Wednesday, April 08, 2009

Frequency dependence & cooperation   posted by Razib @ 4/08/2009 01:56:00 AM

Snowdrift game dynamics and facultative cheating in yeast:
The origin of cooperation is a central challenge to our understanding of evolution...The fact that microbial interactions can be manipulated in ways that animal interactions cannot has led to a growing interest in microbial models of cooperation...and competition...For the budding yeast Saccharomyces cerevisiae to grow on sucrose, the disaccharide must first be hydrolysed by the enzyme invertase...This hydrolysis reaction is performed outside the cytoplasm in the periplasmic space between the plasma membrane and the cell wall. Here we demonstrate that the vast majority (99 per cent) of the monosaccharides created by sucrose hydrolysis diffuse away before they can be imported into the cell, serving to make invertase production and secretion a cooperative behaviour...A mutant cheater strain that does not produce invertase is able to take advantage of and invade a population of wild-type cooperator cells. However, over a wide range of conditions, the wild-type cooperator can also invade a population of cheater cells. Therefore, we observe steady-state coexistence between the two strains in well-mixed culture resulting from the fact that rare strategies outperform common strategies-the defining features of what game theorists call the snowdrift game...A model of the cooperative interaction incorporating nonlinear benefits explains the origin of this coexistence. We are able to alter the outcome of the competition by varying either the cost of cooperation or the glucose concentration in the media. Finally, we note that glucose repression of invertase expression in wild-type cells produces a strategy that is optimal for the snowdrift game—wild-type cells cooperate only when competing against cheater cells.

From ScienceDaily:
Studies have shown that in the wild, yeast carry different numbers of copies of the invertase gene. This genetic diversity in the wild may be similar to the long-term coexistence of cooperators and cheaters observed in the laboratory, says Gore.


Wednesday, September 12, 2007

Popularity of pretty boys and frequency dependence   posted by agnostic @ 9/12/2007 10:10:00 AM

In the comments on a previous post about height of pretty boys, Jason suggested that they're so popular because they appeal to younger girls who value not-so-threatening guys more than do older women, and because their non-threateningness appeals to a given woman throughout the majority of her menstrual cycle, unlike the virile caveman-looking guys who only appeal to her during the narrow window of peak fertility. I think this view confuses pretty boys with safe, gentle father figures, and so do most studies that forces females to choose between safe, non-threatening guys and macho men. That's because there are not just two ideal types of guy -- the tough, polygynous alpha male vs. the supportive, monogamous dad -- but three, including the polygynous pretty boy / rockstar type.

This is a well established typology elsewhere in nature: the Common Side-blotched Lizard shows exactly these three types. The "sneaker" Yellow males, from afar, look just like females, allowing them to fly under the radar and copulate with numerous members of an Orange alpha-male's harem. The monogamous Blue mate-guarders have only one female to watch, and they cooperate with each other due to green beard effects (free text of the published article), so they can defend their female against sneakers. The three morphs play out a frequency-dependent game of rock-paper-scissors that cycles every six years.

Right away, we've discovered one reason why so much genetic variation is maintained in traits relevant to sexual selection in humans, from personality traits to dancing ability to height -- there is likely no Evolutionarily Stable Strategy, since alpha-males can invade a population of demure good dads, fly-by-night types (pretty boys, artists, musicians, etc.) can invade an alpha-male population -- the latter might not worry about a pretty boy being around his girl since he might assume the guy was gay or otherwise unattractive to women -- but monogamous good dads can then clamp down on the fly-by-night debauchery by cooperating to erect morally upright social practices.

There's probably some difference in the ecology of humans and the lizards that accounts for why the frequency of human morphs don't follow a clean, periodic pattern. We probably face greater temporal and spatial heterogeneity, so that there's rarely a winner-take-all moment when one morph displaces just about all members of the prevoius morph. So it's not like rock-paper-scissors where each strategy totally defeats exactly one other, and is totally defeated by exactly one other (separate from who it beats). Also, human females likely have much more choice than the lizard females, and could follow a mixed strategy (i.e., choose different types of males).

Getting back to the datum that women find different types of guys more attractive when they're at their peak fertility during a menstrual cycle, we now see why pretty boys cluster more with alpha-males than good dads. Well, just behaviorally, babyfaced rockstars and actors are more likely to be polygynous than drab but devoted dads -- that's true for the lizards too.

It's an error to suggest that pretty boys have a non-threatening appeal: if that were true, then they would appeal more to older females, who are more concerned about security than cutting loose and throwing caution to the winds. Also, which cohort of females is most likely to date drug dealers, gang members, or other dangerous males? Or to commit crimes and act dangerously and barbarically themselves? Not the older ones. And it's pretty clear that females who dig Johnny Depp or Jared Leto are not thinking what a great father he'd make. Rockstars and dreamy actors have legions of eager groupies throwing themselves at them -- hardly a long-term strategy. That's true even with non-famous rockstars, although the groupies number fewer: think of how many talentless bums with guitars you've seen get the girls.

What is the more abstract trait that pretty boys and rockstars share with macho men, then? They're exciting, risky, volatile, and take-charge. So, the proper dichotomy is not "virile vs. wimpy" as has been supposed, but "exciting vs. drab," with the former having the two distinct sub-groups "macho man vs. pretty boy." Another way to see that this is the right dichotomy is to look around the world: wherever girls really dig macho men, they also dig the peacocky musician type too, finding safe guys a bit boring. And conversely, where devoted dads do the best, it's more difficult for macho men or in-town-for-a-day rockstars to make out like bandits.

Bobbi Low has shown that pathogen stress is positively correlated with polygyny (free PDF). So whatever it is about high-pathogen-load areas that selects for greater polygynous behavior -- maximizing your offspring's genetic diversity to stay ahead of the pathogens? -- will result in an increase in both gorilla-like and peacock-like males, since they're two viable ways to pursue a polygynous mating strategy. In particular, this resolves the paradox of why men of West African descent tend to show higher levels of both the belligerent tough-guy, as well as the high-pitched singer who can dance better than most girls and has a penchant for dapper dress. Scandinavia and Northeast Asia, by contrast, show lower levels of badasses and show-offs but higher levels of good dads. That follows from the pathogen-load hypothesis, combined with the three-part, as opposed to two-part, typology of ideal male types.

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Monday, June 25, 2007

Martin Nowak interview (translated)   posted by agnostic @ 6/25/2007 12:11:00 AM

Martin Nowak, whose new book Evolutionary Dynamics is a must-read, recently gave an interview (PDF) for the Italian magazine Panorama. Unfortunately, it is only in Italian, so to put my break time to good use, I've translated it below. I'm fairly certain that everything is correct, but any errors are due only to me. See here for Razib's posts on Nowak's book.
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[Headline]: Give, and ye shall receive: it's not the Gospel but Darwin
According to the American biologist and mathematician Martin Nowak, the motor of evolution and of survival of the species is altruism.
--by Chiara Palmerini

[Photo caption]: Equations of generosity. Martin Nowak. Below, the Amish, a community founded on mutual aid.

There was a pest that tormented biologists as early as Darwin: how is it that humans, and sometimes animals too, sacrifice themselves for others in a world that evolved according to the law of survival of the fittest? His successors have found some solutions. Richard Dawkins, for example, has maintained that the true selfish agents are genes, willing to do anything -- including to sacrifice an individual, for example the mother, to perpetuate themselves within the child. The problem is that altruism and generosity are not observed only among relatives. What next, then? In a singular reversal of perspective, the biologist and mathematician Martin Nowak maintains that cooperation, far from being a problem for evolution, is one of the laws that propels it, on a par with random mutation of DNA and natural selection. At Harvard, Nowak directs the program for evolutionary dynamics, in which mathematics is applied to the study of evolution. While passing through Varenna, Italy, for the conference Evolvability: the evolution of evolution, organized by the "Piero Caldirola" International Centre for the Promotion of Science, he responds to Panorama.

Why is cooperation a law of evolution?

Because it permits the construction of complexity. For example, it's cooperation between cells that leads to multicellular organisms. Without cooperation, the evolutionary process will not reach the highest levels.

Are cooperation and altruism the same thing?

According to most evolutionists, yes. But I think that the motivation should be important if it's altruism: the true altruist helps without selfish motives.

In your recent work in Science, you describe five mechanisms for the evolution of cooperation. What are they?

The first is cooperation among relatives, summed up by the motto of John B.S. Haldane: "I'd lay down my life for two brothers or eight cousins." But cooperation is also observed between people not related by blood. The rule that matters here is: "I scratch your back, you scratch mine," which still leaves out many aspects of cooperation among humans -- we help even those who will not be able to return the favor.

So then?

Then the rule of indirect reciprocity comes into play, which is typical of human societies. "You scratch my back, I'll scratch the back of someone else." Here reputation counts: helping someone serves to establish a good self-image, which will be rewarded by others. Even though some forms of indirect reciprocity are found among animals, only among humans does it develop fully. It's no accident that as a species we are very interested in gossip. Language could have evolved for acquiring information and spreading gossip, in tandem with indirect reciprocity.

Is not Darwinian generosity an oxymoron?

No, even in a situation where all compete against all, forgiveness and generosity can be a winning strategy.

You cite the Gospel as an example of indirect reciprocity: give, and ye shall receive. Evolution and religion, however, do not agree so much.

If you ask me, it's incorrect to interpret the theory of evolution as though it led necessarily to atheism.

Are you a believer?

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Wednesday, March 14, 2007

The games people play   posted by Razib @ 3/14/2007 12:45:00 AM

I purchased The Origin and Evolution of Cultures and Foundations of Human Sociality as a 1-2 theory + data survey of culture. Two things that have jumped out at me immediately:

1) In Foundations of Human Sociality the authors show that the variance in 'rational' behavior by universities students world wide (Western and non-Western) is very small. In contrast, in the 15 world wide cultures they surveyed there is an enormous variance in the modal outcomes. If man had a paleolithic mind, then it is very diverse indeed. The differences don't seem to track phylogeny well at all (that is, genetically close groups can behave very differently, and similarly to distant groups). Part of the answer seems to be that analogical reasoning plays a big role in how these peoples respond to these "games." Remember the whole schtick about how human universals are what matter, and they're hard-wired by the Stone Age EEA? That is a lot easier to assert when your study subjects are undergraduate college students apparently. On the other hand those barely out of the Stone Age seem to exhibit more behavorial plasticity.

2) In one paper there was a peculiar finding re: the Ultimatum Game, defined by wiki as:
...two parties interact anonymously and only once, so reciprocation is not an issue. The first player proposes how to divide a sum of money with the second party. If the second player rejects this division, neither gets anything. If the second accepts, the first gets her demand and the second gets the rest.

This experiment wasn't that anonymous. The researchers found that when tracking gender, women in mixed players pools tended to accept far lower offers from men than women. Men did not accept low offers from either gender. But, it turns out that on average women in mixed player experiments (that is, repeated plays with different individuals) had the highest overall take. Why? Because half the players were men and they tended to have a higher offer acceptance rate in those cases. The men tended to reject many offers when they felt they were too low no matter the context, though they did net the bigger offer when the giver gambled on their stubborn pride. I don't know what that says for human sociality, but one thing I have mooted on this blog is that males are high risk players, and women keep it safe. The Trivers-Willard Hypothesis is an extension of this insight when considering the nature of reproductive skew: female offspring are a sure bet, but male offspring are a bigger bet.

...more later.

Related: The games genes play.

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