Originally Answered: Why hasn't the human race evolved to a better species?
Your question contains the classical mistake given to evolution that it is progressive... it is NOT progressive. Evolution is based on variance within the population allowing adaptation to fill new environmental niche. Fortunately for humans we are the most generalized mammal on the planet and as of such have found a nearly universal sucess throughout most of the environments that we encounter. That said it is difficult to imagine an environment that would be different enough and that we would habituate for long enough to allow natural selection to act upon the effected population for long enough to create any kind of significant visual change in our species. At the same time we as a species are truly defeating the few acts of natural selection that were acting upon us with advances in medicine. With this in mind the changes that might occur could be much like your thanksgiving Turkey, a creature who has been put under so much unnatural selection that it no longer is capable of mating due to humans artificially breeding them for nothing else but size. If humans were no longer present to do this service for Turkeys, they would go extinct. Through "unnatural selection" humans are doing very similiar things to themselves. Undoubtedly a woman who goes in for infertility medicines has a greater chance of having children who would require the same, children that have severe enough allergies/asthma that may have killed them in the past will now grow to a reproductive age with the assistance of asthma/allergy medicines will also produce offspring who have higher occurances of asthma/allergies. This is a bit of a concern, seeing as at the same time we are making ourselves more susceptible to asthma and allergies we are also taking actions that are converting the environment that we will have to adapt to, to containing more contaminants. If it gets serious enough those who are persevering with the assistance of medicine may be the first to become too sick to be reproductively successful such that the power of natural selection might overbear unnatural medical selection once again in this area? Some areas where medicine is making slow progress, like cancer and Aids, there will be a continued evolutionary process. Already there have been a few case studies that have shown a truckstop in Africa where the prostitutes, despite an alarming exposure rate, are not catching or dying from Aids suggesting that natural selection has already discovered a variant within one ethnic group that appears to have white blood cells which are not as susceptible to the AIDS virus, much like sickle cell trait does the same red blood cells resistance for Malaria. As this gene now becomes more prominent due to the pressure of AIDS we may also see the advance of a new genetic disorder if a person is born with two recessives, much like two recessive copies of sickle cell results in anemia instead of trait. So if people do begin to die from a worse environment that causes worse Asthma/Cancer/AIDS, undoubtedly natural selection will find many variants within our population which now far exceeds 6 billion people and will come up with the needed solutions to combat these problems so that enough people make it to a reproductive age and then go to the doctor to get their fertility drugs. If there is also enough disruption in the sociopolitical structure such that medicine is no longer readilly available, at least to the poor, then natural selection will also reclaim the realm of fertility and those who can't naturally, won't. So that is what our environment has in store for us...
But what about technology, if it is allowed to persist and becomes an enpowered agent towards our development. If on top of the fertility drugs, people take medical-engineering so far that they also have their fetus' genetically altered to bring out the "preferred" characteristics. Well it sounds great on the surface, this is actually a dangerous game of Russian roulette. First of all, if the procedure becomes too widespread we would in essence be eradicating variability within our gene pool. It is this same variance which is the mobilizer of natural selection that gives us our ability to adapt to new environments. Now lets say a bacteria or virus evolves to exploit a now homogenous niche that our scientists have now placed in EVERYONE. Not a single person will have the variance to counteract that virus/bacteria... whoops. Secondly, if genetic altering is only a thing for the rich, then we might find that if two genetically altered children were to fall from the economic status of their parents that either they could not have children at all as they have so many recessives combining inutero that the fetus is continually rejected, or if a new baby is conceived without the same genetic tinkering that the parents underwent, that the resulting child may have severe genetic defects, again by two recessives that matched up in the new baby for the fact that that recessive with a different dominant in both parents had allowed a benificial trait in their parents genetic engineering procedure. Simply put, genetic engineering may create a circumstance where it becomes a neccesary procedure for genetically engineered people to have healthy offspring and the Human race would be up the creek without a paddle if we somehow lost the ability to continue the procedure, once started.
So what are we evolving into... What environment are we heading towards and how will our biology enpowered by natural selection and our sociopolitical and medical progress, which cheat the processes of natural selection, shape our evolution, our sucess within this new environment? This is the question. The answer is