disabilities psychiatrists and sleep medicine specialists to manage sleep disorders, and provide recommendations for future directions.
Physicochemical Properties of Adjuvants: Values and Applications
- David Stock, Geoff Briggs
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- Weed Technology / Volume 14 / Issue 4 / December 2000
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- 20 January 2017, pp. 798-806
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The physicochemical properties of adjuvants determine their function and impact upon biological activity. Various physicochemical parameters are key to modifying both the preretention events and postretention consequences of adjuvant usage, irrespective of whether the adjuvants are tank-mix additives or built into a formulation. This paper discusses several key adjuvant parameters for a range of adjuvant chemistries alone and in mixtures. In addition, the misleading use of terms such as nonionic surfactant and hydrophile–lipophile balance is addressed. From a more coherent understanding of the parameters involved, it can be shown that there are ways of predicting the required properties of an adjuvant to solve specific delivery problems. The recognition that different problems often require quite different approaches illustrates that good adjuvants do not exist per se, only materials that should be rationally selected for specific reasons. The chemistry of the herbicide and the nature of its targets will dictate adjuvant selection criteria.
References
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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- 30 June 2016, pp 482-568
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16 - Flowering plant evolution: advances, challenges and prospects
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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- 05 June 2016
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- 30 June 2016, pp 336-381
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Summary
In a famous section of the Origin Darwin (1859) speculated on the evolution of the variety of organisms. He wrote:
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species … The limbs, divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.
(Darwin, 1859).With regard to the ‘testing’ of this model, in a letter to T. H. Huxley, dated 26 September 1857 he expressed the view that: ‘The time will come, I believe, though I shall not live to see it, when we shall have very fairly true genealogical trees of each great kingdom of Nature’ (Darwin & Seward, 1903).
The devising of phylogenetic trees
Spectacular progress has recently been made in the study of phylogenetic trees from the DNA sequences of living organisms. In order to understand these powerful techniques it is important to consider the historical context in which they developed. But first, we examine the different sorts of classifications that biologists make, and consider the extent to which they might reveal evolutionary pathways.
Classifications
Gilmour (1937, 1940, 1951; Gilmour & Walters, 1963) has stressed that different classifications have been developed for different purposes. Two types of classification may be devised. First, there are the special-purpose classifications, sometimes called artificial classifications, which are based on one or a few characters. Thus, plants may be divided into trees, shrubs and herbs, the characters height and woodiness having been chosen a priori, i.e. before the assignment to class was made. As we saw in Chapter 2, Linnaeus produced a famous classification, his so-called Sexual System based on the number of parts of the flower (the number of stamens and the number of pistils). In this classification species of different families were placed in the same group.
13 - Allopatric speciation and hybridisation
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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- 30 June 2016, pp 250-286
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In Chapter 12 we presented a simple model of gradual speciation. Two populations derived from a common ancestor and occupying different geographical areas (i.e. allopatric) pass through a period of independent change yielding derivatives that are reproductively isolated from each other. In such cases the existence of isolating mechanisms is revealed if the taxa come to occupy the same area (i.e. become sympatric).
Allopatry may arise in many different ways. A comprehensive review of evolutionary plant geography, in relation to geological, climatological and historical factors, is presented in Chapter 17. Here, we note a few of the many possibilities, including vicariance and dispersal. ‘Vicariance is the appearance of a barrier that allows fragmentation of the distribution of an ancestral species, after which the descendent species may evolve in isolation’ (Morrone, 2009). For example, geographical isolation of daughter populations may result from the destruction of land bridges (e.g. the opening of the Irish and North Seas following post-glacial sea-level changes). In the longer geological perspective, vicariance events may be the result of major geological processes, such as continental drift and associated mountain-building.
New isolated populations may also result from long-range dispersal of propagules to new territories, including ‘islands’ of different sorts, whether they be oceanic islands, isolated mountain peaks, landlocked lakes or areas associated with specialised rock types (e.g. serpentine).
To take account of the complexities of different situations likely to be important in nature, a group of different models of allopatric speciation have been devised incorporating a range of assumptions (Grant, 1971; Levin, 2001b; Rieseberg, Church & Morjan, 2003; Rieseberg & Wendel, 2004; Bomblies, 2010; Rieseberg & Blackman, 2010).
While the distribution of the daughter populations might be largely allopatric, the ranges of the two might overlap in certain areas – the so-called parapatric situation – and gene exchange might be possible in the contact zones.
Models also differ in the relative importance they attach to the effects of chance events. For instance, population establishment and development may involve founder effects and genetic drift in the independent evolution of daughter populations. Thus, Mayr (1982) considered the possibility of ‘rapid divergence of peripheral isolates or founders’ (so-called peripatric speciation) (Baldwin, 2006).
Contents
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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12 - Species and speciation: concepts and models
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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- 30 June 2016, pp 242-249
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Summary
Since the time of John Ray, whose own attempt at a definition of species we discussed in Chapter 2, there has been no universally agreed definition of ‘species’; different definitions have been devised by biologists working in different specialist fields. Thus, the word ‘species’ has different meanings for different biologists. Here, we examine five influential definitions that focus on different aspects of pattern and process in evolution. (For thorough reviews of species concepts, see Stuessy, 2009; Wilkins, 2009).
The morphological species concept
Historically, the naming, description and classification of species have been based largely upon morphological details of herbarium specimens, and to a lesser extent living material collected from wild or cultivated sources. This is supplemented by geographical and sometimes ecological information. The aim of the taxonomist is to provide a convenient general-purpose classification of the material, a classification that will serve the needs of biologists in diverse fields.
It is quite obvious that in order to communicate experimental findings to others, by word of mouth, in the literature and through databases, the experimentalist, like any other botanist, must be able to name plants unambiguously. To this end, an International Code of Botanical Nomenclature has been agreed. The development of this Code has a fascinating history (Smith, 1957). By 1900, four rival codes of practice were employed in different herbaria. Discussions of the problem occupied taxonomic sessions at International Botanical Congresses in Vienna (1905), Cambridge (1930) and Amsterdam (1935), and the successive Congresses, now at approximately 5-yearly intervals, are the occasion for continued revision of the Code. The international agreements leading to a unified Code must be recognised as a major achievement.
One meaning of the word ‘species’ is now clarified. We may say that species are convenient classificatory units defined by trained biologists using all the information available. Clearly there is a subjective element in their work, and we must therefore face the fact that there will sometimes be disagreements between taxonomists about the delimitation of particular species, but there is a very large measure of agreement, for all except ‘critical groups’, in regions where the flora has been studied for many years. In the taxonomic process, a type specimen is designated and a diagnosis provided that specifies the important distinguishing characteristics of the ‘new’ species from others of the same group (Stace, 1980; Stuessy, 1990, 2009).
9 - Pattern and process in plant populations
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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The word ‘population’, like so many other familiar terms, seems to present little difficulty until we have to define it. In statistics, the concept of population is an abstraction signifying a theoretically large assemblage of individuals from which a particular group under consideration is a sample. Most biological uses of the term, however, imply the total of organisms belonging to a particular taxonomic group (or ‘taxon’) that are found in a particular place at a particular time. Scientific research involves the testing of models. The population model of outstanding significance, in the study of variation, is quite specific, being the local ‘interbreeding group of individuals sharing a common gene pool’ (Dobzansky, 1935 in Rieger, Michaelis & Green, 1976). In earlier editions of this book, the term ‘gamodeme’ was used for such populations. The deme terminology was devised by Gilmour and associates (Gilmour & Gregor, 1939; Gilmour & Heslop-Harrison, 1954; Gilmour & Walters, 1963; Walters, 1989a, b, c). Details of the system are given in the glossary. It is with some regret that we abandon the use of this term, but must face the fact that the ‘-deme’ terminology has not been accepted or is used in ways not intended by its authors (Briggs & Block, 1981).
Population geneticists have coined other names – Mendelian or panmictic populations – for groups essentially similar to gamodemes. All these units represent idealised ‘model systems’ (see Maynard Smith, 1989; Silvertown & Lovett Doust, 1993; Silvertown & Charlesworth, 2001).
The model underlying the idealised population is the Hardy–Weinberg Law. In a population with two alleles A and a, which are selectively neutral and in which mating is at random, the expected frequencies of A and a (p and q respectively) are
(p + q)2 = p2 + 2pq + q2 = 1
This represents the ‘null’ model (Cockburn, 1991), for a large population of constant number in a diploid organism, with sexual reproduction, non-overlapping generations, the same allele frequencies in male and female, no gene flow or migration and no selection operating. In such a situation the relative frequencies of the alleles will not change from generation to generation.
Clearly, in natural populations ‘in the wild’, many factors will cause deviations from the Hardy–Weinberg equilibrium, and these have been modelled by population geneticists (Silvertown & Lovett Doust, 1993; Silvertown & Charlesworth, 2001).
6 - DNA: towards an understanding of heredity and molecular evolution
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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- 30 June 2016, pp 74-97
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Summary
A revolution in biology is in progress – no less of a revolution than that caused by Darwin's theory of evolution by natural selection. The achievements of molecular biology have been so great and the progress so rapid that an enormous body of information now exists. In this chapter we focus on several key issues that are important in understanding later chapters.
• How the properties of nucleic acids provide the basis of heredity.
• Why molecular insights are changing our view of the mechanisms of evolution.
• How intensive molecular genetic studies of model flowering plant species (particularly Arabidopsis thaliana and a variety of crop plants) are revolutionising studies of many areas of plant biology.
• As the results of molecular investigations into these model and representative organisms for all the major groups are becoming available, do these point to a common origin for life on Earth? And what does the comparison of genomes of different groups indicate about evolutionary changes in gene order, chromosome number, the fate of duplicated DNA segments etc.?
• Finally, what are the key molecular techniques for studying pattern and processes in populations and species, and what properties of DNA have been exploited to allow, through sequencing and computational methods, spectacular insights into the evolution of the flowering plant branches of the Tree of Life?
Obviously, in this short book, we must be content to provide an outline of the ways in which this new knowledge of molecular biology illuminates these key questions. For comprehensive accounts of historical and recent developments in molecular biology, Clark (2005), Allison (2007), Craig et al. (2010) and Watson et al. (2014) should be consulted. For detailed accounts of our present understanding of the molecular biology of plants, see Smith et al. (2010), Jones et al. (2012) and Grotewold, Chappell & Kellogg (2015).
DNA: its structure and properties
The early geneticists visualised genes as ‘beads threaded on a string’. In 1944 Schrödinger, in his fascinating book What Is Life?, suggested that genes could be complex organic molecules in which endless possible variations in detailed atomic structure could be responsible for codes specifying the stages of development. Since the1950s, spectacular progress has been made in our understanding of the structure of the hereditary materials and how they work.
4 - Early work on the basis of individual variation
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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Summary
In the last chapter we saw how the early biometricians found great difficulty in analysing some of their data because they were unable to decide which part of the variation had a genetic basis and which part was environmentally induced. For animal studies it was Galton (1876) who appreciated the unique value of twins in investigations of the relative roles of nature and nurture in the development of the individual. To study genetic and environmental effects in plants, specimens selected for comparison may be cultivated under a standard set of environmental conditions. Experiments, both historical and recent, have been performed on the assumption that residual differences between plants of the same species, collected from the same or different habitats and grown under such standard conditions, might be considered to have a genetic basis. What follows is a brief survey of early studies. In Chapter 8 we will discuss in some detail the design and interpretation of garden experiments.
It is very interesting to see how cultivation techniques have developed as methods of analysing variation in plants. Experimental cultivation of plants undoubtedly arose as an adjunct to gardening and horticulture, and in Chapter 2 we saw how Ray, collecting the striking prostrate variant of Geranium sanguineum from Walney Island, demonstrated its constancy by cultivating plants in different gardens. The most valuable of these experimental tests were undoubtedly those of a comparative nature. For instance, Mendel cultivated two variants of the Lesser Celandine Ficara verna (Ranunculus ficaria), which he called Ficaria calthaefolia and F. ranunculoides, and reported to Dr von Niessl that each remained distinct (Bateson, 1909).
In a paper of quite remarkable scope, Langlet (1971) has reviewed the extent to which foresters in the eighteenth and nineteenth centuries were using experimental cultivation to study adaptive variation in some of the widespread forest trees of Europe. He cites, for example, the neglected (and largely unpublished) work of Duhamel du Monceau, Inspector-General of the French Navy, who, around the time when Linnaeus published his Species plantarum (1753), brought together an impressive collection of samples of Scots Pine, Pinus sylvestris, from Russia, the Baltic countries, Scotland and Central Europe, and established the first experimental provenance tests for any wild plant.
18 - The evolutionary impact of human activities
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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- 30 June 2016, pp 411-438
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This chapter examines the evolutionary influences of human activities, and considers the proposition that many species are threatened with extinction.
Humans: as animals practising extreme niche construction
Anatomically modern humans arose in Africa over 100 000 years ago, and by 40 000 years ago they were perhaps fully human in anatomy, behaviour and language (Diamond, 1992). Increasingly, as humans have migrated to almost all parts of the globe, they have influenced, modified, managed or transformed all the Earth's natural ecosystems by practising what has been termed ‘niche construction’.
Leland (2002) notes that, traditionally, ‘adaptation typically is regarded as a process by which natural selection molds organisms to fit a pre-established environmental template … yet in varying degrees, organisms choose their own habitats, choose and consume resources, generate detritus, construct important components of their own environments … destroy other components and construct environments for their offspring. Thus, organisms not only adapt to their environments but in part also construct them.’ In the case of humans, niche construction may be seen, therefore, as an evolutionary strategy that aims to maximise survival and reproductive success.
Human environmental activities through the development of cultural practices have many intentional effects. As Western (2001) observes, ‘the most universal and ancient features of our “humanscapes” arise from a conscious strategy to improve food supplies, provisions, safety, and comfort – or perhaps to create landscapes we prefer, given our savanna ancestry’. He points out, however, that there are many unintended side effects of human activities that lead to a multiplicity of consequences at every spatial scale, and his list includes: habitat and species loss; loss of keystone species; major changes to ecosystems resulting in reduced ecotones; truncated ecological gradients and changes to soils and accelerated erosion; introductions of invasive non-native species and diseases; side effects of fertilisers, pesticides, herbicides; over-harvesting of natural resources; nutrient leaching and eutrophication; pollution of soils, water and air; and ‘global changes to the lithosphere, hydrosphere, atmosphere and climate’.
Human impact on the environment
The formal geological term for the last 11 700 BP years is the ‘Holocene’, but human niche construction has become so universal and dominating that the present era has been called the ‘Anthropocene’ (Ruddiman, 2013). There has been a great deal of debate about when this era might have begun.
17 - Historical biogeography
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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One of the areas of plant evolutionary studies most enriched and stimulated by advances in molecular phylogeny is historical plant biogeography. ‘Earth history has profoundly influenced the geographic ranges of species … evolutionary histories of areas and lineages are tightly coupled’ (Ree & Smith, 2008). This long-standing botanical discipline has now been invigorated by advances in plate tectonics; the investigation of patterns and processes of dispersal (including the increasing use of analytical models); and the development of phylogeography (Avise, 2000). Phylogeography is an offshoot of the cladistic approaches devised by Hennig. One of the key advances was the realisation that phylogenetic diagrams, which have taxonomic entities at their branch tips, can be overlaid, or considered, together with geographical information, such as country of origin of each taxon, or geographical origins can be substituted for taxon names. Two classes of branching structures are then available for analysis, one with taxa and another, of the same data set, but with geographical details. Also, information on timeframes can be included. These are particularly revealing when estimates employing relaxed molecular clocks are calibrated against fossil evidence (Wikström, Savolainen & Chase, 2007; Magallón & Castillo, 2009). In addition, molecular evidence can be combined with information on the environments and ecosystems of the past.
These approaches, together with new advances in our understanding of Earth's history, have shed new light on many questions that have long intrigued botanists about the role of natural selection and chance events in dispersal, speciation, adaptive radiation, hybridisation and extinction. To appreciate the significant progress that has been made, a brief historical account of several puzzling biogeographical phenomena is helpful (see Lomolino et al., 2005). Here we concentrate on a number of key issues, as they influence plant evolution.
The Deluge and Noah's Ark
In a fascinating book, Browne (1983) reveals that, in the seventeenth century many scholars, convinced of the literal truth of the story of Noah's Ark, raised a number of questions. How did all the animals fit into the Ark? How were they fed, housed and the dung removed? In search of answers, the German Jesuit priest Kircher produced a representation of the Ark as a long rectangular box-like structure of three decks, shaped more like a modern cruise liner than the traditional pyramidal structure commonly reproduced in children's toys and in works of art.
10 - Pattern and process: factors interacting with natural selection
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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Chance has profound effects
It is important first to differentiate between chance events and those that involve natural selection. Ridley (1996) provides an elegant example. He imagines a long line of loaded packhorses toiling, in single file, up a long precipitous mountain track. Clearly, elements of natural selection might be important, as some animals might be inherently more nimble and survive the journey in greater numbers than clumsy horses that fall to their death. However, in his example, the hazards of the alpine journey also include massive falling rocks that sweep horses to their death into the ravine below. Animals are not genetically predisposed to avoid the sudden chance arrival of massive falling rocks. The important point to emphasise is that horses swept off the track by such chance events are likely to be a random sample from the original population that started out on this hazardous journey. Furthermore, a new mutant horse – more sure footed than the average – might by chance be carried away with others of lesser fitness. The potential of this mutation is, therefore, not realised. Chance determines which, of a group of horses, survive the falling rocks.
Chance effects – so-called stochastic events – play a major role in all evolutionary processes, not only in chromosomal and DNA changes, but also in hybridisation, speciation and extinction. In population biology, for instance, chance plays a huge role in which flowers are pollinated by animals/wind, and which seeds/fruits are dispersed to ‘safe sites’ allowing successful germination. Indeed, the effect of chance is highly significant in all the processes of population biology: dispersal, establishment, growth and reproduction.
Random genetic change in populations is known as genetic drift or in the older literature as the ‘Sewall Wright’ effect. ‘Alleles may be fixed or lost, especially from small populations, because of random sampling errors and without regard to their adaptive values’ (King, Stansfield & Mulligan, 2006). Thus, chance effects are particularly important if the population is reduced by a bottleneck effect to a very small size (Fig. 10.1). Wright (1931) was the first to point out that, in such small populations, irregular random fluctuations in gene frequency occur that may result in the fixation or loss of one or more alleles. Such chance effects may be very important also in the declining populations of endangered species.
20 - Conservation: from protection to restoration and beyond
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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It is clear that many thousands of species are at present vulnerable or endangered by human activities (Frankel & Soulé, 1981; Primack, 1993, 2010; Given, 1994; Meffe & Carroll, 1994; Frankel, Brown & Burdon, 1995). Broadly, two conservation options are available. Plants may either be conserved ex situ, in such places as Botanic Gardens, or in situ in their native habitats. Here we consider, briefly, the early history of conservation, and how theory and practice have changed – from protection, management, to reintroduction/restoration and creative conservation.
Ex situ conservation
Endangered species, including those at the very edge of extinction, are often conserved ex situ in Botanic Gardens and arboreta (Briggs, 2009; Hardwick et al., 2011), of which there now more than 2700 worldwide (Ali & Trivedi, 2011). The scale of the holdings of Botanic Gardens is impressive: perhaps 25–30% of all vascular plants are represented in the collections (Wyse Jackson & Sutherland, 2000). Some gardens hold very large general collections (e.g. Kew with c.10% of the world's plants), while others have specialist collections (e.g. The Arnold Arboretum, Boston, USA, grows several hundred species of temperate tree). Botanic Gardens Conservation International (BGCI), Richmond, England, organises and coordinates the conservation efforts of gardens (www.bgci.org/). Regrettably, most Botanic Gardens are located in temperate areas of the world, and it is costly to grow tropical plants in the glasshouses of Europe and North America. However, there are some notable Botanic Gardens in the tropics (Heywood & Wyse Jackson, 1991), and, if the number could be increased, it would be possible to conserve many tropical species of plants cheaply out of doors, and provide important centres for economic development and exploitation of plant biodiversity.
Other specialist gardens also play a key role in ex situ conservation. In England, the National Trust has now relocated its conservation activities from Knightshayes Court, Devon. It was necessary to move to another site because of an outbreak of Sudden Oak Death (caused by the fungus Phytophthora ramorum). The new ex situ facilities are designed to secure the future of the rare and endangered plant species that grow in the Trust's ‘200 gardens, 100 landscape parks’ and ‘in the many wild places it manages’ (Morris, 2012).
Index
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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Preface to the Fourth Edition
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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- Plant Variation and Evolution
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Summary
In writing the earlier editions of Plant Variation and Evolution, with my great friend Max Walters (1920–2005), our approach to this complex subject was clearly set out in the preface to the Third Edition. ‘When it was first proposed to establish laboratories at Cambridge, Todhunter, the mathematician, objected that it was unnecessary for students to see experiments performed, since the results could be vouched for by their teachers, all of them of the highest character, and many of them clergymen of the Church of England’ (Bertrand Russell, 1931). While Russell's mischievously anti-clerical comments do not entirely reflect the views of Todhunter (Todhunter, 1873; Macfarlance, 1916), they do provoke us to take a critical look at the way scientific advances are made, in every historical period, through questioning the opinions of various ‘authorities’. In many texts on evolution a judicious mixture of concepts, mathematical ideas and the results of laboratory and field experiments are combined in an elaborate pastiche to provide a more or less complete edifice. Perhaps one or two areas of uncertainty may be indicated, but the general impression is of a house well built, but awaiting the placing of the last few roof-tiles. Conversations with research biologists, however, quickly reveal a different picture. While the broad outlines of evolution are supported by an increasing body of evidence, almost nothing is completely settled: current views represent a provisional framework, and even some parts of the subject, long held to be clarified, are suddenly overturned by new discoveries. Teaching experience reinforces our view that students of science should be shown the way in which, slowly and painstakingly, our present partial pictures have been built up, how and to what extent they are testable by experiment and observation, and in what way they remain vague or defective. A healthy scepticism in the face of the complexities of organic evolution is the best guarantee of real progress in understanding its patterns and processes.
The aim of this new edition is to provide, as before, an authoritative introductory university text, while at the same time satisfying the general reader with a real interest in the subject, showing how the study of variation and evolution of flowering plants has developed over the last 400 years. This development has been increasingly scientific, leading to the realisation of the crucial importance of hypothesis and experiment.
Acknowledgements
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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Note on names of plants
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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Frontmatter
- David Briggs, University of Cambridge, S. Max Walters, University of Cambridge Botanic Garden
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