big sex questions

This is (a) my list of "why is it like this?" questions, (b) an attempt to trigger VUW's `websense' filter by using the s-word lots on one page (unsuccessful, to date).

-- marcus - 18 Jan 2009

1. Why recombination at all?

Loosely, "why sex?". Why isn't everything female and parthenogenic (self-fertilising)? A gene causing asexual parthenogenesis would leave twice as many descendants as one causing sexual recombination. That's twice as fit!

four ideas:

The first 3 seem to be about variability: you want to be able to produce lots of viable variants: pure mutation makes variation but most of them aren't viable.

• lottery principle (George Williams 1975): changing environment - if things are changing unpredictably, make a variety of solutions so that some work out.
• tangled bank (Ghiselsin 1974): spatial heterogeneity - if you don't know quite where your offspring are going to land, it's good to make a variety (like lottery principle).
• red queen (Van Valen 1973): genetic arms race with parasites - good to be able to flee parasites, yet make viable offspring. Of these, the red queen beats the others since sex seems especially prevalent in unchanging environments as opposed to varied ones.

• DNA repair (Bernstein et al 1989): needed as consequence of Muller's ratchet - with asexual reproduction one fatal error ruins all the other good ideas on the same strand of DNA. With recombination though (a) during meiosis (prior to crossing over) chromosomes line up and the spare copy (one's own...) can be used to repair, and (b) out-crossing allows busted DNA to be replaced (by someone else's...).

The fourth is about damage [and only (b) requires out-crossing / recombination, as opposed to built-in redundancy, I guess]

2. Why are there males?

ie. why isn't everything (that is sexual) hermaphroditic? Assuming there's something bad about self-fertilization, and recombination is therefore a good idea, and we're going to have males and females, why not combine them? Then if two individuals meet they can each fertilize the other (certain deep-sea and reef fish do this, as do many flowering plants), and if there's always the handy option of self-fertilization.

What is a "female" (or "male") anyway, really? Apparently the primary distinction comes down to size of the gamete: females are the bigger ones. Why did this asymmetry develop then?

So there are two questions here really. A micro one about the sizes of gametes, and a macro one, about the exclusive use of only one of the two types of gamete by entire creatures.

ideas:

huh?

To the micro question: a runaway process perhaps, involving smaller riskier morphs "racing" to reach larger (therefore more sedentary) ones?

To the macro question: perhaps the cost of hauling around a dual set is only worth it if probability of meeting is very low. Or perhaps the dynamics of competition for mates means it's better to specialise into one sex or the other, e.g. if there is competition amongst the fertilizers and the more 'differentiated' individuals tend to win significantly more competitions, then it may pay to give up the fertilizable (female) side in order to specialise in being a fertilizer (male).

3. Why are the numbers of males and females so close?

Producing and distributing sperm is cheap and cheerful whereas eggs (and egg-rearing) are risky and expensive, and yet the operational sex ratio is almost always 50:50. In many species, most of those males don't even provide any genetic material! This seems incredibly wasteful.

idea:

• Fisher. Suppose you're about to be born. Would you rather be male or female?
  • If the ratio of females to males is 2:1 then, on average, each male will have 2 descendants in the time that each female has 1. You want to be male.
  • If the ratio of females to males is 1:2 then, on average, each male will have 1/2 a descendant in the time that each female has 1. You want to be female.

End of story, I reckon. [nb. A killer experiment was done with fish that have a temperature-dependent sex ratio. Warm the bath and they have more (say) females, cool it and it's males. But breed them in those baths and you quickly get back to 50:50. b.t.w. presumably this temp-dependence has some adaptive value, e.g. good to have females early in season and males latter, or something like that]

4. Why are males and females so different?

A certain amount of sexual dimorphism (difference, physical specialization) is readily explained in terms of natural selection - women have to have hips wide enough for birth, but men don't. But the differences between how males and females look and act often seem more pronounced than "normal" natural selection would suggest. Eg. males fight each other, and females don't. How come?

idea

(Darwin:) Once natural selection discovers recombination, a second kind of selection starts to operate: sexual selection. To reproduce you now not only have to grow up healthy (natural selection), but when you get there you also have to interact successfully with (a group of one...) the opposite sex, hence "sexual selection". This isn't just one thing though:

1. mates of at least one sex become a limiting resource - with a 50:50 operational sex ratio and long gestation time usually there will be there will be limited availability of females, which drives male-male competition. "male-male combat" (=competition to be able to mate at all). Traits for this include weapons like horns. Anything that males use to compete directly with one another in the context of mating.
2. either sex that has significant investment in any offspring will have strong choosiness about who to mate with. Since females usually bear higher costs, they're usually more choosy. Traits selected by mate choice are called "ornaments". Nb. the female ornamentation seen in humans indicates significant male investment in offspring, doesn't it?
3. where force is possible, there can be sexual conflict. Expect male traits for subduing females, and female traits for defending against this (eg. `cryptic female choice').

[q: what's a species that has very similar looking / acting sexes? how come, given the above?]

5. Why are some (sexually selected) traits so over-the-top?

Some sexually selected traits go beyond what is required to just be good for something directly (horn for fighting, ) - they seem arbitrary, silly, or downright costly, eg:

• • peacock tails - their main predator is the tiger, and pulling on the tail is a really convenient way both to find them and to fish them out of the tree!
  • Rhinoceros beetle - monster super-sized male-male fighting equipment
  • human breasts - bigger than they need to be for feeding babies.
  • human beards - it's known that the deity has one, so possibly that's why male humans got them as well. Otherwise they're a complete mystery.

ideas:

• honest signals, a.k.a. "good genes" - they're signals to other animals concerning some trait that is harder to see... (huge horn with tinsel on, shows males you're in good shape which avoids fights, and shows females you're in good shape so they choose you). Only hard-to-fake traits should become signals / be attended to. [Nb. this (handicap) principle holds for all kinds of signals, not just sexually motivated ones.]
• `Runaway' sexual selection, a.k.a. "sexy sons" - female choice is prone to meaningless specialization: choosiness for its own sake can evolve. They're not signals of anything!