Definition of Sexual Selection?

Question

How do you define Sexual Selection (SS)?

(One might want to subdivide SS into intra- and inter- SS to answer)

• Is SS clearly different from Natural Selection (NS)?
• Is SS nested within NS or are NS and SS two different and (anti- or not) parallel processes?
• Is the evolutionary processes due to sexual conflict are SS processes?

  Sexual conflict arises when the reproductive interests of males and females are not aligned, generating sex-specific selection on shared traits such as mating rate (from this article)

  • (One may want to subdivise sexual conflict into intralocus and interloci conlict to answer)

• Are the evolutionary processes of sexually antagonist genes SS processes?

  Sexually antagonist genes are those genes where one allele is favored in females while another allele is favored in males (my own definition)

• Can SS be accurately defined or is there a continuum of kinds or processes from complete SS processes to complete NS processes?
  • Stated differently, is SS defined as the color orange (it is a continum) or like a square and a rtiangle (no continuum)
  • (I talk here about kinds of processes, not about the relative importance of SS and NS in explaining the evolutionary dynamic of a trait or a locus)

If you define SS based on some other concepts that might be subject to various definition, please define this other concept. For example, if one says "SS occurs on a trait whenever this trait undergoes directional selection due to sexual competition", he will necessarily need to define "sexual competition" as well.

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Below is a list of scenarios where you can answer by yes or no whether SS is involved in the evolution of some feature. In all of these scenarios, we'll consider cases where the males have to find a mate…

1. The males need good ears to find the females before other males. Is SS involved in the adaptation of ears abilities?

2. The males need good flying abilities (supposing we talk about flying animals) to have access to females before another male. Is SS involved in the adaptation to good flying abilities?

3. The males need good flying abilities to have access to females although there is not much between male competition because the vast majority of females won't never be fertilized. Is SS is involved in the adaptation to good flying abilities?

4. The males need good flying abilities to be able to fly a long time without spending to much energy otherwise they will die before finding a female. Is SS is involved in the adaptation to good flying abilities?

5. The males need good flying abilities to be able to fly a long time without spending to much energy otherwise they will die before finding a female. There are quite a few females but there the competition is so rough to find a female that is still unmated Is SS is involved in the adaptation to good flying abilities?

6. The males need good camouflage because when they fly to find a mate they may get eaten by a predator and die before reproducing. Is SS is involved in the adaptation to good flying abilities?

7. The males need good camouflage because when they fly to find a mate they may get eaten by a predator and die before reproducing. Here the difference is that the predators are attracted to the spot if they can sense many males. So the presence of the other males influence the probability of finding a mate of a given male. Is SS is involved in the adaptation to good flying abilities?

Answer 1

Is SS clearly different from Natural Selection (NS)? Is SS nested within NS or are NS and SS two different and (anti- or not) parallel processes?

Darwin, in The Descent of Man, and Selection in Relation to Sex defined sexual selection as a type of selection that "depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction." Throughout, he distinguishes between natural and sexual selection. This suggests to me that Darwin thought of them as different processes. A spot check of various evolutionary texts show the authors all using some variation of Darwin's definition.

In my opinion, the two are are clearly different but not necessarily separate. Both NS and SS affect fitness, a major component of the evolutionary process. I tend to think of SS as nested within NS. NS affects fitness indirectly. Beneficial traits increase survivorship. The longer individuals survive, the more reproductive opportunities they will have and fitness will, on average, increase. But, survivorship does not guarantee reproduction. Thus, I think natural selection acts indirectly on reproductive success.

SS operates directly on reproductive success because males selected by females will reproduce, increasing their fitness. Sex is, after all, a natural process. In my view, female choice is the "environment" of the male (as far as reproduction) so variation of male traits in the female choice "environment" differentially affects the fitness of the males. That is natural selection.

Is the evolutionary processes due to sexual conflict are SS processes?

Absolutely. Sexual selection ultimately arises because of a difference between genders of energy investment in gametes. Males produce lots of gametes and relatively little energetic cost. Females produce relatively few gametes with lots of energetic cost (think of tiny sperm vs large, yolk-filled eggs). It benefits males to mate with as many females as possible because, even if a mating produces no offspring, he lost very little of his energy investment. Females will be choosier because they can't afford to waste gametes. Thus, there is an inherent conflict of interest between males and females.

Are the evolutionary processes of sexually antagonist genes SS processes? Sexually antagonist genes are those genes where one allele is favored in females while another allele is favored in males (my own definition)

Are these genes related exclusively to reproduction? If so, then that would be consistent with the original concept of SS. However, sexually antagonistic genes are not necessarily the result of or a cause of SS. For example, Rice (1992) argued that sexually antagonistic genes may explain the evolutionary reduction of recombination between primitive sex chromosomes.

Rice, W.R. 1992. Sexually antagonistic genes: Experimental evidence. Science 256: 1436-1439.

Can SS be accurately defined or is there a continuum of kinds or processes from complete SS processes to complete NS processes?

I think SS is accurately defined, but whether is is a type of NS or not is (I think) open for debate. As I described above, I think of SS as a particular type of NS, one that operates directly on reproductive success rather than indirectly.

Below is a list of scenarios where you can answer by yes or no whether SS is involved in the evolution of some feature. In all of these scenarios, we'll consider cases where the males have to find a mate…

The males need good ears to find the females before other males. Is SS involved in the adaptation of ears abilities?

Depends. Do males and females also use their ears for other purposes, such as detecting prey or predators? Is hearing ability much greater in males than in females? Darwin specifically addresses such a scenario. (Last sentence of page 256, first part of page 257.)

I think these ideas play out for most of your scenarios. It depends on if the traits differ between the sexes (not specified in your scenarios) and the traits lead directly to increased male reproductive success. But I will address one more.

The males need good camouflage because when they fly to find a mate they may get eaten by a predator and die before reproducing. Here the difference is that the predators are attracted to the spot if they can sense many males. So the presence of the other males influence the probability of finding a mate of a given male. Is SS is involved in the adaptation to good flying abilities?

In this case, I would say not SS. The camoflage is necessary for survival, not direct reproductive success. This is natural selection. Increasing the probability of finding a mate is not the same as guaranteed reproduction.