- Inclusive Fitness

The evolutionary ecology of early weaning in Kilimanjaro, Tanzania

- Does Parental Investment Theory explain variation in weaning time in Tanzania? - Breastfeeding duration as a measure of PI? - Variables: 1) Mothers reproductive value: How many more offspring can a mother produce before menopause? Age, Birth order 2) Offspring reproductive value: How likely is it that this offspring will survive to reproductive age? Birth Weight Results: - Later born offspring are breastfed longer - Larger infants are breastfed longer.

The Cinderella Effect

- Important to remember: the rate of child homicide is very low overall - Lightcap et al. studied abuse in blended families: -- 2 parents; each with one biological and step child -- 0/20 biological children were abused -- 12/21 step children were abused Childhood abuse is more common when a household contains non-genetically related adults A child witha non-kin adult in the home is 100x more likely to be killed than a child living with 2 biological parents

Hamilton's Rule

- Kin selection is formalized as Hamilton's Rule - A formal model of kin selection (total fitness advantage must take into account costs, benefits, and the probability the same gene is in the other individual) - If rb > c, altruism is favored

The dark side of families

- Neglect and infanticide - Abuse by non-genetic relative - Paternity Uncertainty

Neglect and Infanticide

- Parental Investment (PI) is facultative - Is PI facultatively dependent on anything? 1) Conditions predictive of success or failure (young moms, low SES, education, resources) -- Young mothers / mothers without a mate are more likely to commit infanticide -- Mothers are more likely to commit infanticide in the first 2 years of life 2) Whose offspring is it? (Non-investing mate) 3) Burden on Mother 4) Offspring quality

Parent-Offspring Conflict

- Parents value offspring equally, therefore parents will prefer an equitable distribution of investment (all other things being equal) - Each offspring values itself 2 (to 4) times more than it values its sibling - Offspring will prefer an inequitable distribution of parental investment - P-O conflict also manifests as sibling rivalry

P-O Conflicts (types)

- Weaning conflict - Carrying conflict - Temper Tantrums

Human Social Groups

Human societies are kin-based - In principle, individuals in social groups could aggregate any way, including randomly - but they don't - We see vastly more cooperation/altruism/investment among family members than among non-family members

Kin Selection

One of two explanations for apparent altruism (the other is reciprocal altruism)

Coefficient of relatedness (r)

The probability that two people share a gene by virtue of their sharing a common ancestor - 0.5 - full siblings, parents - 0.25 - half-siblings, g-parents, full aunt/uncle - 0.125 - cousins, half aunt/uncle

Inclusive Fitness

W.D. Hamilton observed that genes have 2 routes to the next generation: 1) Direct - Producing more offspring by meeting the challenges of the environment 2) Indirect - Aiming the benefits of altruism at individuals that carry the same genes you do. Inclusive Fitness: The combined fitness benefits of direct and indirect reproduction. - Kin selection (indirect): The evolutionary strategy that favors the reproductive success of an organism's relatives, even at a cost to one's own reproductive success.

Conflict of interest in families

We can also use the logic of relatedness (from kin selection theory) to discover the roots of familial discord: 'parent-offspring conflict'