Sexual Selection, Stasis, Runaway Selection, Dimorphism, & Human Evolution

On sexual selection...A interesting and relevant post, if a bit lengthy.Many people concentrate so much on natural selection (i.e. 'survival of the fittest'), they forget that sexual selection is also a potentially potent evolutionary force, and was also put forward by Darwin.However, it is important to distinguish between 2 kinds of sexual selection: 'male combat' and 'female choice'.Male combat can be important in polygynous species whenever males can monopolize either the females themselves (e.g. herding in ungulates) or the resources needed by females for reproduction (as in the Orians-Verner polygeny-threshold model). It tends to be common in polygynous species (one male gets many females) and accounts for the evolution of a lot of sexual dimorphism (males larger than females, antlers for fighting etc.)On the other hand, female choice was less well accepted as a concept, originally. Recently, however, 'choosy' females have been identified in many species. It is this type of sexual selection that can produce what Fisher termed the 'runaway effect'. When the gene for choosing in females becomes linked to the gene for the exaggerated trait in the male, the trait can 'runaway' to greatly exaggerated proportions. Fisher showed mathematically that this occurs because of the fitness benefits accruing to choosy females via the mating advantages achieved by their sons in the subsequent generation, once the choosy gene predominates in the population. This is the effect at work in producing the peacocks tail, and the long tails of the widowbirds studied by Andersson in Africa (the tail-lengthening experiment you refer to).

You are correct in that both forms of sexual selection can potentially act on a single trait. So you can have a character evolve that is, say, first useful in male combat, and then secondarily becomes a choice criterum for females.Also, traits sexually attractive to females may also be evaluated by males for size, quality etc. and have a cryptic effect on male competition (Don't mess with me - do you see the size of this tail ? Why bother - I'll get all the females anyway)However, it is generally accepted that females are usually passive participants in the male-combat driven situation. Thus, where male combat is evident, it is usually considered a sufficient explanation by simple parsimony. It becomes encumbent on the observer to demonstrate *active* choosing on the part of the females, just as Andersonn did with the widowbirds. Here it was easy. Males observe territorial boundaries, so male combat only occurs when territories are established early in the season. When females arrive to choose territories, they 'like' males with the longest tails. However, females may also choose the territory (based on its quality and food supply), rather than the male himself. This type of choice can be much harder to demonstrate - it is harder to experimentally manipulate territory quality than it is tail length. However, nuptial gifts are another way males can induce females to choose them (based on the quality of the gift).Where female choice becomes especially important is in monogamous mating systems that evolve primarily because both parents are needed to rear a clutch to maturity (typical of most Passerine birds). This is the most precarious and unstable of all mating systems, because there are always incentives to cheat for both male and female. A female will choose as good a male as she can get but will always be limited by her own apparent quality to the male (Unless she is Pamela Anderson). The male also wants as good a quality female as he can get to make his genetic investment with, but will readily mate with any other female regardless of quality, because he has nothing to lose (unless discovered) and everything to gain from any offspring that he doesn't have to rear. For the female, she can improve her fitness by mating with a male of higher (perceived) genetic quality than her own, but only if she can cuckold her mate into believing the offspring are his own so he will help to rear them. Some very insightful parallels for human relationships, I think, and the evolution of jealousy.Here is a link to a recent article about courting avian mistresses...http://www.newscientist.com/channel/sex/mg18624935.900This message has been edited by EZscience, 04-27-2005 02:15 PM

Yes, the difference between male combat and female choice (the 'old' terminology) is the same as the difference between intra- and inter-sexual selection. Sorry for not noting your recognition of this.But I still argue that the distinction is an important one because some traits and phenomona are not explainable by intra-sexual selection and require female choice.
For other traits, intra sexual selection is the sufficient (parsimonious) explanation). Still other traits may be affected by both forces.Also, Fisherian 'runaway' selection refers SPECIFICALLY to female choice - it is a diallelic model - one gene for preference, another for the trait being preferred.So as far as sexual dimporphism, I guess we can agree that either form of SS can cause it. In the case of larger male size, it is usually male combat that is the driving force. Note that in bird species with sex role reversal like pharalopes and jacanas, females are much larger than males because each defends a territory (and several males within it) from other females. For the herding ungulates, the social structure probably arises from mutual safety - the 'selfish herd' concept - which a dominant male can the take advantage of by driving out weaker adult males.Now, you final comment about 'sneaky males' raises an entirely different topic - alternative male mating strategies.
Actually, there are species of salmon with as many as 3 or more male mating strategies.1. 'conventional' males (large, build nests, court females)
2. 'satellite' males (mimic females in coloration and spawn together with mating pairs)
3. 'sneaker' males (very small and cryptic, dart in on mating pairs to squirt a bit of sperm in the mix, and then quickly escape).These 'secondary' mating strategies are all held at low frequency in the population by frequency-dependent selection. Because females can only be induced to spawn with conventional males, the latter can never be supplanted by either of the other types.This message has been edited by EZscience, 04-29-2005 02:35 PM

Ah yes, indeed.
The "expendability of males" is another under-appreciated factor that has far-reaching evolutionary consequences for sexual dimorphism, mating strategies, and sex-specific behavior.
Ever since the strategy of "maleness" evolved at the unicellular level (anisogamy and oogamy), it has been one of "produce the most gametes possible with the least investment in each". This enables the production of more and more gametes by putting fewer and fewer resources in each. While this might seem like a form of genetic parasitism (it is), it cannot displace the female strategy because really small gamates must fuse with a large one to produce a viable zygote.
So we must keep in mind that the success of males in producing "cheap sperm" was also the driving force behind the evolution of truly female characters - few large eggs with all the cytoplasmic resources necesssary for embryo development, internal fertilization (to control male parentage), and parental care to maximize post-natal survival. Females would not be 'as female' as they are without selection for a strategy to counter 'maleness', the original form of genetic parasitism.
But you are right - males are expendable simply because one can do the job of 100, if needed, so population growth is always determined by the number of females, not the number of males. The result is that genes go through a harder form of selection when in male form than in female. Males have always had to to take on the risker tasks (family defence, etc.) so no wonder they have the greater propensity for violence. I also think this has led to the human male brain functioning quite differently from the female brain in various cognitive processes.

Actually, viruses are already emerging as natural mechanisms of genetic transformation, not just human manipulated ones.On-going work by one of my colleagues is showing that sorghum plants naturally-infected with certain plant viruses can have their entire genome 'scrambled' by the infection, and in a way that gives rise to heritable changes in the progeny. The progeny represent a startling array of divergent morphologies, each with heritable genetic differences. Although many are, of course, non-viable, certain others have unique and potentially desirable characteristics.Interestingly, the same work shows that the genetic sequence of the virus itself can be permanently changed by passing it through a particular host plant.It is almost Lamarckian (but not quite).

Yes, hermaphrodites are the "special cases" of sexual evolution.Two kinds exist:simultaneous herm's (like your snails and earthworms)
sequential herm's (first one sex, then the other)Simultaneous hermaphrodites are thought to arise in sexual species when the organism has very low mobility and may occur at very low population densities. So the advantage is, you can mate with any conspecific you encounter - even if you only encounter one in your entire reproductive life.Sequential hermaphroditism is a bit more tricky and may have a sexual selection component to its evolution.
There are 2 kinds: protogynous (beginning as female) and protandrous (beginning as male - much rarer)It occurs in certain fish species, and a few other taxa. In the case of the fish, female fitness increases as a linear fucntion of age, simply because large females can produce more eggs.The female fish all prefer to spawn with large males for qualitative reasons: large males are 'proven' fit by virtue of their age alone. Thus male fitness is a non-linear (actually sigmoidal) function of age. This means the best way to maximize lifetime fitness is to start out female when you are small and then 'switch' to being male once you reach a certain size, because big males can achieve much more fitness than females of the same size.The interesting thing is that when you have a school of these fish in a tank and remove the male, the largest female will then turn into a male. It has been shown that male aggressive behavior, posturing, coloration, and other signals actually suppresses the development of 'maleness' in the females.Here are a couple of good links:Page Not Found (404 Error) | Port Townsend Marine Science CenterHermaphroditism(editted for typos - EZS)This message has been edited by EZscience, 05-04-2005 01:37 PM

Well it is questionable what degree of interchange of genetic information occurs between plant and virus, so phylogenies of the actual organisms will probably still remain independent of one another.However, it kind of deflates arguments objecting to 'genetic engineering' of crops as an unnatural process.
Apparently, genetic engineering by viruses is a natural phenomenon.What it does show is that extrinsic forces (such as a virus infection) can produce heritable genetic changes in an organism - hence my comment about Lamarack.

Actually, the theory of 'endosymbiosis', first advanced by Lynn Margulis in 1981 to explain the origin of mitochondria *and* chloroplasts in eukariotic cells remains the most appealing explanation to date regarding the origin of these cellular organelles.After all, both these organelles have double, rather than single, membranes suggesting they were phagocyticly engulfed by the ancestor cell, but never consumed. There is little in the way of direct evidence, apart from the double membranes and both possessing autonomous DNA, but no better explanation has yet been put forward.This message has been edited by EZscience, 05-05-2005 03:45 PM

Ah, but this was not a combining of *conspecific* genomes, but rather *heterospecific* genomes, chromasomes that had never functioned in tandem before.It was an act of predation (phagocytic engulfing) that ultimately resulted in a 'truce' between predator and prey. That would have likely been the first step. In the case of mitochondria, the 'prey' was capable of generating ATP, and if an opportunistic 'predator' happened to be able to avail itself of this energy source, and also provide some raw materials for the process, it would have benefited much more from sharing fortunes with its prey than by consuming it.Possibly some lessons there for our politicians today.But can we seriously expect those currently in power to grasp the relevancy of these concepts to human destiny ?This message has been edited by EZscience, 05-07-2005 09:52 PM

Here is a perfect example of what we were discussing earlier in this thread. In this case, its all about female choice - no male combat involved."A study of mosquitofish guppy-like creatures that feed on mosquito larvae shows that females definitely prefer well-endowed males to their shrimpier brothers.""But sex aside, being bigger isn't necessarily better. Male mosquitofish with large genitalia have a greater risk of dying even if it is with a smile. That's because mosquitofish with large genitalia known as a gonopodium can't swim as quickly...""All females had the same preference..."-as predicted by the Fisherian 'runaway' model-
fixation of a female preference in a population leading to exaggeration of a male trait that is then ultimately limited by natural selection when it reaches the point where it reduces survival.I wonder how an ID theorist would model this?

Hey Raz - good stuff.I enjoy your posts because they make me think beyond the hexapoda of my daily existence !I think you have a very well substantiated story on how the evolution of 'bareness' in humans
has evolved in response to thermoregulatory constraints and the cultural evolution of 'clothing' etc.You have convinced me that we are approaching the biological limits of nakedness.
Now if we could just get rid of the clothes, heh heh ...NO - wait, please, not *everyone* !But seriously, what is lacking in your thesis is tying this into some sort of mating advantage for hairless as opposed to hairy men.(we can safely assume that with mammals, only males usually benefit from polygamy because females are far more tied to the offspring because of lactation and thus more subject to abandonment than vice versa.For the hairless trait to be useful to females in SS, it would have to affect their fitness via the quality of their offspring.The only way to make this argument work would be if it was a male choice situation, but then you would need to show that males giving birth to hairless daughters achieved greeater reproductive success than those giving rise to hairy daughters.While this might seem logical on the surface, we need to rememeber that reproductive success in human females varies little.
Even the uglist are sometimes the most fecund !Otherwise, for this to be truly categorized as Fisherian 'runaway' SS you need to demonstrate a minimum of two things:1. Females choose mates based on degree of hairlessness in males.Hmm... opening a recent copy of GQ magazine one might expect that to be the case...but no.Rationale human beings have more cerebral mate choice criteria (one would certainly hope !).2. Hairless males leave more progeny (or more fertile progeny) than hairy males. Not likely, I think you will agree.So a great story, but not a Fisherian Runaway story.minor content added in edit - EZThis message has been edited by EZscience, 05-13-2005 10:39 PM

OK - now I owe you an apology.My edit on the post added that possibility, but not fast enough to head off you reply.So we can go from here on what it would take to make the male choice model work....As stated below:1. Females would have to vary significantly in reproductive success based on degree of hairiness (No evidence, I suggest)2. Males would have to gain fitness by selecting hairless females via the increased fitness of their hairless daughters.Entirely contingent on No. 1, but could be true if No. 1 is true.EZ

I would say that I agree with pretty well all your observations and the correlations you have drawn among them.However, I might explain them a little differently.No doubt that hairlessness is a sexually desirable trait in the human female and that it is pretty much at the extreme level possible. But I would argue that the use of clothing has diminished what would otherwise be significant dis-advantages (in terms of thermal insulation) under natural selection.
This wopuld not constitue selection for hairlessness, but it would relax selection against it in both sexes equally.To invoke runaway SS as a cause for the high level of the trait, it is not sufficient to demonstrate the the trait is a desirable feature of the opposite sex.
One must show that it is directly linked to increased reproductive success in the sex that is the subject of mate choice.
In other words, you would need to show that hairy women *leave fewer offspring* than hairless women, and that hairless women achieve higher fitness through the increased fecundity of their hairless daughters.This might be difficult for two reasons.
First, almost all women get mated and leave some offspring, no matter how hairy or how ugly, unlike males for whom variation of reproductive success can be considerable (in polygynous systems, at least, some get no matings, others get large numbers).
Secondly, there is not a lot of quantitative variation in RS among human females, at least not compared to fish or insects. Each usually leaves only 5-10 progeny max, so its not a lot of variation to work with.
So it is MUCH harder to get the model to work for male choice than for female choice.But I won't argue that it can't work.
Rather I will hypothesize a scenario in which it could.Your best evidence is the dimorphism between male and female in this regard.
It can only be explained by some selective force acting in a sex-specific manner.Since we agree the trait has been decoupled from survival criteria, we are inclined to exclude NS as a cause for the dimorphism.
The problem becomes how to formulate the reproductive advantages when there is very little *quantitative* variation in female Rep. Success.I propose that the answer is to infer *qualitative* differences in reproductive success that are conceivable in a monogamous mating system where both parents share parental repsonsibility for ensuring offspring survival. This type of mating system has been prevalent in many human societies for a long time, I suspect.So here is how it could work.
In a monogamous mating system, there is opportunity for both sexes to be choosy, since both make an almost equivalent investment in reproduction.
Hairlessness in females attracts males of 'high quality' (females are free to select males on the basis of other criteria such as size strength etc. - doesn't matter for the females trait) so they get paired up with better quality mates than hairy females and this then improves the survival probability of their offspring, rather than their number, because they have better quality fathers contributing to their care.
The hairless females then experience improved RS in terms of *fertility* (offspring quality) rather than fecundity (offspring number) and their choosey mates benefit via the increased RS of their hairless daughters. THEN you have a runaway effect, so that very soon all males are choosing females based on hairlessness and all females are virtually hairless in the extreme.So I am now convinced that it can work, provided the following conditions are met.1. A monogamous mating system.
(a pre-requiste for any male choice model, I would contend)2. Quality of male parental care is important for offspring survival.3. Female fertility varies significantly as a function of the quality of parental care contributed by her mate.Lots of fun, but now I have to do some real work

only works for female choice in the Fisherian model You lost me. I won't say no because I don't understand yet,
(you already caught me that way)
but if so, it wouldn't follow the Fisherian preference-choice linkage model. Not hard to imagine significant variation in this behavior This message has been edited by EZscience, 05-14-2005 08:01 PM

Let me digest this and respond over the weekend.