A simple argument suggests that sexual selection is
generally responsible for almost twice as many features
of organisms as natural selection is ;-)

The argument compares the volume of information added to the 
population by each process in each generation.

I'll define the term "natural selection" - in this context -
to refer primarily to deaths.

There are usually about as many births as deaths in the 
population.

The information needed to describe the deaths in a given
small period of time essentially consisits essentially of a
list of who died.

The information needed to describe the births in the same
period of time essentially consisits essentially of a list 
of which females conceived children with which males (along
with a whole bunch of information about meiotic divsions
which I will consider to be random).

This can be represented by two numbers between 1 and half 
the population size.

That's twice as much information as is needed to represent
the deaths (minus two bits per event).  Two bits per
event is probably negligible.

Unfortunately, the list of (dubious) assumptions underlying
this argument is rather embarassingly long:

Steady state population;
Random mating;
No senescence;
Equal numbers of each sex at conception;
The resulting changes don't interfere;
Mutation is neutral with respect to the resulting changes;

However some of these assumptions cancel each other out somewhat.

For example, senescence means the death list is compressible -
and non-random mating means that the list of mates in compressible.

I think the conclusion is fun enough to be worth pointing out ;-)

No wonder some organisms appear to spend more time thinking about
sex than about where their next meal is coming from ;-)

Lastly, I note that the argument ignores sperm competition and failed 
conceptions - on the grounds that they fall into a grey area between 
natural and sexual selection.  However I don't for one moment doubt
that their significance in evolution is substantial.
-- 
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