L14- C+C in Humans

Evolutionary Psychology (sociobiology)-

• the study of the effects of genes on human behaviour

• has caused controversies in past e.g. Darwin’s theory:

  • simplistic arguments were made that made no sense

  • abuse of darwinism (social darwinism)- eugenics movement in late 1800-1960s was the philosophical justification for apartheids, etc.

  • motivation- don’t think about inclusive fitness when behaving

• instead we are talking about evolved behaviours consistent with some kind of adaptive functions

Menopause-

• life history theory→ natural selection should make soma (body) and germ (reproductive) function run out at the same time

• menopause→ loss of reproductive potential followed by long post-reproductive life

  

  • isn’t a product of extended lifespan, same pattern is seen in current hunter-gatherers and historical populations

• few other species that experience menopause e.g.

  

• menopause is seen in one population of chimpanzees-

  • new population in West Africa found that female Ngogo live long after they stop reproduction, whilst other populations of chimps die at age of end of reproduction

    

Theories:

1. Mother hypothesis

   • Avoid risky reproduction and mortality in later life and ensure survival of existing offspring

2. Grandmother hypothesis

   • Kin selection – gain inclusive fitness by helping existing offspring to reproduce 

Study:

• used life history data on pre-modern population of Finns and Canadians (before modern contraception and medicine)

Mother hypothesis-

• what was the effect of mother’s death on fitness of offspring?

  • very little effect, slight reduction of lifespan on pre-weened offspring in Finland

  • little affect on breeding success of offspring

  

  → no strong evidence for the mother hypothesis

• beyond weening age, the care from mother is compensated for by other family members

Grandmother hypothesis-

• how many additional grandchildren are there in relation to lifespan of grandmother after menopause?

  • the longer a grandmother stays alive, the more offspring their children have

  • 2 extra grandchildren for every 10 years post-menopause

  

• having a mother alive means:

  • higher fecundity

  • higher lifetime reproductive success of offspring

  • more kids if grandmother remained local

  • earlier reproduction

  • shorter intervals between successive births, less effect with the more children they have

  • increased survival to 15

    

  → grandmother can relieve burden on mother

• similar results found in orcas→ surviving matriarch has a positive effect on survival on grand-offspring, especially grandsons

• grandfathers→

  • no significant difference between grandfather alive or dead on lifetime reproductive success of offspring

  • survival probability is slightly lower

  

Is there more to the grandmother hypothesis?

• argue kin selection of helping as a grandmother did not outweigh continued reproduction

• suggest menopause was also driven by reproductive competition between mothers and daughters/daughters in law

• analysis on the fitness effects of grandmothers in pre-modern Finn populations:

  • there is nearly no overlap between timing of reproduction by grandmothers and children

    → there is a mutually exclusive reproductive activity

  • negative effect on productivity (offspring survival) when there is an overlap

    

  → grandmother effect is true but with the added conflict

Cooperative Breeding

• historical Finn population-

  • do unmarried siblings act as helpers? NO

  • do aunts and uncles help? NO

  • had lots of co-breeding (siblings marrying and living in same house)- did co-breeding wives positively affect offspring? NO, if they overlapped, there was actually a negative effect:

    

→ no evidence for cooperative breeding in historical Finn populations

• categorised parental care patterns across societies:

  • some societies have strong evidence that individuals other than parents provide significant amounts of offspring care

    

• is the pattern of care influenced by ecology?

  • cooperative care is more frequent in unpredictable climates with low rainfall and low temperatures, and when starvation risk is low

    

Mating Systems-

• males typically have higher reproductive potentials than females

• highest reproductive successes in human history:

  

• mating system diversity→ in 849 societies:

  • 0.5% are polyandrous

  • 16% are monogamous

  • 84% are polygynous

  → most societies have some level of polygyny

Polyandry:

• very rare

• most investigated in Buddhist communities in Kashmir+Tibet:

  • usually have 2-3 co-husbands per wife, usually brothers

  • in typical community→ 33%of males aged 10-59 in polyandrous marriages, 31% of females aged 20-59 unmarried

  • big effect on reproductive potentials→ 0.7 children per unmarried woman, 3.3 children per married woman  

• is influenced by 2 factors:

  • environment→ live in harsh, high altitude environments, only agriculture is possible in small community deltas

    → maintaining integrity of units of land

    

  • culture→ buddhism, some sons were dispatched to the monastery, usually second son

    • measured the number of children produced by brothers, sisters and grandfathers if the father dispatched a son to be a monk:

      • for all→ if no monks were sent they had low fitness, if 1 or more sent off were sent they had higher fitness

      

• polyandry is so stable because:

  • kinship→ marrying brothers reduces conflict

  • age hierarchy→ second son is often dispatched as they are the most likely to compete for paternity

Monogamy:

• common in hunter-gatherer and western societies

• is not strict due to:

  • extra care paternity- ~1% of offspring

  • serial monogamy

    • measured average number of children according to number of spouses at age 40-47:

      • when women had more marriages they had no significant effect on the number of children she had

      • but men had a significant increase of number of children with more marriages

      

Polygyny:

• is the norm in most cultures e.g. mormons, ishmael the bloodthirsty

• is described as resource defence polygyny

• e.g. Kipsigis of Kenya:

  • polygyny is higher in wealthier men

  

• measured patterns of financial transactions at the time of marriage and inheritance of wealth in families:

  • in olden european societies- wife’s family paid a dowry to groom

  • now→ groom’s family paying bride’s family is more common

    • can see that bridewealth payments are more common as polygyny increases

    • this affects the way resources are inherited between sexes→ is fairly even in monogamy, polygyny is predominantly by sons

      

Social Evolution-

• human societies are more complex and operate at a much larger scale than any other organism

• study→ used dataset from pacific and SE Asia

  • classified every society according to 4 levels of social complexity:

    • acephalous- no head person, families living together

    • simple chiefdom→ single chief

    • complex chiefdom→ multiple layers below chiefs

    • state→ civil service

  • investigated transitions between social states using language phylogenies

    

  • asked if these transitions were all equally likely:

    

    • increases in political complexity→ occur in sequential jumps, no big transitions

    • decreases in political complexity→ some are sequential but there are also major jumps in societal structure- were typically associated with major environmental destruction by people e.g. Easter Island

    

→ ruining the environment can get a total collapse in society

Summary-

• Menopause is an unusual feature that is likely to have resulted from kin selection and reproductive competition

• Cooperative breeding in humans depends on environment

• Variable human mating system, related to cultural and ecological factors

• Human social transitions follow predictable pathways (to some degree)