Specialization goes hand-in-hand with a redistribution of
complexity. The individual elements become simpler and the
orchestration of these simpler elements becomes more complex.
For example, single-cell organisms maintain a wide range of complex behavior in order to respond to the range of possible environmental conditions they may encounter. Most of those capabilities are seldom used. Cells in multicellular organisms can instead rely on the whole organism adapting to changing conditions via a multicellular response. So the individual cells can dispense with the rarely used functions. Social insects demonstrate the same sort of tradeoff: “...individuals of highly social ant species are less complex than individuals from [socially] simple ant species.[2]”
Specialization is deeply intertwined with the other three multicellular organizing principles in part because the orchestration of many kinds of simple elements requires more careful messaging mechanisms and a different sort of communication strategy called stigmergy.
Specialization begins at the level of individual cells in
multicellular organisms and that specialization is almost always
permanent. Bacterial cells capable of participating in biofilms
(cooperative communities of single-cell organisms) usually must
also be able to live as isolated cells. Only when single-cell
organisms participate in a cooperative biofilm do they
temporarily specialize. Metazoan cells, i.e., the cells in
“true” multicellular organisms, don’t just temporarily
specialize, they permanently differentiate as the organism
develops from a fertilized egg to an adult.
“The different cell types in a
multicellular organism differ dramatically in both structure and
function. If we compare a mammalian neuron with a lymphocyte,
for example, the differences are so extreme that it is difficult
to imagine that the two cells contain the same genome." (see here
for more detail)
Occasionally, through some mischance, a cell loses some of its differentiation. The cells in which this occurs are called neoplastic (i.e., newly changeable). Neoplastic cells are not just abnormal, they form tumors. Tumors may be benign, but are often a hallmark of cancer.
Multicellular specialization at the cellular level is possible because the environment faced by a cell in a multicellular organism is quite different from and more benign than that faced by a single-cell organism in its natural environment. Metazoan cells live in a cooperative, nearly homeostatic environment protected and nourished by the whole organism. In contrast, a single-cell organism must be prepared to deal with all sorts of unfavorable circumstances such as predators, changing abundance of nutrients, and toxic chemicals. That flexibility requires each cell to support a large complex repertoire of behavior.
Multicellular specialization at the cellular level is necessary because maintaining the full complement of all possible behavior is costly, dangerous, and/or incompatible with requirements of the specialized functions. For individual cells, one obvious cost is energy consumption; the maintenance of all the unnecessary cellular machinery is not free. Since each specialized Metazoan cell uses only a small fraction of the total genome, its energy costs can be dramatically reduced. But also cell specialization induces very different and incompatible cellular biochemistry, shape, and function. A nerve cell could not function as a reliable communication channel between point A and point B if it also builds bone around itself, fills itself with hemoglobin to carry oxygen, accumulates fat, and secretes stomach acid.
Perhaps the most important benefit is that specialization reduces the number and type of messages to which the cell can respond. There are thousands of different types of molecular messages active in a complex multicellular organism. Each cell responds to just a small subset. It would be more than meaningless for a cell to retain the ability to respond to all these messages - it would be chaos. Worse, a general-purpose cell would be susceptible to all viruses. Viruses infect a cell by binding to particular surface proteins. Since different surface proteins characterize different specialized cells, each kind of virus can infect only certain types of cells -- those that have the right binding sites. The cold virus infects cells in the nasal passages, the hepatitis virus infects certain liver cells, the HIV virus infects certain cells in the immune system, and so forth. If every cell expressed all cell markers, any virus could bind to and infect all cells. Catching a cold would be fatal.
In this coevolutionary world of today's Internet where computer
viruses and worms run rampant, a similar necessity for specialization in
computing should be obvious.
[1] A Complexity Drain on Cells in the Evolution of Multicellularity, McShea, D. W., Evolution, Vol 56, No. 3 Pages: 441-452, 1997. And, “ The hierarchical structure of organisms: a scale and documentation of a trend in the maximum.” McShea, D.W., Paleobiology 27:405-423, 2001.
[2] “Individual versus social complexity, with particular reference to ant colonies,” Anderson, C & McShea, D. W. Biol. Rev., vol 76, pp. 211-237, 2001. Note: the individual ants of highly social species are not only less complex, but more specialized and more varied in size and shape. Yet, as with specialized metazoan cells, they all have the same genetic complement.