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Biological diversity or biodiversity has many aspects which include genetic,
biochemical, physiological, structural, taxonomic and ecological components.
Summary of Section 1.1
Diversity at the genetic and molecular levels is often hidden (with no visible,
e.g. structural, counterpart) but may be linked to ecological or physiological diversity. Populations of common bent-grass tolerant of one or more heavy metals and growing on toxic soils such as mine spoil-heaps provide one example of hidden diversity.
Summary of Section 1.1
The uniformity of life is reflected in a common core of properties at the level of whole organisms and of cells and molecules.
Summary of Section 1.1
Biologists may ask two sorts of questions which require different kinds of explanations. ‘How’ questions (usually asked by ecologists, physiologists and biochemists) require a proximate explanation; ‘why’ questions (usually asked by evolutionary biologists) require an ultimate explanation in terms of genetic properties and evolutionary processes.
Summary of Section 1.1
Summary of Section 1.2
Summary of Section 1.2
Most taxonomists aim to classify organisms in a natural way that reflects the common origin of any named group from a single common ancestor and can be used to construct family trees or phylogenies which reflect evolutionary relationships.
Summary of Section 1.2
Species are the smallest taxa to be universally recognized and are given a unique, Latin binomial name. Their biological definition relates to the capacity to interbreed to produce fertile offspring.
Summary of Section 1.2
The biological species definition is often impossible to apply because breeding tests cannot be carried out. Hence structural and other characteristics are commonly used to delimit species. Among the problems that arise are: (a) identical appearance of species, sometimes as a result of convergent evolution; (b) disagreements among taxonomists (splitters and lumpers); (c) lack of structural markers in prokaryotes, and (d) gene transfer between species.
Summary of Section 1.2
The taxonomic hierarchy groups organisms into progressively larger units: genus, family, order, class, phylum (or division), kingdom and domain. Each unit should be monophyletic. The higher the taxonomic category shared by two organisms, the further back in time is their common ancestor.
Summary of Section 1.2
Characters that are used to estimate relatedness for higher taxonomic units should reflect deep, underlying similarities (homologies) of structure or physiology or molecules and avoid superficial similarities that have arisen through convergent evolution. Fossil evidence provides the best method of determining the time at which groups diverged in the past but is unavailable for many groups. Molecular sequence comparisons are increasingly used to delimit higher taxonomic groups and determine their evolutionary relationships.
Summary of Section 1.2
The deepest division of living organisms (the highest taxonomic grouping) is
into three domains: Bacteria, Archaea and Eukarya. These domains separated
from a universal common ancestor shortly after the origin of life, about
4000 Ma ago.
Summary of Section 1.3
Domains and, in particular, the existence of Archaea were first recognized on
the basis of molecular sequence comparisons of a gene for rRNA. Other
distinguishing characteristics relate to cell structure, chemical linkages in
membrane lipids, packaging of DNA and the protein synthesis machinery.
Summary of Section 1.3
Bacteria and Archaea are prokaryotes (lacking membrane-bound organelles) whereas Eukarya attain a higher grade of organization with membrane-bound organelles, that is, a nucleus and, usually, mitochondria and/or chloroplasts. The first eukaryotic cells may have evolved from the Archaea (the Archaea hypothesis) or may have arisen through a fusion between a bacterial and an archaeon cell (the fusion hypothesis). Mitochondria and chloroplasts were obtained later from Bacteria, by endosymbiosis.
Summary of Section 1.3
Summary of Section 1.4
The degree of difference necessary to separate organisms into kingdoms is still disputed and so, therefore, is the number of kingdoms. Here we recognize seven kingdoms, three prokaryotic and four eukaryotic. More prokaryotic kingdoms are likely to be recognized in the future as further molecular sequence data are obtained.
Summary of Section 1.4
In the prokaryote domain Archaea, two kingdoms are usually recognized on the basis of rRNA sequence comparisons: Crenarchaeota and Euryarchaeota. The most ancient lineages occur within the Crenarchaeota and include microbes that live in hot, acid conditions (such as volcanic springs) and have a sulfur-dependent metabolism. Euryarchaeota include extreme halophiles
and methanogens (methane producers). All archaeons appear to be structurally simple unicells and, although the majority of known species occur in extreme habitats, the use of archaeon rRNA probes is revealing their widespread occurrence (especially of crenarchaeotes) in normal habitats such as seawater.
Summary of Section 1.4
The domain Bacteria with its single kingdom shows a greater range of structural and metabolic diversity than the Archaea. As in the Archaea, some of the oldest lineages are thermophilic. The ability to use light energy (photoautotrophy), multicellularity and spore production have evolved in several lineages. Some unicellular species from animal guts can be very large. Gene transfer is probably responsible for much of the metabolic diversity in bacteria.
Summary of Section 1.4
Summary of Section 1.5
Non-vascular plants or bryophytes. These plants were the first to evolve from aquatic algal ancestors and, although generally regarded as rather poorly adapted to land, mosses in particular are diverse and abundant. Bog-mosses (Sphagnum spp.) dominate acid peat bog vegetation and their dead remains are the main component of peat; some mosses tolerate severe cold and are dominant in polar regions and above the treeline on mountains; and the moist western parts of the British Isles probably has the richest bryophyte flora in Europe. The distinguishing features of bryophytes are summarized below:
Summary of Section 1.5
They have no woody cells, that is, cells in which the cell walls are stiffened and strengthened by deposition of the polymer lignin. As a result, bryophytes are all low-growing and have either a leafy shoot (mosses and some liverworts) or a flat thallus (hornworts and some liverworts), with rhizoids for attachment.
Summary of Section 1.5
The dominant generation, that is, the plant you see, is the haploid gametophyte. It produces flagellated male gametes (sperm) which swim over the moist surface and fuse with much larger female gametes (egg cells) which remain embedded in parent tissue. The diploid sporophyte develops from a zygote on the gametophyte and is visible as a stalk and spore-containing capsule (Figue 1.32).
Summary of Section 1.5
Vascular plants or tracheophytes. All plants except bryophytes are described as vascular plants because they possess cells (forming the vascular system) specialized for the transport of water and dissolved materials around the plant. The leaf veins are part of this system. The water-conducting cells have tough lignified walls (see (1) above) and, together with thick-walled lignified fibre cells they perform a support function, allowing plants to reach a large size. In contrast to bryophytes, the dominant generation in all modern vascular plants is the diploid sporophyte. The haploid gametophyte is much smaller and structurally simpler; in later-evolved groups it is greatly reduced.
Summary of Section 1.5
Seedless vascular plants are an artificial group comprising five phyla, of which the only ones you are likely to have seen are ferns and horsetails (Sphenophyta). The common horsetail (Equisetum arvense) is a weed of gardens and disturbed land. Apart from the ferns, which are relicts of a glorious past, all other phyla in this group contain relatively few modern species: the dense forests of the Carboniferous and Permian periods were dominated by tree lycopods and sphenophytes that are now a major component of coal deposits.
Members of this group release haploid spores from a variety of specialized structures (e.g. see Figure 1.33) and the spores germinate to give an independent, usually small and delicate gametophyte, where sexual reproduction occurs. Independently within lycopods and ferns, some plants have evolved heterospory, which means that there are two types of spores: large megaspores which develop into a female gametophyte, producing egg cells; and small microspores which develop into a male gametophyte, producing male gametes.
Summary of Section 1.5
The evolution of seeds, which are the defining characteristic of seed plants, is regarded as a major advance in the adaptation of plants to life on land. A seed is a structure borne on the diploid sporophyte and contains a diploid embryo plus stored food and protective seed coats. It functions to disperse, protect and nourish the sporophyte embryo. Independent gametophytes do not occur in seed plants and, instead, there is an extreme form of heterospory (see (3) above), with the megaspore retained and developing on the parent sporophyte within an ovule. The microspores are also retained and male gametophytes are dispersed from the parent sporophyte in the form of thickwalled pollen grains, which germinate to produce a tube through which a male gamete can reach the egg cell. A simplified version of sexual reproduction in angiosperms is shown in Figure 1.34. The end result, however, is that all requirements for a moist habitat in which sperm swim to egg cells are abolished.
Summary of Section 1.5
Gymnosperms include four phyla of seed plants. Most are trees and they dominated vegetation in the Jurassic and early Cretaceous but are today represented by fewer than 1000 species. So this group is another with a glorious past and, like the seedless vascular plants, contributed substantially to coal deposits. Conifers are the best known group and are also ecologically important, dominating large areas of northern boreal forest as well as being widely planted for timber in Europe. Ovules and pollen are produced in flower-like structures (which are not true flowers) and the seeds are produced in cones. The name gymnosperm means ‘naked seeds’ and reflects the fact that the seeds are not surrounded by layers of sporophyte tissue as they are in angiosperms.
Summary of Section 1.5
Flowering plants or angiosperms are the crowning glory of the plant kingdom and largely displaced gymnosperms during the Cretaceous period. They are more diverse in terms of species, structure and ecology than any other plant group and central to their success was the evolution of the following characteristics:

structures (flowers) that facilitate sexual reproduction and allow animals, particularly insects, to transport pollen between flowers;

very well protected and nourished seeds that are surrounded by maternal tissues which form a fruit. Seeds are usually dispersed within a fruit and animals may be involved in dispersal;

a wide array of chemical compounds known as secondary products, which include alkaloids such as nicotine. Other plant phyla make secondary products but they are less diverse than in angiosperms. The main function of these chemicals appears to be deterrence of animal grazers which, in turn, have evolved mechanisms for detoxifying or tolerating the deterrents.
All three of these characteristics have involved coevolution between animals and plants, that is, a process in which two species exert selective forces on each other so that both undergo evolutionary change. You should therefore be able to appreciate that much of the diversity seen in angiosperms, and especially the variety and beauty of flowers, have their origins in animalplant coevolution.
Summary of Section 1.5
There are four eukaryotic kingdoms. Protoctista are the most ancient kingdom (that is, first to evolve) and include all the unicellular eukaryotes and any multicellular descendants which are neither plants, animals nor fungi. They show enormous diversity of structure and mode of life and are often classified into several kingdoms. The majority possess flagella or cilia at some stage of their life cycle.
Summary of Section 1.5
Protoctist lineages that evolved before the endosymbiotic acquisition of mitochondria are called the Archezoa. All are confined to habitats where there
is little or no oxygen and they include the gut parasite Giardia lamblia
Summary of Section 1.5
Protoctists that acquired chloroplasts by endosymbiosis are called algae. Two phyla acquired chloroplasts from a cyanobacterium (primary endosymbiosis) and other phyla acquired chloroplasts later from algal (eukaryotic) endosymbionts (secondary endosymbiosis).
Summary of Section 1.5
Plants (kingdom Plantae) are multicellular, eukaryotic photosynthesizers adapted primarily to life on land. They usually have sexual reproduction, develop from a diploid embryo and show an alternation of multicellular haploid and diploid generations (gametophyte and sporophyte, respectively). Mature plants are non-motile and have cells with rigid walls strengthened by cellulose.
Summary of Section 1.5
Plants evolved from green algae and, except in some bryophytes, have an upright leafy shoot (the photosynthetic part) and non-green underground parts for anchorage and absorption (roots, rhizoids or modified shoots). Plant diversification and classification relate primarily to reproductive structures, life cycles and support or transport tissues.
Summary of Section 1.5
Twelve plant phyla are recognized here. The oldest (i.e. first to evolve) are the non-vascular plants or bryophytes, whose three phyla have no woody or vascular conducting tissues and in which the haploid gametophyte is the dominant generation. The remaining nine phyla (vascular plants or tracheophytes) develop woody, including vascular, tissues, and are able to attain much larger sizes. The sporophyte generation is dominant and the gametophyte shows a progressive reduction from the earliest to the most recently evolved and diverse tracheophytes, the flowering plants.
Summary of Section 1.5
The fungi are eukaryotic, multicellular or coenocytic heterotrophs which feed by absorption and, apart from yeasts, have a filamentous, hyphal construction. Hyphae have rigid walls strengthened by chitin, and reproduction involves spores as dispersal agents. For most of their life cycle, fungi are haploid and, like plants, they diversified mainly on land.
Summary of Section 1.5
Fungi probably evolved from flagellated protoctist ancestors and are classified, mainly on the basis of their life cycle and reproductive structures, into four phyla. The earliest to evolve (chytrids) produce flagellated spores but all other fungi lack flagella.
Summary of Section 1.5
Animals are multicellular, eukaryotic heterotrophs that feed mainly by consuming particulate organic matter. Their cells lack a rigid wall and may become highly specialized. Most animals are diploid and have sexual reproduction with embryo development into a blastula. Diversification has occurred mainly in the sea.
Summary of Section 1.5
Animals probably originated 900–1200 Ma ago. They are classified on the basis of the arrangement of body tissues and organs, with each of the 35 (or so) phyla having a distinctive body plan which was established before or during the Cambrian.
Summary of Section 1.5
The earliest animals include sponges and the radially symmetrical Cnidaria (corals and jellyfish). Other animals are bilaterally symmetrical (the Bilateria), the majority of species occurring in the arthropod phylum, mainly in the class Insecta.
Summary of Section 1.5
Biological innovations and environmental changes acted together to promote diversification in protoctists.
Summary of Section 2.1
When protoctists first appeared and started to diversify, the atmosphere was
still anoxic despite the innovation of oxygenic photosynthesis some 1000 Ma
earlier. Cyanobacteria dominated the oceans but amphiaerobic Bacteria were
probably also present.
Summary of Section 2.1
The nature of early protoctists and which of their characteristics are now
present in archezoans remains uncertain. They may have possessed
hydrogenosomes (acquired by endosymbiosis) which later evolved into
mitochondria. Or they may have contained no organelles except nuclei.
Summary of Section 2.1
Acquisition of mitochondria allowed early protoctists to exploit the increasing number of aerobic habitats, improved energy metabolism and increased metabolic flexibility. At a later stage of evolution some protoctists recolonized certain anaerobic habitats and lost mitochondria.
Summary of Section 2.2
Defence systems against oxygen toxicity via free radical formation evolved as atmospheric oxygen levels increased.
Summary of Section 2.2
The acquisition of chloroplasts allowed a switch to autotrophy and access to light as an abundant energy source. It also provided oxygen for aerobic metabolism and new biosynthetic abilities (e.g. starch synthesis).
Summary of Section 2.2
Because of the need to stay in the light, many open-water autotrophs remained as small unicells but some shallow-water species evolved into large, anchored multicellular types.
Summary of Section 2.2
Osmotrophy and phagocytosis are the two main methods of feeding in heterotrophic protoctists.
Summary of Section 2.3
Osmotrophy involves absorption of soluble organic molecules across the cell surface and, although it may supplement feeding in many species, is used exclusively only by internal parasites and by groups such as the oomycetes and slime nets which break down solid food by extracellular digestion before absorbing the soluble products. No special type of movement is required for osmotrophy.
Summary of Section 2.3
Phagocytosis involves engulfment of food particles (endocytosis) and digestion within the cell inside food vacuoles. It requires a flexible cell surface which, in amoebae, occupies the whole surface. Amoebae feed and move by amoeboid movement using pseudopodia.
Summary of Section 2.3
Other phagotrophs use flagella or cilia either to move towards prey or to create feeding currents. All have a definite shape which is produced by stiffening of the outer cytoplasm through development of the cytoskeleton. An unstiffened pocket (the gullet) may be used for phagocytic feeding.
Summary of Section 2.3
Some later-evolved phyla (e.g. forams, actinopods) have lost flagella and become secondarily amoeboid but have rigid outer coverings. Their pseudopodia protrude through openings in the covering and are used for feeding, active swimming or walking.
Summary of Section 2.3
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