A very frequent statement we hear concerning biological systems can be expressed as follow:
The degree of functional redundancy observed in a subsystem reflects the importance of this subsystem within the system it belongs to.
This idea is very anthropomorphic, and based on what an engineer would consider a good design: Any critical subsystem should be redundant, so if one instance fails, the others maintain the system in a functional state. But a biological system has not been designed. It evolved. And the two processes are completely different.If I am an engineer who wants to build a new type of aircraft, I can hardly afford to loose one of them when packed with passengers. Or actually even one with only the test pilot on board. I will therefore create all the critical subsystems redundant, so the plane succeeds at least to land before a general failure makes it unflyable.
However, if I am an oyster the situation is quite different. How then, can I be sure to preserve my offspring from transmitting any negative deviations I would happen to have? If the critical subsystems are redundant, they will be able to survive the effects of those minor deleterious variations. As a result they will transmit those variations to their own offspring. Following the neutral theory of molecular evolution (Motoo Kimura), these variations can invade the population by genetic drift. Now, if my species encounters situations where an optimal function of the all redundant subsystems is required, it will be wiped-out from the surface of the earth. Unlucky. An alternative scenario: The redundancy of my critical subsystems has been kept to its minimum. If any of my offsprings experiences a small deviation in one of those subsystems, it will die quickly. But I do not care, because I am an oyster. I spawn millions of eggs. I can afford to loose 90% or more of them.
In fact, it seems very few of the critical subsystems in a cell are redundant. For instance, most of the enzymes producing the energy in the cell are unique. Same for the RNA production machinery. If you happend to have a problem in one of those enzymes, you die at a very early age, so you cannot “polute” the genome of the species. Redundancy appears only for “secondary” subsystems, typically dealing with feeding, signalling etc. Yes those subsystems are important for the proper life of higher organisms, and probably provide a selective advantage in a stable environment. But they are not crucial for life itself. Furthermore the diversity observed is rarely a true redundancy but allows to feed on a larger variety of substrates, sense more compounds etc. The redundancy only appears when the systems is stretched by the disappearance of other subsystems. True redundant systems would most probably just be eliminated (for a more informed discussion of gene duplications and losses, read this recent review)
Continuing on the topics of “importance”, one of the most irritating remarks one can still hear too often from hard-core molecular biologists is mutatis mutandis:
This gene is not important because the knock-outed mice display no phenotype.
A term has even been coined for that: the essentiality.
Gene essentiality is a pretty busy domain of research, and I am as far as it is possible to be an expert. So I am not going to discuss it. But I ressent the notion that equate “important” with “phenotype immediately apparent”. The problem comes from the way we analyse mutant animals (which is designed this way for very good reasons, that is not the issue here).
Consider the case of a car. What is a car supposed to do? To progress on a flat surface propulsed by its own engine. So we set-up an experimental environment, with a perfect flat surface. To eliminate any uncontrolled variables, we place this surface indoor, under a constant temperature and illumination. And of course we remove all the other vehicles from the environment. On my right, a control car. On my left, the same car without chock absorbers, ABS, without lighting at all, without hooter, with all doors but the driver’s one fused to the frame, and with pure water in the cooling system. Let’s start both engines, and drive the cars during 50 metres. Noticed the difference? No? Therefore none of the parts we removed was important. Well, how long do you thing you will drive the modified car at night on the London Orbital when it’s -10 degree Celsius and the surface is covered in ice? I will tell you: that day, you will find the ABS, lightning, anti-freeze etc. damned important. Even essential.
I once worked in a team studying a mouse mutant strain “without phenotype” (*). Until someone (**) decided to study aged mice (what a weird idea) and discovered that the brain degenerated quickly in those animals. Hard to find out, when for practical reason one uses only young animals. See: Zoli M, Picciotto MR, Ferrari R, Cocchi D, Changeux JP. Increased neurodegeneration during ageing in mice lacking high-affinity nicotine receptors. EMBO J. 1999 Mar 1;18(5):1235-44.
(*) Well, with very a very mild phenotype.