Recently I was reading an old article, The Croonian Lecture 1991 about Genostasis and the Limits to Evolution , and I realized that the fact that evolutionary biology is a historical science (in the general sense of the word) colors how some of the reasoning about it. In this article, A. D Bradshaw remind us that even though the current natural world is the result of the different evolutionary processes, we have to remember that it only represents the species of organisms that survived, and that the majority of the species in the history of the planet are actually dead.
One of the main logical errors that people commit when considering the theory of evolution is to ascribe a teleological function to it. That is, that the evolutionary process is a "force", that guides "purposefully" the organisms to be better adapted to their environment. This goes against the concept of genostasis, and against most of what we know about evolution per se. It is very hard to conceive that the directionality of the evolutionary results is due to the fact that the results that were not compatible with the environment simply died off. They disappeared. Therefore the one remaining, the one that we are able to see, seems to have received the effect of the "force" of evolution. To this species belong the spoils.
How does the concept of genostasis illustrates this fallacy's errors? Well, A.D. Bradshaw shows it very nicely, with several examples taken from real environments. Some of the environments described have very tough conditions for the organisms (extreme salinity, copper toxicity, etc), and even though the organisms around have had all the time in the world (literally) to adapt to those conditions, they haven't. Because evolution is not purposefully trying to push the organism to adapt to that particular condition. If the organism has a mutation that gives it a differential reproductive advantage over others when it is placed in the tough environment, well, then it will survive. But if it doesn't (and most of the times we could say they don't), then simply the organism will die off, or just never go into that environment, even thought the resources in that environment could help the organism to thrive.
If there was a teleological purpose behind evolution, we would expect that it would be pushing the organisms to evolve towards a better optimization with the environment. What happens in real life is completely different. In a real environment, the organisms may or may not adapt to a change in the condition. It all depends on preexisting variations, or in newly formed mutations that give place to new variation. But as A. D. Bradshaw shows, in a lot of organisms, this variation never happens, either because it is too hard to evolve the characteristic, because it never had enough time to evolve it, or because the variability for that characteristic implies a worsening in other life sustaining features, therefore making it harmful (at least in the short run) to evolve in that direction. And when they are not able to
So while we do have to marvel to all the "power" of evolution in our planet, we also have to remember that evolution always hides its failures under a mantle of dirt.