Introduction Plant and animal interactions Cells and receptors Blood water and sodium concentrations
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Introduction. In The Origin of Species Darwin stated: "If it could be demonstrated that any complex organ existedwhich could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down".
Professor Michael Behe of Lehigh University, Pennsylvania, recently wrote a book called Darwin's Black box in which he demonstrates that complex biological systems could not be formed by numerous modifications and such systems are termed "irreducibly complex".
At the end of Michael Denton's book Evolution: A Theory in Crisis he says ".....everywhere we look, to whatever depth we look, we find an elegance and ingenuity of absolutely transcending quality, which so mitigates against chance".
"[The instructions within the DNA of a single cell] if written out would fill a thousand 600 page books. Each cell is a world brimming with as many as two hundred trillion tiny groups of atoms called molecules. Our 46 [human] chromosome 'threads' linked together would measure more than six feet. Yet the [cell] nucleus that contains them is less than four ten-thousandths of an inch in diameter." Rick Gore, "The Awesome Worlds within a Cell" in National Geographic, September 1976, pp. 357-358, 360.
For a recent detailed essay on this topic seeOn the Origins of Life
What is irreducible complexity? Have you ever had the experience of trying to build one of those pieces of furniture or other household item that come as a package of components you have to assemble (usually with very sketchy instructions) and you find one of the components missing? Quite often this means the item cannot function. If this has happened to you, you have experienced “irreducible complexity,” i.e. the fact that a complex item made of many parts will not function until all the component parts are present and working together.
Biological systems, from individual cells to whole ecosystems, abound with irreducible complexity. Below are some examples.
Plant and animal interactions. Some orchids are pollinated by male insects that try to mate with the flowers. This bizarre behaviour occurs because the flowers produce chemicals that are the same as chemicals emitted by female insects. Usually a mixture of common chemicals is involved. An Australian orchid named Chiloglottis trapeziformis is only pollinated by a wasp named Neozeleboria cryptoides. Scientist who studied the wasp and orchid were surprised to find that the chemical signal involved was “one unique compound, requiring a rigid biosynthetic process and a highly specific receptor a system with seemingly limited evolutionary flexibility.” (Science, vol. 302, p437, 17 Oct 2003.)
“Limited evolutionary flexibility” means that if either the orchid or the wasp got any of the steps wrong in making this compound, emitting it at the right time and making the receptors that detect the chemical in the air, then the orchid would die out for lack of pollinators. The highly specific relationship between some plants and their pollinators is the classic evolutionary problem – both orchid and wasp had to evolve their part of the system at the same time or it wouldn’t work at all.
The wasp orchid relationship depends on chemical signalling. The orchid makes a chemical with a very precise shape, and the male moth has a receptor for it – a much larger molecule containing a part that is the complementary shape to the signal chemical. The receptor and signal molecules fit together like a lock and key. When the signal molecule fits into the receptor, the receptor changes shape and that starts off a whole series of reactions that change the wasp’s behaviour.
Cells and receptors. Although the results are usually far more mundane than sex crazed wasps (see above), chemical signalling is essential for all biological processes. Living cells have numerous receptor molecules on their surfaces which are turned on and off by chemical signals sent out by other cells, or in the case of the wasp, another living organism. Quite often the interaction between one chemical signal and receptor is merely one step in a multi-step process that involves other types of physical and chemical reactions.
An effective signalling mechanism needs the signal, the receptor and the response mechanism to all to be present and functioning at the same time before it is functional. If any of these are missing the system is useless.
Blood water and sodium concentrations. Being able to detect and respond to changes in the environment, (e.g. the presence of the pheromones) and within the body is essential for biological systems, and involves irreducible complexity. An example from within the body is the way the body responds to changes in the amount of water and sodium in the blood.
When the blood volume, blood pressure or sodium levels are low the flow of fluid and sodium through kidney tubules decreases, which is detected by cells in the kidney. Cells in the kidney produce tiny amounts of a chemical called rennin into the blood. In the blood rennin interacts with a protein called angiotensinogen, which is made by the liver. Rennin cuts off a string of a string of ten amino acids called angiotensin I from the angiotensinogen. As angiotensin I passes through the lungs (and some other body tissues) an enzyme called Angiotensin Converting Enzyme (ACE) cuts two amino acids from it converting to a string of eight amino acids called angiotensin II.
This is a powerful chemical signal that stimulates the heart, blood vessels, hypothalamus, pituitary gland and adrenal gland to each ply their part in raising the blood pressure and retaining water and sodium. Neither the rennin, angiotensinogen or the ACE are of any use on their own but the system will not work if one of them is missing.