Natural selection is the process by which certain heritable traits – those that make it more likely for an organism to survive and successfully reproduce – become more common in a population over successive generations.
Natural selection is frequently referred to as “survival of the fittest” for sake of simplicity.
Let me give a hypothetical example. There are two varieties of moths in a population – A and B. A flies faster and has a better sense of smell; B is slower and cannot smell as well. The moths feed on flower nectar for survival. Also, they are equally likely to successfully reproduce. These two varieties live in an area where there is a scarcity of flowers. Let this flower be called the O variety. Common sense would suggest that A should be able to get to flowers faster – both because of better sense of smell and faster flying. Thus, fewer of the B variety would be able to survive, and gradually their numbers would decrease. There would come a time when the ratio of both the varieties in the population would stabilize in the region. This because, despite A’s ability to reach the flowers faster, pure chance would allow the B variety to also feed on few flowers. Moreover, B-moth has to mate only once to pass down its genes even if it dies of starvation.
Now imagine, someone introduces a new plant (N variety) in the region such that its flower produces a very strong fragrance. The problem with this plant is, it is insectivorous (click), i.e., its flower traps and digests (kills) insects to fulfill its protein requirements. Now obviously, A-moths would reach the N-flowers faster and die. Gradually, the ratio of A:B moths would decrease. Whether B-moths outnumber the A-moths would depend on the relative strengths of fragrances of the O- and N-flowers. If the N-flower has stronger fragrance, then most of the A-moths would get attracted to it and die without reproducing. The situation for B-moths would be more complex. Whether their absolute numbers decrease significantly or not would depend on the number of N-flowers available. Meaning, if N-flowers are themselves not very successful in surviving, then they would only be able to decrease the number of A-moths, which would leave B-moths free to feed on O-flowers and mate gaily (with moth of the opposite sex, of course!). But if N-flowers increase significantly in number, the entire moth population might get eliminated from the region.
The important thing I wanted to illustrate was when talking of fittest in the sense of survival, the said traits need not be what we ordinarily think of as good (e.g., better sense of smell and faster speed of flying). Those traits must help the individual organisms survive and reproduce as well. So as part of evolution, species on the whole do not ‘improve’, but become better suited for the environment.
Human species has come a long way from having to struggle for basic survival. ‘Success’ in many societies is not even linked to ability to reproduce. Ability to attract a suitable mate does not necessarily transform into passing down of genes, thanks to contraception and family planning. Many inheritable diseases may manifest after the affected individual reproduces. With possibility of cloning in the future, even those unable to reproduce conventionally may be able to pass on their genes.
All these factors have made a few people concerned that the ‘quality’ of human beings would worsen. But there is inherent paradox in this idea. ‘Quality’ of human beings has nothing to do with human conceptions of desirable or undesirable traits. From perspective of natural selection, fit v/s unfit had always been about ability to pass on ones genes.
So will the human species stop evolving?
This question cannot be answered in simplistic terms. Unfortunately, because of being exposed to pictures that show evolution of species in a linear fashion with various stages (e.g. tree-dwelling lemurs getting transformed into upright walking humans), we have come to believe that evolution is all about gross physical changes. But then many changes could be subtle and not manifest physically. What allows new traits to emerge in a population are gene mutations (click), i.e., an inheritable change in an existing trait or appearance of a new one. So in case of humans, though new mutations would keep on emerging, they are unlikely to displace people of pre-existing traits, because, they will not necessarily make them reproduce at faster rate.
Surging ahead in time, when societal pressures to reproduce to continue the family would diminish, factors like inherent attraction to children and nurturing behavior, both of which are enhanced by a hormone called oxytocin (click) could become important. Thus in future, not necessarily physically stronger or better looking or more intelligent people, but those with higher oxytocin levels and desire to rear children would be evolutionarily more “successful”! Of course, all this is merely my conjecture.