Positive Selection

Positive selection is one of the great driving forces in evolution.  It is probably the thing we think of most when we're talking about natural selection and survival of the fittest.

Positive selection is a force that acts on a beneficial mutation and causes it to occur more often in the population.

Let's say that an individual in a population has a mutation that gives it some advantage over others.  Maybe it gives it slightly better eye sight so that it can see predators coming more easily, or maybe it helps it survive harsh conditions, or improves fertility.  There are all sort of reasons that a mutation could be beneficial.

In each of these cases, the new mutation helps the individual survive to pass on its DNA to more offspring.  That's basically the point of life from a genetic point of view.  The fitter you are, the more you pass on your DNA.

So, if the individual manages to pass on their DNA more because of a new mutation that they have, this mutation will enter the population more times in the next generation that a non-beneficial mutation would have (because they have more offspring).  And then their children who have this mutation, will pass on their DNA more often as well.

The mechanism causes the mutation to rapidly increase in frequency in a population.  This is called a sweep (and is researched in great depth in the lab I work in).  The speed at which the mutation sweeps through a population depends on how beneficial it is compared to other alleles.  If it is only slightly better, it will take a long time to increase in frequency, if it is much better this process will occur much faster.

If a mutation is particularly beneficial it will probably reach fixation in the population.  This means that all individuals in the population now have the beneficial mutation and there are no other versions.  It is through this process that new species can evolve.