Altruism (problem, and possible solutions)
- Action whose average effect is to benefit someone else at some cost to the actor measured in expected fitness
- Problem: It couldn't evolve because the genes wouldn't increase in frequency in any subsequent generation
- 3 Possible Solutions to this problem:
1) Kin Selection: Help those who share your genes
2) Reciporical Altruism: Help those who will have helped you
3) Costly Signaling: Help because it makes you look good.
- Altruism can increase under selection if it is kin-biased in accordance with Hamilton's Rule (r >c/b, c c = cost to altruist b = benefit to the individual to whom the altruism is directed r = relatedness between the altruist and beneficiary
c = cost to altruist
b = benefit to the individual to whom the altruism is directed
r = relatedness between the altruist and beneficiary
Wright's Coefficient of Relatedness (r)
- Relatedness of two parties from a common ancestor - The probability that a randomly selected autosomal allele in one individuals genotype is also a component of the others from a recent ancestor.
- 2 ways you can seem like your are matching
1) Identical by descent: alleles identical due to inheritance
2) Identical by state: look the same, or have similar DNA but do not come from known common ancestor.
- Even though we share 99% of DNA with chimps, they aren't from a common ancestor so it doesn't count
- You have a 50% chance of sharing at least some (.5 alleles) with sibling, 25% for 2 or 0
- Over many replications
(Definition, satisfying rules and see chart)
- Darwinian fitness = reproductiveness success = fitness
- Inclusive fitness = direct fitness + indirect fitness
- Altruism may become an evolutionary stable strategy if it satisfies Hamilton's rule:
1 ) Hamiltonian mutant can invade a population whose members allocate their altruism indiscriminately or by any other rule
2) a population of Hamiltonians can not be invaded by any non-Hamiltonian mutant
- Chart: Any benefit that an an indiscriminate helper gives to a Hamiltonian helper they preferentially give it to their kin. They get more resources and survive better
(4 predictions kin selection would find and experimental evidence)
1 ) More likely to provide costly help to kin than to non-kin, Inheritance of wealth, Burnstein et al., 1994, Stewart-Williams, 2007 (see cards)
2) More tolerant of inequity in social exchanges with kin than with non-kin (Hunter-Gatherers and garden prep sharing)
3) Inhibited against harming kin relative to non-kin in conflict situations (Homicide, Donner Party)
- An inhibition of harm for those related to us would impede our own fitness
4) Respond positively to cues that are statistical indicators of kinship (Phenotypic matching, DeBruine, Smell, Weisfield 2003, MHC, Wedekind, Inbreeding Avoidance, Westermarck effect
Inheritance of Wealth
- 92.3% of death estates left to kin
- 50 were .5 relatives, x4 more to children than siblings (because the gain of giving those resources to children increases fitness more)
- 8% to .25, < 1% to 0.0125
Burnstein et al., 1994
- 2 manipulations: every day scenario, life/death scenario
- Life Death: Who would you save from a burning building 7y/o cousin, 10 y/o bro, 75 y/o g-ma, 21 y/o old acquaintance
- Everyday: Who would you help 7y/o cousin, 10 y/o bro, 75 y/o g-ma, 21 y/o old acquaintance asks to pick up items from the grocery store
- Results: In both, we help genetically related more
- Age was also manipulated: we help 75 y/o relatives way less in life and death. In everday we preferentially help old/young relatives because they can't provision for themselves as well, but middle age can so we help less.
- Health was also manipulated health (high/low quality). In order we help: 1) We help sick relatives in everday 2) healthy in life and death (because it can more directly impact our fitness if they die) 3) then sick in life and death 4) healthy everday (healthy everyday because they can provide it for themselves more than likely)
- Asked people how much and who they helped in the past 2 months
- 3 levels of helping:
1) Low-cost (emotional support)
2) Medium (help during illness/crisis/errands)
3) High-cost (hypothetical) help (donate kidney/rescuing from burning building)
- Results: We help siblings less in low cost and more in high cost, but friends more for low cost
- Indicates that reciprocity is really important for maintaining relationships where there is no relatedness.
- We do low-cost as feelers for later opportunities to trust.
Hunter-Gatherers in Venezuela
(what nepotism rule does it support, hint: gardens)
- Inequity in Social Exchanges (rule 2)
- Walked around camps at random times in the day and wrote down what everyone was doing
- Computed inter-household relatedness and the level of exchange imbalance
- Difference between % of household a's labour that went to household b, and vice versa
Correlation = .76
- The amount of work that goes into one household is being guided by who gave to them and who they are related to - very high correlation
- help for closer kin is undeterred by debt, non relatives have tighter recriprocity.
(Detroit, 13th century and what rule)
- 3) Inhibited against harming kin relative to non-kin in conflict situations
- Detroit Homicides, huge proportion of homicides are spouse and non-reletives.
- 13th century: 20% of Co-offenders in a murder were much more likely to be genetic relatives
- Victims were rarely related to their killers andt hose who were were usually marital relatives
- Most likely to kill spouse
(Story, what rule)
- 3) Inhibited against harming kin relative to non-kin in conflict situations
- Caravan in 1840 that got stuck in winter without proper equipment and training. Lots of journals were kept, and correlated family size and demographics to death rate.
- 57% of men died, 29 % of women (less women died)
- Individuals that survived and did better had a much larger family and there was a strong between family size and days until death
- Also there was a protective effect of being a woman.
1) Phenotype matching. This individual looks/smells/sounds like me/known relative
- Sound phenotype matching doesn't exist because we hear ourselves through our own skulls
2) Circumstantial cues: sources of experience or sources of information
- E.x. This baby emerged from my birth canal, so it's mine
Debruine Nepotism Experiments
- 2 players, where sum of money was involved - Economic game.
- Player 1 is given a choice: they can divide $10 in the way they see fit or let player 2 and the pot of money that can be divided increases to 20.
- Created face morphs of themselves to look more or less like them
- Results: People passed (trusted player 2) more often than if morphed to look different. They were cued to relatedness
- Were more likely to give option to P2 and gave more as P2
- Second experiment: had a control where they had a second set of individuals to see if they responded in the same way (they didnt)
- Third Experiment: Checked to make sure familiarity wasn't doing this by using face morphs of celebrities, it's not.
- Fourth Experiment: Also tested if this was just a sexual cue: self-resembling faces are rated as more attractive, and unattractive for a ST, no effect for LT
- Inhibited for mating and suggests context specificity, it's not mating is kinship/prosocial
- We can dscriminate between indivduals through it
- Blindfolded mothers can ID their infant, but can't discriminate against twins
- (Breastfed only) infants prefer moms smell
- Strangers can match the smell of twins reared apart higher than chance
- MHC (Weisfield et al. 2003), T-Shirts (Wedekind 1995)
Weisfield et al., 2003
- Mother, father, sibling, familiar unrelated person, stranger and yourself wore 2 tshirts for 2 nights
- Could smell mother, and stranger the highest (stranger may because it's so different) and father/sib better than chance
- Could not smell self
- Mothers can Id own child, but not step
- Association & shared environment not sufficient for smell recognition (4-7 y/o can id sibs but not .5/step)
(Wedekind, 1995 again)
- Major Histocompatibility Complex: Large cluster of highly polymorphic genes
- Has a role in immunity and causes a unique scent
- Participants wore shirts and MHC-typed
- Women smelled shirts and scored on pleasantness and intensity (3 were MHC similar, 3 were not)
- Women who were on BCP found similar men more sexy than dissimilar (opposite for non BCP). May be because they are "hormonally pregnant" and are looking for familial support
- Couples who share more MHC have a harder time getting pregnant and have natal development issues
- Non BCP found intense smells less pleasant for MHc dissimilar
- Female sexual responsiveness/atisfaction is predicted by MHC dissimilarity - which therefore means more likely to conceive and have healthy baby
- Disimilar reminds a woman of their ex or mate, but no effect for reminds me of relative (inhibition of preference may explain lack of some effects).
- Send letters out to people matching in first, last, both or no names in common
- Females were more likely to reply if they had less common names (most for both, then less then first)
- No effect of common names and for men
- Because men keep their names usually, women don't, paternity uncertainty, and women seem to be more involved in geneology
(2 reasons why, example and effect)
- Invest avoidance is a calculation of costs and benefits. The less related mate will produce fitter offpsring. But mating with a relative is better than not mating at all.
- 1) Pathogens: red queen hypothesis. Heterozygocity confers a selective advantage
- DALY index: high levels of incest relationship have higher mortality levels at a national scale, and predicts TB and hepB at an individual level (deltrious recessives may be causing this)
- 2) Deletrious Recessives: When the autosome is recessive, it causes less damage, but carriers can maintain the gene frequency
- If it's dominant, it can destroy populations
- 3-5% of our alleles are lethal equivalents
- Example: Amish and Ellis-Van Creveld Syndrome
- Westermark Effect
- Ellis-Van Creveld Syndrome: Founder effect causing high mortality and hand mutatinos
- Due to increased endogamy
- 50% of infants do not survive
Cost & Benefits of Inbreeding
- Vary by degree of relatedness and sex of the mate (more costly for females due to biological investment. Females typically have more opportunities for more non-relatives)
- Net benefit for males desire for incest is higher than for females
2 Psychological Mechanisms for Inbreeding Avoidance
1) ID potential relatives - Kin recognition
2) Inhibition of preference/sex - Disgust
- Traditional Israeli Kibbutz are sharing of infant raising.
- Infants are grouped together by age and live with each other for laong portino of life
- Very few individuals married within their age group even though it was culturally encouraged
- Critical period exists between 2-6 y/o
- Taiwanese aranged marriages had 3 types: Patrilocal (wife moved in to husbands, Uxorilocal: husband to wife's, minor: bride given to -in-laws as infants)
- If became a minor before 3 y/o, 2x more EPC, higher divorce and 30% RS (RS correlated to marriage satisfaction)
- After 3 years, no effect found (critical period)
(And Fessler & Navarrette, Lieberman et al.)
- Original function was to avoid toxic materials
- Universally recognized expression and inhibits harmful behaviour
- Incest avoidance may be so strong that disgust reaction is a by-product or adaptive of the strong selection pressure (Disinterest is not as effective as repulsion)
- May have been co-opted to further motivate aversionn, also suggest that disgust is not context-specific
- Measured responses to an incest story (2 siblings who lived together to save money and began experimenting with sexual incest)
- Stronger reactions among: females with male siblings, males with female siblings (greater with less age difference and each additional sister)
- Women call their mother more during high fertility than father and for longer. Women call their father at low fertility at the same amounts.
Why do coanguienous marriages occur at all?
1) Avoid/reduce dowry/bride wealth
2) maintain familial wealth (think kingdoms and royalty)
3) Geographic or cultural isolation - Amish, by-product of other things
4) Adaptatino to LOCAL parasites; Sickle-cell. Mobility across cultures is very new
- Incest = high when 1 virus/pathogen hits a group constant and hard
-Not an arms race but a shift to cost/benefits depending on the ecology
Inclusive Fitness and Kinship Summary
- Inbreeding depression stems from increased susceptibility to 1) Infectious disease and 2) recessive deltrious disorders
- Potential fitness cost of inbreeding vary with 1) degree of relatedness and 2) biological sex
- Psychological adaptations function to increase nepotism and may also serve to avoid inbreeding.