Categories of Social Behavior. Actor. Cooperative behaviors can be favored when advantage to actor > receiver. +. Selfish behaviors are always favorable. Selfish. Cooperative. +. -. Recipient. Spiteful. Altruism.
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Cooperative behaviors can be favored when advantage to actor > receiver
Selfishbehaviors are always favorable
Spiteful behaviors are theoretically possible if the harm to receiver >harm to actor
Altruistic behaviors are NEVER (by definition) favored through individual RS
Altruism presents a paradox for natural selection:
If natural selection favors traits that increase individual fitness, how can we explain behaviors that cost an individual fitness while helping another?
Darwin hinted at an answer:
Selection could favor traits that result in a decrease of individual fitness if they INCREASE a RELATIVE’S fitness
r = coefficient of relatedness
r = the probability that two alleles in two individuals are identical by descent
By helping a relative--> more of “your” genes are passed to the next generation
An individual's fitness can be partitioned into direct and indirect components:
Inclusive fitness = Direct fitness + Indirect fitness
Direct W = personal reproductive success
Indirect W = RS of individuals that share alleles (weighted by the probability of sharing alleles, that is, relatedness)
1/2 parents-offspring; full-sibs
1/4 grandparents-grandkids; half-sibs
1/8 cousins (first cousins), uncle/aunt/niece/nephew
Belding’s Ground Squirrels (work by Paul Sherman)
Provide alarm calls to warn others of approaching predators.
Cost to actor -higher predation risk-->Benefit to receiver, reduced predation risk
males disperse from the natal territory, so that females in a colony tend to be closely related whereas males are unrelated to the females
Females more than expected
Males less than expected
Females are more likely to give alarm calls when close relatives are close by
Females are more likely to help close relatives chase away trespassers than nonrelatives
In order to direct altruistic behavior towards relatives, individuals must be able to recognize relatives
genes that code for membrane proteins that display antigens. It is thought that a greater diverstiy of MHC types allows more proteins to be recognized and therefore resistance to diseases is higher
MHC is a great example of how many processes work together:
Cooperation among individuals in raising young
Specialized castes of individuals that are nonreproductive
Found in many insects (hymenoptera, termites, thrips), one group of mammals, and snapping shrimp
Why would sterile castes give up all direct reproduction?
Males - 1n, develop from unfertilized eggs
Females - 2n develop from fertilized eggs
females are more closely related to their sisters than their own offspring(3/4 vs 1/2, assuming the same dad)!
an allele spreads faster by helping mom reproduce than by reproducing itself!
• Some haplo-diploid species are not eusocial
• Some eusocial species are not haplo-diploid
Haplo-diploidy may allow eusociality to evolve more easily, but it neither necessary nor sufficient for eusociality to evolve
Naked Mole Rats!
• Reproduction is by a single queen and 2-3 males
• Most matings are between parents & offspring or full-sibs--> r = 0.81
• Workers care for young, dig tunnels, defend colony
But workers would still be more related to their own offspring, so why don’t they reproduce?
Queens beat workers into submission
• Queens shove non-relatives more than relatives
Shoving by the queen increases effort by workers
Can altruistic behavior to evolve even when directed to nonrelatives?
repeated interactions with other individuals
many opportunities (and an unpredictable number) to be altruistic
symmetrical costs and benefits among the interactants
When these conditions exist, what type of actions will natural selection favor?
Vampires - forage at night for blood meals on large mammals
33% of young bats and 7% of adults fail to feed on any given night - 3 consecutive bloodless nights and a bat dies
vampires roost in small groups, and membership in groups changes, some members are regular associates and others aren’t
vampires will share bloodmeals each other, preferentially to related bats, but also to those with whom they have some experience
Bats preferentially share with nonrelatives that they are frequent roostmates with
Bats preferentially share with relatives, especially those related by more than 1/4