A favourite theory within evolution is that the sole purpose of DNA, of genes, is to propagate, and that the organisms they form exist for the sole purpose of accomplishing this goal. This explains why an organism will favour its own survival above that of others, unless it can protect its offspring, and also the incredible urge all organisms possess to procreate, even at the cost of their own lives, such as with salmon. It also may be the downfall of more intelligent species.
No species before has been able to manipulate its own evolution and genetic makeup the way humans now can. We already have shown with various animals including dogs (tame wolves), cats and cattle that we understand how to make certain characteristics in the phenotype more pronounced, even if it goes right in against the normal process of natural selection. So are for example virtually all kinds of dogs riddled with defects, such as the limited (~5 year) lifespan of Danish Dogs and their ability to crush their own internal organs if they ever end up on their back, or the serious hip issues with German Shepherd dogs, among other issues: http://en.wikipedia.org/wiki/German_Shepherd_Dog#Health
Evolution originally relied on natural selection and a bit of luck to see which traits would lead to the best chances at survival. Now that we humans have changed the game by largely isolating ourselves from our original environment, we are at risk from many factors in this new environment we created, ranging from various chemical compounds, (growth) hormones, lack of exposure to certain types of bacteria and illnesses leading to various auto-immune diseases and sometimes cancer, the excess of available and largely unhealthy food, and so on and on. This should be a very familiar story to most of us.
The real question at this point is: how are we affecting our evolution as a species and where will it lead us? The number of random mutations in modern society is relatively high due to exposure to carcinogenic substances, whether it's from household chemicals, unhealthy habits like smoking or tanning, or other factors. This introduces a lot of potential deviations within the genetic makeup of the population as a whole, and thanks to society's tendency to preserve individuals even when afflicted by life-threatening diseases, the genetic material has become muddled with a lot of 'weak' genes which otherwise would have been selected against.
Normally the selection process would follow a more or less strict course, not allowing for deviations straying too far from the general direction. Nullifying this effect can have unintended and potentially disastrous consequences. Weak genes in a population will eventually propagate to a large percentage if given the chance to keep existing. This will mean more individuals afflicted by genetic diseases, more deformities at birth, more spontaneous abortions and above all a significantly reduced average lifespan.
Another result of these weak genes and environmental factors is reduced fertility, causing many to resort to artificial methods to induce pregnancy, whether it is through taking extra hormones or by having the conception take place outside the body through In-Vitro Fertilization (IVF). The dangerous thing about these methods is that the effects of them haven't been studied properly or at all. We know for example that children conceived through IVF have a significantly higher number of genetic defects within their DNA and the possibility of an IVF child being born with genetic diseases is thus higher. All taken together the quality of the genes within the population is thus further reduced.
The question thus being raised is whether this process will continue, leading to an ever-weakening gene pool within the human population, or that there exist certain factors which act to curb or limit the process. One thing is for certain, though. Humans will want to keep propagating, no matter how, thanks to their selfish genes. Even if it destroys humanity as a whole.