Paul sent us this...
My dad is having trouble totally accepting evolution. He brings up complex chemical reactions such as the Kreb Cycle and how half a Kreb cycle would be useless. I've tried to explain that for things to evolve, each intermediate step must be useful. I've managed to convince him of the eye but microbiology still stymies my attempts to get him to completely accept evolution. Any suggestions?
It is always difficult to explain why evolution makes sense, and of course we can't alwasys explain everything. That does not mean that things have not evolved, but just because physicists cannot (yet) explain how black holes operate it does not mean that they do notobey the laws of physics. In the same way evolution has occured and does occur, although we may not always be able to see what has happened in the case of some things.
The trap your father has fallen into is assuming that the origin of the Krebs cycle was somehow 'aimed' at producing energy for a cell. But the basis of the cycle probably had a function beyond this. Organisms billions of years ago may have made different uses of the chemical pathways to produce useful products. Evolution added new steps to this (each of whichwould have been advantagous to the organism at the time) and this eventually resulted in the Krebs cycle as we see it now.
It is often said that the components of a structure, for example the eye, or the flagellum of a bacterium, are only worth having if they are part of an eye, or flagellum. There is no point evolving a lens, if you don't have the rest of the eye to go with it. However, the eye has been shown to have been built up from a simple light sensitive spot seen in all animals. It is throught the addition of structures like a lens which makes it into a really specialised organ that can allow the brain to differentiate the tiniest variation in colour and shape of a structure. We see (no pun intended) that the complex camera eye that we have, and also the Cephalopod molluscs, has evolved independently in both of these groups. The animals in between have a variety of simpler eyes from which our sort can be seen to have evolved.
It is interesting that all eyes whatever their morphology, be they camera eyes, or compound eyes seen in the arthropods, are all patterned by the same set of genes, the Pax6 genes.
Likewise the individual components of the flagellum have been shown to have an individual function, before they came together to form the Bacterium's propulsion system.
The is a series of links to various books and papers here and some more information on the origins of complex pathways here (scroll down a bit in both cases!).