How, exactly, does the evolutionary process decide which traits are to stay with a species?

As in do we know exactly how it happens, in terms of cause and effect? Could you give a specific causal example?





(I'm not religious etc, just reading Jerry Fodor..)|||It's relatively random. However, traits that impart a disadvantage to survival are frequently selected against, because those who have the trait are more likely to die out.





For example, if a mutation of an animal genome provides it with bright colouration of its coat, so that it will not be able to hide effectively from predators, it will likely be caught and killed. This means the genome will be selected against and will slowly die out. You will get carriers in the offspring, but eventually, the number of carriers becomes so low that the gene pool is effectively restored to its pre-mutation stage.





If, on the other hand, the mutation provides better camouflage, the animal is more likely to survive. If more and more individuals with the enhanced camouflage are born, they will slowly increase in number until they outnumber the original colouring. The new gene will be selected for in the gene pool, and slowly build in number.





Some mutations have neither an advantage or disadvantage. They can pass on without any definite selection happening, they just occur.





And humans have ways of keeping unfavourable mutations in the survival race,meaning the genes normally selected against are not killed off. Many plants are examples. Some are bred because they look nice or have other desirable traits such as scent, but the plant itself may be very sickly and would not survive in the wild.|||The traits that are successful and give those members of a species an advantage over others.|||It doesn't explicity "decide" which ones stay there. Basically, ones that help the species survive are more likely to stay, whereas others are not. An example is probably the best way to describe it:





This is a real example, which has been seen both in the lab and in the population generally (and is now causing serious problems for doctors in some parts of the world).





Suppose you have a colony of bacteria. Because of mutations and genetic drift, a small fraction of these bacteria are resistant to a certain antibiotic. If that colony is exposed to that antibiotic, then the non-resistant ones are destroyed, but the resistant ones survive and multiply. Their descendants carry those resistant genes, so the next generation of bacteria has far more resistant bacteria than before.





The basic logic is this- if a trait helps an organism survive to the point where it can reproduce, then organisms with that trait are more likely to reproduce than others, and so that trait will be more common in the next generation, which over many generations leads to long term-changes.





There are a few consequences of this process (known as "natural selection):





* It doesn't consciously decide what traits survive, and it can't force a trait to appear. It can only make some traits statistically more likely to appear than others.





* It can't make traits out of thin air- it's dependent very much on genetic factors, particularly what genes are available in the population, how prevalent they are, and how they interact with each other. These processes are extremely complicated and can lead to strange results.





* Remember that only traits which help the species survive *and reproduce* are given an advantage. For instance, you'll note that humans only start to age significantly beyond about 60, and most cancers only become a problem beyond that age. This is because most humans will have had their children by then, so living past this age has no evolutionary advantage. As such, traits which let you survive longer are not very common, as they don't spread faster than average.|||Jerry Fodor argues with a strawman version of evolution - it's certainly not the same evolution that biologists study - so you shouldn't be surprised if it doesn't hold water. If you'd like to know the exact cause and effect sequence, a degree in genetics is helpful, but if you're into books, here's one that can spell it out for you:





Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom


http://books.google.com/books?id=-SqwP8C鈥?/a>