This topic has already been mentioned briefly (see 1.9) and must now be enlarged. At any systems level, there will be properties that do not exist at the lower levels. These properties are called 'emergent properties', or just 'emergents', because they emerge, and can be seen and studied, only at their own systems level. But they do not emerge, and they cannot be seen or studied, at any lower level.
For example, a word has the two properties of spelling and meaning. Its spelling is its structure, the system itself. Its meaning, however, is an emergent property, which is entirely dependent on the arrangement of letters. Change the spelling in the smallest extent, and the meaning is likely to be altered beyond recognition. For example, try reversing the order of the letters in words such as 'dog' or 'tar'. Furthermore, the emergent, the meaning of the word, is apparent only at that systems level. Go to the lower systems level of the individual letters, and the emergent property is absent.
Perhaps the best biological example of an emergent is seen in both the 'schooling' behaviour of fish, and the 'flocking' behaviour of birds. Individuals within the school or flock behave according to a few simple rules, which can be simulated on a computer. The emergent is a sort of 'super organism' in which all the individuals behave as one, and this provides considerable protection from predators. The fact that this emergent has evolved in so many different species of both fish and birds, as well as in the migrating butterflies, and stampeding herbivores, is a clear example of both group selection, and the suggestion that the basic mechanism of evolution is natural selection acting on emergents (see 2.10).
An emergent such as schooling or flocking can be observed only at the systems level of the population. It cannot possibly be observed, studied, or analysed at the systems level of the solitary individual. A scientist, whose studies are confined to a single fish in an aquarium, or a single pigeon in an aviary, cannot study the emergents of schooling or flocking. This point becomes crucially important when we consider the system of locking that is an emergent of the gene-for-gene relationship (see 4.14) and the n/2 model (see 4.15).
"The whole is greater than the sum of the parts" is the essence of complex systems thinking. There are thus two components of a given systems level. There are the parts, on the one hand, which add up to the 'whole'. And, on the other hand, there are those additional components, the emergents, which make the whole greater than just the sum of those parts. It is these additional components that are called the emergent properties. For example, a book may have the emergent property of being great literature. But if the book is studied solely in terms of single words, unrelated to all the other words, the qualities that make it great literature are not discernible. It is in this sense that a Shakespeare play is greater than the sum of its parts.
Another example is the difference between a dead body and a living body. The dead body might be perfect in all respects, except that it has stopped living. The dead body is the 'sum of the parts'. The property of living, of life itself, is the emergent property. All those aspects of life, that used to be called 'vital forces', are emergent properties. Death is the loss of the emergent property. Decay, however, is the loss of the parts. An alternative description is to call the parts 'structure', and to call the emergent properties 'behaviour'. In living systems, death is then an irreversible loss of behaviour. Decay is an irreversible loss of structure.
A further practical example is a motor car engine. If the engine is running, it produces a stated horsepower. This horsepower is an emergent property. It means that the whole is greater than the sum of the parts. But if the engine is switched off, the emergent is lost. There is then only the sum of the parts, only a dead engine. An engine, of course, is a very simple system, and it can easily be switched on again. A living body is a highly complex system and, if it stops functioning, the loss of the emergent property of life is normally irreversible.
The basis of reductionism (see 1.10) is the belief that 'the whole' can be analysed in terms of the 'the parts'. There is a further weakness of reductionism, because it is blind to emergent properties at the higher systems levels. For example, Dawkins (1982) totally rejected the Gaia hypothesis, because he apparently did not comprehend the importance of the holistic approach, of self-organisation (see 2.4), and of the emergents that can emanate at the higher systems levels. What systems (or holistic) thinking has done is to reverse the relationship between the parts and the whole. Systems scientists recognise that living systems cannot be explained only in terms of their parts. They must be explained in terms of both their parts and their emergents. Reductionist science tends to be interested in the parts, and to neglect the emergents.
In the context of plant pathosystems, the most prominent emergent is the system of locking that emerges from the gene-forgene relationship (see 4.15). This very important emergent is apparent only at the systems level of the pathosystem. That is, at the level of the two interacting populations of host and parasite.
For much of the twentieth century, this emergent was not apparent to crop scientists, because they were working at too low a systems level. They were studying individuals only. They could no more see the emergent of the system of locking than someone studying a single fish, or a single bird, could see the emergent of schooling or flocking. It is this suboptimisation in crop science, and the damage it has caused (see 5.3), that emphasises, more than anything, the absolute necessity for the holistic approach when studying nonlinear systems.
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