Re: Why would more than one complement pathway have evolved?

Why do we have taste and smell? Vision and hearing?

It might actually be more surprising that we don't have dozens or more different complement pathways. It seems that anything which gives the organism a selective advantage soon becomes fixed in the population. Things like changes in skin color to protect us from too much sun, or help us make vitamin D in areas with little sun exposure, are small in comparison to being able to fight off infections.

There are many factors which influence whether or not something can or does evolve. How many genes are required? How complex is the system? What are the costs as well as the benefits of the feature? For example, an immune system that is too sensitive may too often lead to allergies, toxic shock syndrome, lupus and other auto-immune diseases.

Another very interesting aspect of studying the evolution of a feature such as complement pathways in the immune system, is to find out when the system(s) evolved. Do all vertebrates have both complement pathways? Or did the second one only appear recently in mammals or even just the primates? Is it possible that most mammals do have many more complement pathways, and humans have just lost most of them to end up with only 2? Most such questions are only now beginning to be studied. We now know for example, pretty much exactly when the ability to produce vitamin C was lost in the primate lineage.

The general rule is that evolution is not exactly intelligent design. A well designed system might have 3 or more complement pathways and the ability to produce vitamin C when needed. Random mutations in fact do not lead to evolution, it takes random mutations plus reproduction and selection to generate an evolving system. The gain of a second complement pathway, or the loss of vitamin C production may themselves not have resulted in an increase in fitness, but may have been rather accidentally linked to some other trait that did have a large fitness gain.

It may also be quite possible for organisms to be "too fit". A virus that kills its host too rapidly goes extinct. Likewise a predator that eats all the available prey too quickly would go extinct. It may be beneficial for organisms to be somewhat susceptible to diseases which keep their population size in check, in the long run. Contemplating such theories gets into questions of "group selection". How can something that is detrimental to the individual (susceptibility to disease) be beneficial to the group (smaller population, so more resources for each individual)? The answers are likely to be very different for different types of organisms (mammals vs fungi or ants, for example; of even rodents vs primates).

With our modern databases of genes and complete genomes, you yourself can easily go where no man (or woman) has gone before in exploring some of these questions. For example the GenBank entry with accession number U19253 is the gene sequence for the C3 component of the complement pathway in the frog, Xenopus laevis. Using the protein sequence from the translation of that gene, you can the use BLAST to find that the C3 and C4 complement components are the result of an ancient gene duplication event. All vertebrates have both C3 and C4, and some also have C5.