What are the 4 factors that must be present for natural selection to occur?

The factors
involved in natural selection are not necessarily fixed at 4; in many ways, it's a matter of
perspective and context, ie. what is taken for granted, and what is assumed, about the
population and ecosystem being inspected. We might interpret this question to say "what
four factors most directly influence natural selection occurring
as opposed to another form of selection, such as sexual or
artificial."

The four primary factors are;

1. Variation exists within a species. This is
   largely the result of mutations and meiotic gene shuffling. Variation allows for the possibility
   of certain variations being more fit than others.
2. Species
   overproduce. This means that they have more offspring than their environment can
   support. If they had less offspring than the environment could support, then a surplus of
   resources or lack of competition would not place the same selective pressure.


3. Competition exists. This can be intraspecific or
   interspecific competition, but in most ecosystems it is likely a little of both. Competition
   causes certain variations to be more capable of succeeding in the competition than others; a
   lack of competition would make the variations largely irrelevant.
4. 
   Inheritance. If the offspring of individuals inherit their parent's
   traits, this ensures that certain aspects of variation will endure; fit individuals will produce
   fit offspring, and unfit individuals will not produce magical, super-fit babies for no apparent
   reason.

We must be careful to frame some of these factors in the
proper persepective; for example, competition among plants does not mean that two plants are
eyeing the same grassy hilltop and shooting each other dirty looks. In fact, most of the time,
there is no "intention" in the human sense behind most of these factors, but rather
they are simply the way the world is described from the perspective of long-term ecological
change.