[schrodingersCat]

Its generally understood that traits that encourage reproduction regardless of whether or not they encourage fitness will outperform other traits (https://en.wikipedia.org/wiki/Sexual_selection). This is why you will see the appearance of coloration and seemingly useless adaptations faster than actually beneficial traits within a population (i.e. a giraffe's neck; its not for reaching those high leaves). Natural selection is generally concerning traits that promote survivability over time, a much slower process. A major hypothesis is that sexual selection may be a driving force behind differentiation of the sexes, probably originating from genetic information sharing among single-celled organisms. A competing hypothesis is that there is an intrinsic advantage to sexual reproduction versus asexual reproduction. I generally favor the former hypothesis because it generally fits the "short-sightedness" principle of evolution.

[arketyp]

I agree with you. I wrote an essay once about the need to be sentient about what characteristics of biological evolution one adopts in genetic programming, arguing that sex is arguably just a serendipitous side-effect or at least a sub-optimal solution to the problem of effectively mixing gene material. Much points to this, such as the short-sightedness principle of evolution, and that group or species selection argument is generally hard to make.

However, I am still reluctant to accept your distinction between natural selection and sexual selection. The article you link to even calls sexual selection "a mode of natural selection". The giraffe neck does not have fitness value in the sense of making it a better food processor, quite the opposite. But it does have fitness value in the sense of prospering those very genes, indeed otherwise the giraffe would not exist. Such a definition of fitness does tend to be tautological, but the alternative is no better, because it is inevitably based on a measure of survivability that in the end is necessarily arbitrary.

[PeterWhittaker]

The basic distinction is this: Natural selection refers to one organism having superior reproduction prospects due to inherent physical factors (strength, speed, height, weight, ability to digest specific foods, ability to detect and respond appropriately to threats, etc.). Natural selection takes place without the awareness of the hosts involved: By the time their relative advantages or disadvantages are in evidence, they are long dead.

In other words, natural selection is about fit, that is, how well an organism "fits" or is suited to and environment. (That it what Darwin meant by fitness: Environmental suitability.)

Sexual selection refers to overt preferences in mating: A female bower bird chooses the male that builds the "best" bower (a completely useless artifice neither ever use), a human male prefers women with larger breasts (pretty much ditto), a peahen prefers the peacock with the more extravagant tail (ditto for sure, in fact, more extravagant tails are liabilities).

Examples are many. The term "runaway sexual selection" is used to refer to exaggerated traits that become prevalent and possibly dominant in specific, local populations (think of human characteristics you associate with certain groups: Sometime, somewhere, someone preferred those slightly larger eyes or slightly curvier hips and over time those became prevalent). These traits are generally completely irrelevant, with zero fitness value.

They exist because we think they look/smell good. Why did we come to prefer one over another? The same random variation that causes changes to fitness causes changes to fitness indicators, but those indicators are not necessarily honest signals.

[schrodingersCat]

Sexual selection is a huge area of research in evolutionary biology. You don't have to like the wikipedia article, but a quick search on google scholar on the term reveals over 2M+ results. Check it out. Peter's description of sexual selection was much better than mine, so thanks! I brought up the Giraffe's neck because its one of the best examples of bad scientific method leading to incorrect evolutionary conclusions. Giraffe males joust as a dominance ritual prior to mating, and generally the giraffe that can exert more force will win and hence mate. There are no advantages to the neck outside of this ritual as far as anyone has been able to determine, hence it is understood to be a purely sexually selected trait.