Stephen Boyden[*]

Uniformities
Biodiversity
    Food intake in plants
    Food intake in animals
    Reproduction
Comment
Further reading

We have only to look around us anywhere in the natural environment to be struck by the amazing diversity among living organisms – diversity in habitat, size, shape and colour; and, among animals, diversity in means of locomotion and patterns of behaviour. Animal species also differ widely in their food sources, and in their resistance to heat, cold, dryness and wetness; and some are at home on the land, some in the water, some in the soil, and some in the air. Each is adapted, in its inheritable characteristics, to its own particular ecological niche.

It is impossible to state precisely how many different kinds of life now exist on Earth, but it has been roughly estimated that there are some 7 to 15 million species of living organisms (excluding bacteria, fungi and viruses). Of these, around 400 000 are plant species and around 50 000 are vertebrates.

Uniformities

Underlying all this diversity, however, there are some remarkable and essential uniformities. One of these fundamental universals is the fact that all forms of life depend on a continual supply of energy. Except in the case of a small proportion of microbial organisms, this energy was initially captured from sunlight by photosynthesis in green plants, converted into chemical energy and stored in organic molecules.

There is also a basic similarity in the complex chemical processes by which this energy is used in living cells, be they animal, plant or microbial. For example, a common denominator at the molecular level is adenosine triphosphate (ATP) which, in every kind of living organism, plays an essential part in the chemical reactions involved in the storage of energy and its eventual release ­ for example, in the synthesis of complex molecules or the contraction of muscles.

The organic molecules of which organisms are made up also share the same basic characteristics right across the board. These molecules fall into four classes: carbohydrates, proteins, lipids (fats) and nucleic acids. However, within these four main classes there is immense diversity. In the case of proteins, for example, every species of animal and plant has very many different proteins with different functions ­ for example, as enzymes, hormones, or playing specific structural roles ­ and the proteins of each species are distinguishable from those of all other species. Indeed, subtle differences exist in the structure of proteins between individual members of the same species. This is why skin, or other organs, can only rarely be successfully grafted from one individual to another (except in the case of identical twins), unless special steps are taken to depress the immune system of the recipient. The rejection of the tissue graft from another individual is due to the fact that the immune system recognises the cells of the donor as "foreign", and consequently sets up an inflammatory response which ultimately destroys them.

Genetic inheritance - Another universal is the fact that all life forms, with the exception of sub-microscopic viruses, have a cellular structure ­ ranging from single-celled organisms, like bacteria and amoebae, to the large multicellular plants and animals, which may be made up of hundreds of billions of separate cells with many different functions. But every one of these multicellular animals, and most of the multicellular plants, begin life as a single cell, formed by the union of two cells ­ the ovum and the sperm.

This leads us to note another universal among living organisms, and that is the means by which genetic information is passed from parents to their progeny, providing the instructions that result in the new organisms developing and functioning as members of the species to which their parents belong, and that determine all their other inherited characteristics.

The essential agent in this process is the genetic material of the cell, deoxyribonucleic acid (DNA). In animal and plant cells chains of DNA are located in the cell nucleus, and in this situation (and, in some laboratory situations, outside the living cell) DNA is itself capable of self replication. It contains, in coded form, most of the information necessary for the formation of the new individual.

The inheritable characteristics of any organism are determined by the arrangement of four nucleotides (cytosine, thymine, adenine, and guanine) in the genes, which are discrete areas or regions on the DNA chains.

Sexual reproduction - Almost universal among plants and animals is the involvement of the sexual process at some stage in the reproductive cycle. This consists of the fusion of two separate cells (gametes) which, in the case of multicellular organisms, usually come from two different individuals (but in some species, from different parts of the same individual). In some very simple organisms the two gametes may be identical, but in all higher species of plants and animals they are clearly different. One, the male gamete, or sperm, is motile. The other, the female gamete, or ovum, is larger and sessile. The fusion of the two cells results in the new fertilised egg, or zygote, which contains twice the amount of DNA contained in each of the gametes. However, there is a mechanism, known as meiosis, by which the amount of genetic material is halved at a specific stage in the formation of the gametes as a consequence of which is that the total amount of genetic material does not double continually at each generation.

The fertilised egg thus contains genetic material from two different sources (i.e. from both parents). Since it is very unlikely that the material from each parent will be identical, it follows that the offspring will be different, even if only slightly, from either parent.

As a result of this sexual process, the genetic material in a population is being constantly reshuffled. From the evolutionary point of view, the importance of sexual reproduction lies in the fact that, unlike in asexual reproduction, the precise genetic make-up of the new individual is different from that of either parent. This has the effect of maximising the number of genetic combinations in the population, thereby enhancing the potential of the population to adapt to environmental change though natural selection.

Gene mutations - While the mechanism of sexual reproduction explains the continual rearranging of genetic material in populations, it does not explain how entirely new genetic characteristics come into existence. The process responsible for such change, known as mutation, involves a chemical change in a gene that is perpetuated when the gene replicates in cell division. The change then affects the particular characteristic of the organism for which the gene is responsible. Mutations are normally rare events, but their frequency can be increased by certain physical and chemical agents, such as ultraviolet light, radioactive radiation and mustard gas.

The great majority of mutations are deleterious, so that cells that carry them do not survive. Occasionally, however, a mutation arises which, by chance, increases the likelihood of the organism surviving and successfully reproducing in the habitat in which it lives.

Top

Biodiversity

Despite these fundamental uniformities, the processes of evolution have given rise to an amazing variety of structural forms, physiological mechanisms and ways of life. It is not possible here to attempt even a summary of all the kinds of adaptations to different habitats found in the plant and animal kingdoms. Let us simply use a few examples to give some idea of the extent of diversity that exists and to illustrate some important biological principles. We will consider diversity with respect particularly to two main aspects of life: food intake; and reproduction.

Food intake in plants

The range of ecological niches exploited by plants is vast, and is reflected in the wonderful array of different forms of vegetation that can be found, for instance, in the deciduous and coniferous forests of the northern hemisphere, in the dense evergreen forests of the tropical and sub-tropical zones, in the eucalyptus and acacia forests of Australia, in the mountainous terrains, savannah country, low-lying marshlands, deserts, heathlands, and sand-dunes of Africa, Australia, and South America, as well as in the meadows of the established agricultural systems in temperate regions of the world.

Each plant form is adapted, through evolution, to certain conditions of temperature, humidity, soil quality, soil wetness, light and wind.

While some water is essential for the survival and growth of all plants, enormous variation exists in the amount of water that different plants need. Some forms, like most reeds and bulrushes, cannot survive in soil that does not have a high water content, while others are adapted to extraordinarily dry conditions. Plants found in dry habitats often have small, leathery leaves. An extreme example is provided by the desert cacti in which the leaves are hard, spiny structures which do not support photosynthesis. In these plants the photosynthetic process takes place in the fleshy stems, which are also organs for storing water. Their water content may account for up to 98 percent of their weight.

There are many other kinds of adaptation in plants to dry conditions. One of these takes the form of very short life-cycles. Parts of the Australian desert may receive a reasonable rainfall only once in every few years. When this occurs, the previously parched and apparently lifeless ground suddenly becomes an amazing mass of small flowering plants, and in a very short time seeds are produced. If there is no further rain, the soil soon returns to its state of desiccation, but containing myriads of drought-resistant seeds which lie dormant until next time it rains.

In most leafy plants the size of the pores, or stomata, on the leaves can be varied in response to changes in the moisture content of the soil and the humidity of the atmosphere, thereby controlling the rate of water loss by evaporation. In some plants that live naturally in dry regions, the stomata are permanently sunken into the surface of the leaf, minimising evaporation, while in others the leaves are covered with hairs which have the same effect. In many plants the leaves fold up when conditions become dry, and in some forms the leaves fold regularly after dark and sometimes in the late afternoon. In most plants only about 1 or 2 percent of the water taken up by the roots is used in photosynthesis: the rest is released through the stomata into the atmosphere - a process known as transpiration.

A particularly interesting adaptation to nutrient deficiency in soils is seen in the carnivorous plants, of which there are at least 350 different species. These plants are usually found in swamps, bogs and peat marshes where acids have leached the soil of nutrients; their prey may consist of insects and other invertebrates and sometimes even small birds and amphibians. The sundews, for example, are very small plants, usually not more than 5cm across, and they have tentacles on the upper side of the leaf which secrete a clear sticky fluid that attracts insects. As soon as an insect is caught by one tentacle, the others bend inwards towards it, so that the animal is thoroughly trapped. The tentacles also secrete enzymes which digest the insect tissues, and the soluble nutrients are then absorbed by the leaf surface. Among other carnivorous plants is the well-known Venus flytrap, which occurs naturally only on the coastal plain of North and South Carolina, in North America. Unlike carnivorous animals, carnivorous plants do not use their prey as a source of energy, but rather as a supplementary source of certain nutrients, especially nitrogen and phosphorus.

A much more common way of acquiring nitrogen is that which operates in the legumes, like clovers, vetches, lucernes, peas, beans, and acacias, and which involves a symbiotic relationship between the plant and certain nitrogen-fixing bacteria. When the plants are seedlings their root hairs are invaded by the bacteria (Rhizobia), and eventually these give rise to small nodules in which the bacteria live and multiply. These micro-organisms fix free nitrogen and release it in the form of ammonia, which combines with carbon compounds in the plant cells to produce amino acids.

In agricultural systems the beneficial effects of growing legumes has been appreciated for at least two hundred years. Some of the fixed nitrogen is released into the soil around the legumes and so becomes available to other plants. If the leguminous plants are ploughed back into the soil, much of the nitrogen incorporated in the tissue of the legumes becomes available for other crops. A crop of lucerne ploughed back into a field may add as much as 350 kilograms of nitrogen to the soil per hectare.

Top

Turning to the procurement and assimilation of food in animals, the basic arrangement of the digestive tract (a single mouth, a stomach and intestines containing digestive juices, and a single anus) is common to all multicellular animals, from mosquitoes to elephants, with the exception only of some simple forms like the sponges, coelenterates and flatworms. The extent of variation on this common theme, however, is enormous.

First, the great range of different kinds of food sources has resulted in wide variation in the structure of the mouth parts. The following examples illustrate this point: the grinding molars of herbivores (e.g. ox, horse); the sharp cutting teeth of carnivores (e.g. dog, tiger); the beaks of the sparrow and the pelican; the sucking mouthparts of leeches; the powerful biting and chewing jaws of the preying mantis; the proboscis of the mosquito; the fly-catching tongue of the chameleon; and the simple oral cavity of the earthworm.

Digestive organs - There is also great diversity in the various organs concerned in the digestive process, and in the biochemical properties of the digestive juices. Because of the specificity of these adaptations, if animals are forced to feed on a diet that is significantly different from that to which they are adapted through evolution, it is likely that they will show signs of ill health. Tigers will not last long on a diet of honey, and bee larvae cannot survive on a diet of meat.

The following few examples illustrate the range of adaptations in the internal digestive organs. Termites eat mainly wood. However, like other animals, they do not produce any enzymes in their digestive juices capable of breaking down cellulose, which is the chief component of their diet. They are entirely dependent for their nutrition and survival on certain protozoa which they harbour in the stomach and which produce an enzyme that splits the lignin into soluble carbohydrate molecules which can be utilised by the termite.

Birds' digestion - There is wide variation in the structure and physiology of the gastro-intestinal tract among birds, depending on the kind of diet to which they have become adapted through evolution. In most birds the lower end of the oesophagus swells into a large storage chamber, the crop, where the food remains, sometimes for as long as two days, until the stomach can accommodate it. In pigeons the crop takes the form of a large double sac which not only stores grain, but which also secretes 'pigeon's milk' for feeding the young birds. Crops are generally prominent in grain-eating birds, allowing them to swallow a relatively large volume of food in a hurry, so shortening their time of exposure to predators.

The actual stomach of birds consists of two parts, the anterior glandular stomach, which secretes digestive juices, and the posterior muscular stomach, or gizzard. The gizzard is especially well-developed in grain-eating birds, and it is lined with horny plates or ridges that serve as millstones for grinding the food. This process is often furthered by the abrasive action of small pieces of grit that the birds have swallowed. The gizzard of the domestic goose may contain 30 grams of grit. In carnivorous birds the gizzard usually has much thinner walls and has a completely different function. In owls, gulls, swifts, grouse and some hawks it operates as a trap that stops sharp bits of bone and other non-digestible fragments from passing on through the alimentary canal. This material is rolled up into elongated 'pellets' which are regurgitated through the mouth.

Ruminant digestion - A further example of an alimentary adaptation to a specific kind of diet is provided by the four 'stomachs' of cattle, giraffes and other ruminants. These animals tear the leaves off the plant they are eating with their incisors and swallow them almost immediately, without making any attempt to chew them up. The food bypasses the 'first stomach', or rumen, and goes directly to the smaller 'second stomach' or reticulum, where it is compacted into balls. At a later time, when the animal has stopped feeding, these balls, which are referred to as the cud, are regurgitated to the mouth. The cud is then properly chewed by the grinding action of the animal's molars, before being swallowed a second time, this time to be retained in the rumen. This organ is very large, and represents about 80 percent of the total volume of the four stomachs. It is colonised by bacteria and protozoa which not only break down cellulose, as in termites, but also synthesise proteins, using urea and ammonia as nitrogen sources. Some of these micro-organisms pass on down the alimentary canal and are themselves digested, so contributing to the animal's intake of amino-acids. Some of the products of the fermentation are absorbed directly by the lining of the rumen. The rest of the food passes into the omasum, or third stomach, which basically functions as a strainer, and then on to the abomasum. This is the true stomach, where peptic enzymes are secreted. Anatomically, the rumen, reticulum and omasum are actually expansions of the oesophagus.

Finding food - There is also a great deal of variation among animals in the ways that they find and procure their food. A few examples will illustrate the extraordinary range of different kinds of adaptation.

Many species locate their food simply by going around looking for it, in much the same way as we would ourselves, using especially the senses of sight, smell and hearing. Clearly there is a broad distinction between the techniques of herbivores and carnivores, in that the latter (except in the case of scavengers) have not only to locate their food source, but also to catch it. Some groups of animals, however, have very specialised modes of food location and procurement. Bats, for instance, have evolved a special mechanism for detecting their prey in the night sky, known as echolocation. It involves the emission of sounds at very high frequencies and the detection, by means of highly specialised listening devices, of echoes of these sounds coming from objects in the environment. When the returning signal indicates that the object detected is of an appropriate size and is moving in the air, the bat flies rapidly and unerringly towards it, and catches it. The bat is able to discern from the signal whether the object is flying towards or away from it. A similar mechanism has evolved independently in dolphins which also emit ultrasonic pulses, and the pattern of returning echoes provides them with a picture of the world around them.

In some carnivorous animals that feed in water, special receptors have evolved that detect very small electric impulses generated by the muscular movements of their prey. The platypus, which is effectively blind under water, detects small crustaceans and worms that form its diet in this way. Frog tadpoles and some fish make use of similar mechanisms.

Ant 'farmers' - We cannot leave the subject of food acquisition in animals without reference to the farming practices by certain kinds of ant that live in tropical and sub-tropical regions on the American continent. Some of these species collect pieces of leaves or flowers from living plants and carry them back to the nest, where they cut them up into smaller pieces and mix them with saliva and faeces. The ants spread out the resulting compost in an underground garden, and then place pieces of mycelium from a certain kind of fungus on top of it. The fungus, digesting and deriving nourishment and energy from the cellulose in the leaves or flowers, grows profusely. As the mycelium grows, the ants continually make cuts in it, and at the site of each cut the fungus develops a nodular proliferation. These nodular proliferations are eventually harvested by the ants as a major food source. Some other more primitive ants in the region make use of the same principle, but use insect faeces, or dead insects, as a substrate for the fungal mycelium instead of plant material.

Top

Reproduction

The ability to reproduce and so perpetuate the species is, of course, an essential feature of all groups of living organisms. Reproduction ranges from the simple division of one-celled organisms through to the very complicated structural, physiological and behavioural processes that occur in higher plants and animals.

Asexual reproduction - Despite the underlying uniformities at the molecular level mentioned at the beginning of this section, the actual details of the processes of reproduction at the level of whole organisms vary enormously. First, let us note the all-important distinction between sexual and asexual reproduction. In asexual reproduction there is only one parent, which splits, buds or fragments to give rise to two or more new individuals, each of which have hereditary characteristics identical with those of the parent. Asexual reproduction is common among simpler forms of life, including bacteria, algae, fungi, mosses, protozoa, coelenterates and flatworms. In the case of the last group, if the animal becomes fragmented into several pieces, each may develop into a new whole animal. If a starfish is cut in two, each part will regenerate tissue to form a complete new starfish.

Among plants, even the higher seed plants (angiosperms) are capable of reproducing asexually. Some species, such as English elms and Lombardy poplars, may propagate by putting out 'suckers', so that new trees grow up from the distal roots of the parent trees. Reproduction by rhizomes (actually stems growing laterally underground) and by tubers is also common, as horticulturists have appreciated for many thousands of years. Propagation of plants by means of cuttings is another example of asexual reproduction. Indeed, asexual reproduction also occurs in higher animals, including humans, when a newly fertilised egg divides in the uterus to give rise to two or more genetically identical eggs, each of which develops as an independent organism. Today, as an outcome of scientific advances, it is now possible to bring about asexual reproduction artificially in mammals by means of cloning techniques.

Turning to sexual reproduction, we have already noted some basic differences between the simpler, more ancient plants, such as mosses and ferns, and the more recent conifers and flowering plants. Let us look at a few of the adaptations that have evolved in this last group.

Pollinators - The most striking feature of the reproductive processes of the flowering plants is the fact that, while the wind sometimes plays a part in transporting pollen from flower to flower, the great majority of species rely entirely on insects, or in some cases on small birds or mammals, to bring about pollination. For this to work, the insects have first to be attracted to the flowers, so that they pick up pollen and later drop it off when they visit other flowers of the same species, where it can bring about fertilisation. The basic attractant for insects in the great majority of plants is food, in the form of nectar, which is produced at the base of the flower solely for this purpose. Another feature of the adaptation of the flowering plant is the development of petals, which are often displayed conspicuously and in bright colours, signalling to insects the presence of nectar.

While this basic pattern is very common, there are many interesting and sometimes bizarre variations on the general theme. The orchids, as Darwin noted in his remarkable book on these plants, are especially interesting from this point of view. In one species the shape and colour of the flower bears a strong resemblance to the female of a particular species of wasp, complete with eyes, antennae and wings. It even gives off an odour which is the same as that emitted by a female wasp that is ready to mate. Male wasps, deceived by this arrangement, attempt to copulate with the flower. In doing so, they pick up pollen, which they inadvertently deposit on the next flower with which they try the same thing.

Another interesting example is a plant known as the dung lily, which gives off an odour similar to that of herbivore dung. When a dung beetle happens to fly overhead, it responds to the dung-like stimuli by dropping head first into the funnel-shaped flower. Because the inside of the flower is lined by small hairs pointing downwards, the beetle is unable to climb out, and if it happens to be carrying pollen from a previous encounter with a dung lily, some of this will come off and fertilise the ova. By morning, the flower tips over and the one-way hairs no longer prevent the beetle from escaping ­ which it does. On the way out it picks up some pollen that had not been available when it entered.

Aquatic animals - In multi-cellular animals, two main mechanisms exist for achieving union of egg with sperm. The first operates only in the case of animals that live, or at least mate, in water, and it involves the male liberating sperm into the water in the region where the female has recently laid her unfertilised eggs. Usually this act is preceded by certain courtship behaviours which ensure that the male is at the right place at the right time. This method operates in most marine animals, from molluscs to true fishes, as well as in amphibians, which return to the water to mate. In frogs, for instance, the male arranges himself on the back of the egg-laden female, keeping firmly in place by means of special clasping pads on the front of his forelimbs. He remains in this position until the female begins to lay her eggs, at which time he ejects spermatozoa into the water, a small proportion of which find, and unite with, ova.

The pattern in newts and salamanders is somewhat different. For example, in the common newt of north-western Europe, Triturus vulgaris, the male courts the female with a dance display involving a rapid waving movement of the end of his tail, which is turned back on itself, and so points forward. When the female is appropriately aroused, apparently partly as a result of a hormone discharged into the water from the male cloaca, the male newt deposits a mucilaginous bundle of spermatozoa, which the female picks up with her hind limbs and inserts into her cloaca, so that fertilisation takes place internally.

Land animals - The main mechanism for bringing sperm and eggs in contact in land animals involves the insertion of a male copulatory organ into the genital tract of the female, and the ejection from the male organ of sperm, which then swim their way to the ova. This mechanism exists in most insects, in some birds, and in all reptiles and mammals, although different procedures operate in worms and some arthropods.

The reproductive pattern of the earthworm is particularly complex. Earthworms are hermaphrodite, and during mating the two worms, heading in opposite directions, lie with their ventral surfaces in opposition and are held together by a sticky secretion. Each worm donates sperm to the other, and these are temporarily stored in a seminal receptacle. After the worms have separated, a glandular ring of thickened skin called the clitellum secretes a membranous cocoon. As the worm frees itself from this cocoon, it discharges into it both ova produced in its own body as well as the sperms contributed by the other worm. As the cocoon slips off the worm, its two openings constrict, and the fertilised eggs then develop inside it to produce new worms.

Another interesting mechanism has been observed in certain species of peripatus, which are curious caterpillar-like animals that live in moist forests in Africa, Asia, Australia and South America. They have many pairs of legs, and they share characteristics of both the annelid worms and arthropods. In some species of peripatus, males have a special protuberance on their head which is used to carry around a drop of semen, as the animal searches for a female. When a female is found, the male deposits the semen somewhere on the surface of her body, and a cellular reaction immediately takes place inside the female, as a result of which some specialised cells in her body transport the sperm to the ova in the uterus.

Spiders - Reproduction in spiders is rather similar to the first part of this peripatus procedure. The male produces a ball of sperm-containing material which he picks up with one of his pedipalps, which are limb-like structures situated just in front of his four sets of legs, He then sets out in search of a female which, in most cases, he must approach with considerable caution, identifying himself by certain species-specific signals in order to avoid being attacked and eaten. On reaching the female, the male inserts the spermatozoa into the female genital tract. In most cases, he then quickly makes his get-away, although in some spiders the female consumes the male as soon as mating is completed.

Attracting mates - A great variety of procedures exist among different animals for ensuring that males and females find each other for mating purposes. In many instances the female gives off a specific odour which attracts males. In some moths, the males are exquisitely sensitive to such odours ­ responding when there are only about a hundred molecules of the specific substance per millilitre of air. It has been estimated that, in some kinds of moth, the male can detect a female over 4000 metres away, if a gentle breeze is blowing in the right direction.

In other species, the male attracts the female to a particular place or territory by emitting a distinctive call. This pattern is common among birds and frogs. In some bird species, the peacock and the Australian lyrebird being notable examples, males attract females by extending and displaying their tail feathers. In bowerbirds the males achieve the same objective by decorating their ‘bowers’ with all sorts of colourful objects.

In the great majority of mammals, from rats, mice and shrews, to dogs, zebras, elephants and monkeys, females undergo a hormonally controlled cycle, and they are sexually attractive, or receptive, to males only at the certain periods that coincide with ovulation. Biologically, the important consequence of this mechanism is that mating takes place only at times when fertilisable ova exist in the female genital tract. An outstanding exception to this generalisation is Homo sapiens, in which females can be sexually attractive to males at all times, and in which female receptivity is not restricted to a short period in the hormonal cycle.

Mothers' milk - A feature relevant to reproduction and common to all mammals is the production of milk in the mammary glands of females, which is the only source of food for their new-born offspring. Only one species of mammal is known in which new-born animals can survive without milk, eating solid food immediately after birth, and this is the guinea pig. Nevertheless young guinea pigs do drink milk from their mothers if it is available. At the other extreme is the young grey kangaroo, which weighs less than a gram when it is born and which, although it makes its own way from the urogenital opening of the mother to the pouch, is otherwise completely helpless. Once in the pouch it immediately becomes attached to one of the nipples, and it does not leave the pouch, even for short periods, for 9 months.

Top

Comment

The examples given above only touch the surface of the vast range of different life forms that exist on earth. The shelves of science libraries hold countless volumes providing detailed information on the structural, physiological and behavioural diversity encountered among living organisms. And apart from all that has already been described, there is much more yet to be discovered.

Appreciation of this biodiversity is crucial to our understanding of life, of the human place in nature and of the urgent need to support, rather than disrupt, the complex web of life on which we depend and of which we are a part.

An outstanding feature of the present day is the extraordinary rate of loss of biodiversity due to the activities of humankind. Extinctions resulting from human activities have been estimated at up to 140 000 species a year. At this rate about half the existing species will be wiped out in 70 years.

Top

Further reading

For further information see our paper Loss of biodiversity

and

E.O.Wilson, 1989, Biodiversity. National Academies Press, Washington DC.