Evolution and KinshipANTH 321: Kinship and Social Organization

Kimberly Porter Martin, Ph.D.

Two Approaches to the Evolution of Kinship

1. The Study of Our Primate Relatives.2. Evolutionary Psychology

Our Primate RelativesProsimians (Lemurs, Lorises, Galagos)Monkeys (Baboons, Tamarins)Apes

Gibbons and Siamangs (Lesser Apes)OrangutansGorillasChimpanzees

Common ChimpsBonobo Chimps

Prosimians

Lemur – multimale and multifemale groups

Galago – solitary and promiscuous

Loris- monogamous when associate with another animal

Primate Characteristics

Binocular, stereoscopic vision. Opposable thumb/toe Grasping hand/foot Flat nails rather than claws Prolongation of gestation Prolongation of infancy and childhood Enlarged and elaborated brain

Monkeys: Old World and New World

Baboon – Polygynous – one male controls multiple females who mate exclusively with him

Tamarin – Polyandrous – one female mates with multiple males who all stay and care for young.

Gibbons and Siamangs

Monogamous for life – territorial and drive our same sex offspring at adolescence.

OrangutansSolitary in the wild except for mother infant/child pairs.

Gorilla98% Identical to Humans Genetically

Male-Centered groups – polygynous – with dominant males having mating priority in groups andcompetition between males for mating.

Chimpanzees98.6% Identical Genetically to Humans

www.slwaldron.com/BonoboGallery/

Bonobo

Common Chimp

Chimps98.6% Identical to Humans Genetically Common Chimps

Male-centered groups

Females leave natal group at adolescence to join another group

Males stay in the same group for life

Bonobos

Female-centered groups

Males leave natal group at adolescence to join another group

Females stay in the same group for life

WHY THE DIFFERENCE?????

Chimp/Bonobo Differences HYPOTHESIS One uniform species of chimp lived in the same regions as gorillas, who

dominated resources located on land, relegating chimps to the resources in the trees.

Drought separated the region into two areas.

Gorillas became extinct in one area; both gorillas and chimps survived in the other area.

In the area where both chimps and gorillas survived, the patterns of ground and tree resource use continued and chimps retained male dominance as a major organizing principle. Feeding for chimps took place in the trees, making alliances between females difficult and reinforcing male dominance.

Without competition for resources on the ground, chimps were free to utilize these resources and spent great amounts of time on the ground. This allowed females to unite against males, making them equal to or dominant over them.

Bonobos time on the ground meant that they spent more time bipedally.

Evolutionary Psychology

Sociobiology Evolutionary Psychology

Biologists

Primatologists

Ethologists

Anthropologists

Psychologists

Linguists

Lots of Controversy

Fear of Social Darwinism

Arguments about human free will

Arguments about whether humans have instinctive (genetically programmed behavioral tendencies)

First attempts to apply the theories to humans

Edward O. Wilson

Sociobiology: The New Synthesis1975

Some Types of Evolutionary Psychology Theory

Systems Theory

Kin Selection/Inclusive Fitness Theory

Parental Investment TheoryAltruism Theory

Sociability Theory

Multilevel Evolutionary Theory

Kin Diagram Basics

=

male

female

marriage

siblings

= offspring

Kin Selection/Inclusive FitnessIndividuals will tend to invest in

others in proportion to their degree of relatedness.

Degree of relatedness is calculated using the proportion of genes shared in common.

Kin Selection/Inclusive Fitness Theory

== = = =

1/2

= =1/2

1/2

1/2 1/2

Cousins Brother EGO Sister Cousins

1/2

1/21/2 FaSi FaBr Father Mother MoSi MoBr

Parent-Child ½ Grandparent-Grandchild ¼ (½ x ½)

Aunt/Uncle-Niece/Nephew ¼ (½ x ½) Cousin-Cousin 1/8 (½ x ½ x ½)

Males Females

Parental Investment TheoryRelative Contributions of Males and Females to Offspring

Gamete size and contribution to embryo

Minimum time investment to birth of a child

Minimum emotional investment to autonomous healthy child

Minimum economic investment to autonomous healthy child

1/200,000 of an egg

A few minutes

None

None

200,000 times larger

Nine months

5 to 7 years

5 to 7 years

Parental Investment TheoryMale vs. Female Mating Strategies

What characteristics are most sexy?

Males FemalesYouth, Beauty Power, Wealth

Very important. The female needs help with the large time, emotional and economic investments she must make to produce a healthy adult offspring. She would like her mate to stick around and help.

It varies. Commitment to a single female is one of many strategies for a male. The male can offer fidelity to a single female to help support their offspring. This is only one of the male options. At the other end of the spectrum is “free copulation” leaving females to care for offspring.

How important is exclusivity

and

commitment?

Parental Investment TheoryHypotheses about The Range of Male Strategies

The Swinger: Males who attract lots of females who are willing to have sex with them are better off not making a commitment to a single female. They will hope that some of the women with whom they copulate will get pregnant, and that the women will find a way to support/raise the child, either alone, or with the help of another male. This way they have the potential for having large numbers of children in whom they will have invested very little.

The Nerd: Males who do not attract lots of females who are willing to have sex with them can offer commitment as an inducement for sex. They promise long term fidelity and support for the female and her offspring, allowing the female to have more children, or to raise more successful children because she has the time, emotional and economic investments of a male on whom she can count.

Parental Investment TheoryHypotheses about Female Strategies

Females will be attracted to wealthy, powerful males who can help them support and raise their children. This will be especially true in cultures where males control resources. They may tolerate such men having multiple wives or partners as long as they and their children are supported.

Females will be less exclusively monogamous when they have access to economic resources or earning power themselves. However, they will never be as promiscuous as the most promiscuous males.

The risk of pregnancy and the investment that follows conception for women will make them more careful about sexual partners, and more interested in commitment from partners than will ever be true for males.

Hypotheses about the Evolution ofHuman Mating Patterns

Upright, bipedal anatomy

Enlarged brain size

Birth earlier in the developmental cycle

More helpless infants

Prolonged period of dependence

Hidden ovulation in human females

Enlarged breasts, lip size and color

Pair bonding

THE FIVE YEAR ITCH!!!