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Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
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Author | Topic: Sexual Selection, Stasis, Runaway Selection, Dimorphism, & Human Evolution | |||||||||||||||||||||||
macaroniandcheese  Suspended Member (Idle past 3928 days) Posts: 4258 Joined: |
i got yelled at a lot the last time i tried to discuss this. cause i got into race as sexual selection and they all flipped out.
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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
do you remember the thread? just so I can be prepared for the onslaught?
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macaroniandcheese  Suspended Member (Idle past 3928 days) Posts: 4258 Joined: |
no lol sorry
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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
ah okay.
so I should be careful about saying things like the impetus for mating with impossibly young females leads in its extreme expression in certain individuals to pedophilia? goo....oops! we are limited in our ability to understand by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}
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EZscience Member (Idle past 5154 days) Posts: 961 From: A wheatfield in Kansas Joined: |
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).
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Chiroptera Inactive Member |
Are these two all that different? In species that practice "male combat", the males don't force the females to mate with them. The females "choose" to mate with the winners of the combat ritual.
In fact, the peacock's tail is just another form of "male combat", except that the males strut around displaying their tails rather than butt heads. My guess is that if we think real hard, we could probably come up with a fairly smooth continuum of behaviors from rams butting heads to peacocks displaying tails. (Song birds come to mind, for example.)
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EZscience Member (Idle past 5154 days) Posts: 961 From: A wheatfield in Kansas Joined: |
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/mg18624935.900 This message has been edited by EZscience, 04-27-2005 02:15 PM
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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
EZScience writes: However, it is important to distinguish between 2 kinds of sexual selection: 'male combat' and 'female choice'. I refer you to the first line of the post:
This is cursory and a little simplified (no distinction between intra- and inter- sex selection) on purpose to keep it short (??) and to the point. Male combat can be important in polygynous species whenever males can monopolize ... 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.) but not always (hyenas). again I already said (under dimoprphism):
There are also species where there is a large difference in {size\weight\strength}, and where the larger sex usually bullies the smaller one (and the smaller immature members of the same sex) in order to dominate sexually: gorillas, sea lions, hyenas, elk, etcetera. and there are also species that have dominant male types and sneaky male types.
EZscience, msg 22 writes: However, it is generally accepted that females are usually passive participants in the male-combat driven situation. But are the females herded by the males or do they chose to follow one until a better choice comes along? ps -- sorry for the delay, it appears I was a victim of a drive by suspension. we are limited in our ability to understand by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}
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EZscience Member (Idle past 5154 days) Posts: 961 From: A wheatfield in Kansas Joined: |
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
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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
the other aspect of sexual selection that drives (usually) male dimporphic seperation from {female\previous\average} features is that the males are expendible: you need fewer of them for species survival (note relation of this statement to the genetic evidence of less male diversity than female in the "adam" and "eve" concepts)
thus if a choice is made for defending the species, it is usually the males that do the {defend and\or die} routine and if a choice is made for extravagant decorations, it is usually the males that end up decked out to the nines (with striped shirts and plaid pants). and yes, selection of female features is rarer, but not non-existent. we are limited in our ability to understand by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}
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EZscience Member (Idle past 5154 days) Posts: 961 From: A wheatfield in Kansas Joined: |
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.
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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
Interesting too, are the hermaphrodite species like some snails that can be either {sperm\giver} or {egg\reciever} and their mating behavior (tag-your-it).
In some {cases\species} the female has become able to do without the male altogether which makes all the offspring clones, independence in mating at the expense of loss of (most) variability. One wonders if another mechanism of sharing DNA would evolve given sufficient time: virus? we are limited in our ability to understand by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}
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mick Member (Idle past 4986 days) Posts: 913 Joined: |
In some {cases\species} the female has become able to do without the male altogether which makes all the offspring clones, independence in mating at the expense of loss of (most) variability. One wonders if another mechanism of sharing DNA would evolve given sufficient time: virus? Very nice idea! It would have to a virus that only infected gametes, though, as it wouldn't be very effective to attempt to infect all your I-dont-know-how-many trillion cells.
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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
that would tend to select for sexually transmitted virus in order to have opportunity (as well as motive?) ... first riding on the "coat-tails" of male sperm?
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mick Member (Idle past 4986 days) Posts: 913 Joined: |
razd, that is genius! Turn that into a sci-fi novel and you will be rich!
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