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Unformatted text preview: Discussion 4: An early challenge for Darwin: How does cooperation evolve by natural selection? natural I. Why is cooperation hard to explain? I. I. Why is cooperation hard to explain?
Cooperation appears to involve ___alturism__________________: ___alturism__________________: ___alturism______________is the sacrifice of a portion of an is individual’s reproductive success (or fitness) in order to increase that of another individual’s fitness. increase Why is this a paradox? If altruism reduces an individual’s fitness, Why natural selection should select against the altruistic trait and eventually reduce its representation within a population to zero eventually I. Why is cooperation hard to explain?
Even if a population started with only altruists, it would be vulnerable to ___cheating_____________from within: ___cheating_____________ • Imagine a single, mutant selfish Imagine individual that could _exploit________the altruistic _exploit________the tendencies of its neighbors tendencies • It would gain the benefits of others It cooperating, but avoid the __cost_____ (in terms of reducing its own fitness by cooperating itself) its Such a mutant would eventually drive Such the altruistic trait to extinction the __nothing can evolve for the good of __nothing the species__ the I. Why is cooperation hard to explain? A. Eusocial Insects A.
All cooperation is hard to explain, but the evolution of eusociality_ All is particularly hard to explain particularly Typical social structure in eusocial insects (details vary among spp)
1. Queens are reproductive females, _workers_are sterile females, and Queens _workers_are _drones_are reproductive males _drones_ 2. The queen is the mother to all the workers and drones in her hive The 3. A female’s caste is determined by the food she’s given during the larval stage female’s by her sisters, not to a genetic difference not 1. The hive will produce queens The and drones, which will leave to go found their own hives 2. Eusociality is common in 2. Eusociality hymenoptera (ants, bees, & wasps), but occurs in termites, too (isoptera) too
Worker Worker sterile female female Queen Queen fertile female female Drone Drone fertile male fertile I. Why is cooperation hard to explain? I. A. Eusocial Insects A.
“There is one special difficulty, which at There first appeared to me insuperable, and actually fatal to the whole theory [of evolution by natural selection]. “I allude to the neuters or sterile females allude in insect-communities; for these neuters in for often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind.” cannot Darwin, Origin of Species Origin I. Why is cooperation hard to explain? I. A. Eusocial Insects A.
If sterile individuals can’t pass on their genes, then how do genes responsible for producing the sterile individuals get transmitted? responsible
Darwin’s answer: “This difficulty, though appearing insuperable, is lessened, “This or, as I believe, disappears, when it is remembered that or, selection may be applied __to the family__, as well as to the individual.” selection __to In modern terms... 1. genes associated with producing sterile castes (workers) are transmitted, genes ___but not expressed__, by the fertile castes (queens and drones) ___but • We all carry genes that we don’t express, e.g. for sex-specific traits 2. fertile individuals enjoy very high fitness (ie, reproductive success), and fertile they pass along the genes to make the sterile castes they 3. sterile workers receive __indirect fitness benefit__ by rearing fertile sterile __indirect relatives with whom they share genes with 4. the genes possessed by the sterile castes get passed on through their the fertile relatives (_____kin selection__) fertile II. Kin Selection
So how does kin selection work? And how close do relatives need So kin how to be for “altruism” to be favored? We can use _Hamilton’s We Rule to calculate this... to An equation that gives the An conditions for cooperation, depending on the cost (c), cost benefit (b) and benefit relatedness (r) relatedness ...predicts that if costs ...predicts are high, then benefits AND relatedness must be _high_ for cooperation to evolve evolve _br >c_ _br W. D. “Bill” Hamilton (1936-2000) II. Kin Selection, A. Eusocial insects again... II.
Why is helping behavior and reproductive altruism so common in Hymenoptera? in
Partial Answer: _haplodiploid__sex _haplodiploid__sex determination determination • Males are haploid (1N) – from unfertilized eggs unfertilized • Females are diploid (2N) – from Females fertilized eggs fertilized ecause their father is haploid, sisters ALL have exactly the same paternal ecause genotype (all of his genes go to all of his kids) so females share more genotype all genes with their sisters than they would be to their own kids! genes o, r between full sisters is much higher than “normal”, favoring _extreme o, II. Kin Selection, B. Other Eusocial Animals II. Eusociality is a rare behavior Eusociality found in __naked mole rats__, __naked termites, some shrimp, some aphids, and a few other arthropods... arthropods... Note: these are not haplodiploid! Note: not II. Kin Selection, C. Beyond Eusociality II.
This does NOT only happen in eusocial animals: E.g. Cooperative courtship in turkeys (A. Krakauer):
Brothers form coalitions for courtship, but only the alpha male mates Beta males help their brothers because they gain more from _idirect Beta fitness benefits by kin selection than they would from courting females alone alone _kin selection_ doesn’t always involve one individual giving up reproduction entirely! It’s usually more subtle... reproduction Any behavior that reduces the actors fitness (c) but gives a benefit to another (b) Any can evolve by this mechanism (e.g. food sharing, alarm calling, etc.) can II. Kin Selection, C. Beyond Eusociality In order for kin selection to work, animals must be able to recognize kin_; kin How they do this is an ongoing area of research research Can involve “imprinting” Can on visual markings, scent, vocalizations, etc. scent, III. Other paths to cooperation III.
Much of the cooperation we see in the animal world is favored by kin Much selection, but there are other ways for cooperation to be favored by selection selection What appears to be “altruism” can evolve because: 1) _direct fitness benefit_ (the behavior is good for the “altruist”) 1) (the 2) __reciprocity___(“you scratch my back, I’ll scratch yours”) 2) __reciprocity___ III. Other paths to cooperation III. In Western Grebes... Males and females cooperate in raising Males the young to increase their own fitness, so they gain _direct fitness__benefits___, Cooperative fitness__benefits___ Cooperative dances strengthen the pair bond (and allows assessment). allows In Blue Manakins... Beta and Gamma males cooperate to Beta gain matings (direct fitness benefits__), gain but it’s slow going... Males may wait years as beta or Gamma males before the Alpha dies, then they can move up in the hierarchy. in IV. Cooperation Summary IV. Nothing can evolve “for the good of the species” But cooperation (apparent altruism) can evolve by Kin selection Direct benefits to the “altruist” Or through reciprocity (reciprocal alturism) ...
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