In Evolutionary ecology: Novelty makes the heart grow fonder, they say:
Thus, it seems that the diverse coloration in this species is promoted by both natural and sexual selection.
I had always thought of sexual selection as a subset of natural selection. What's the right way to think about the relationship between the two?
You're not wrong, per se, but in practice they refer to two different concepts. I honestly think the Wikipedia article does a good job, in particular this sentence:
In summary, while natural selection results from the struggle to survive, sexual selection emerges from the struggle to reproduce.
It also cites Darwin:
The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.
The whole argument is moot when dealing with individuals that aren't reproducing sexually, of course.
To Darwin's point of view, sexual and natural selection are two different mechanisms. Then, we tended to consider that sexual selection is part of natural selection. The part of natural selection that is not attributed to sexual selection is sometimes called ecological selection. Today, our understanding of these mechanisms brought us to consider the meaning of these words slightly differently.
Natural selection is not anymore the result of a struggle for survival. It is any process yielding to a change in allele frequency through a long-enough term difference in fitness. Fitness is a index of how much an individual, a gene or whatever, leave copies of itself after a long and short enough amount of time (one might not agree with this definition I guess). Take two strains of bacteria in two petri dish, consider they are one population and let the bacteria grow. One strain will grow faster and the allele frequency will vary over time. It is the result of natural selection although there were no competition, no struggle at all given that they were not in contact.
Sexual selection is a type of natural selection that involves sexual competition. There are two types of sexual competition, the so-called inter- and intra-sexual competition. Inter-sexual competition means that one sex chose the other one, while for intra-sexual there is no choice from the other sex. For example, if you see two animals fighting, they either fight to have the physical acces to the other sex (intra-) or they fight and then, let the female decide which one is the best to mate with (inter-). These two concept of intra- and inter-sexual competition might somehow overlap in some specific case. I would argue that the concept of sexual competition and therefore of sexual selection have never been accurately defined. Therefore, I would not complain if one prefers to give up the concept of sexual selection to use only the concept of natural selection.
Many articles state that sexual and natural selection act on a trait with the same or opposite directions. Given the above argumentations, you could as well say that there are two different forces of natural selection. if the trait is at state 'A', then the individual is very attractive to the other sex but is likely to not be able to escape a predator. This is the case of most of the trait we think of when talking about trait evolving through sexual selection. See Zahavi's handicap principle for more information.
I have not posted or read much on this forum before but thought I would throw in my two cents. Perhaps a more accurate and comprehensive term is "reproductive selection". This would include "sexual selection (eg qualities making an individual more attractive to the opposite sex) and "survival selection" - to the extent that survival allows one to produce more offspring. (Dead organisms don't tend to reproduce). Reproductive selection would correctly exclude fitness traits promoting post-reproductive survival that did not contribute to the reproductive success of progeny, eg a longer lifespan spent away from mates or offspring. Reproductive selection would apply to a range of replicative mechanisms, spanning sexual, asexual, mitotic and even possible early-stage RNA-strand replication. It would also include both natural and artificial selection. Finally, it has the advantage of being a more intuitive term that IMHO gets more at the core principle driving this aspect of evolution (as opposed to genetic drift, etc). Not sure if this term is in common usage. As a clinical physician I am not very familiar with the literature on evolution.
According to a theory proposed by Amotz Zahavi (formulator of the so called Handicap Principle), and others, sexual selection can be seen as a subtype of signal selection, which in it's turn is a subtype of natural selection.
The well known example of a signal that is selected for is the tail of the peacock, which signals the high quality of it's owner. The interesting thing is that to maximise fitness it would be better for the peacock to have a smaller and less colorfull tail. Signal selection and efficiency selection (for want of a better word) hence often work in opposite directions!
Some quotes from the abstract of the article "The Logic of Analog Signaling and the Theory of Signal Selection" by Amotz Zahavi & Avishag Zahavi:
... evolution by natural selection is a consequence of two selection mechanisms: the selection for efficiency on the one hand, and signal selection that reduces efficiency in order to provide reliability on the other hand.
The fundamental difference between signal selection and efficiency selection is that
when the investment required for signalling is reduced to the extent that everyone can signal alike, the trait loses its value as a signal. This loss of value is unique to biological analog signals
For example, lace used to be a signal of wealth, but it's effect as a signal diminished once lace machines took over manual production.
The exciting thing about this approach is that it could explain the evolution of features that would be hard to explain by efficiency selection only:
We further suggest that the interaction between the selection of signals and the selection of other traits enabled the evolution of new traits that require the accumulation of many mutations that reduce efficiency before they can start contributing to new efficient adaptations e.g. antlers and feathers.
