Sociobiology 2

Module 4 (SB2) - ALTRUISM & RELATEDNESS:

THE IMPORTANCE OF KINSHIP

0. OBJECTIVES

From this module you should learn

the mechanism involved in Hamilton's theory of the evolution of altruism
how nepotistic altruism is manifested among animals (e.g., kin-recognition)
how nepotistic altruism is manifested in humans (e.g., nepotism, avuncular relationship, ethnocentrism, higher risk of children raised by unrelated adults)
the sociobiological explanation of incest avoidance as contrasted with traditional cultural explanations, and the three "natural experiments" that support the sociobiological explanation

1. HAMILTON'S THEORY

In 1965 Edward O. Wilson, a well-known professor of biology at Harvard and a specialist of ants, reads a pair of articles by British graduate student William Hamilton (1964) in the train from Boston to the Florida Keys. Wilson is at first skeptical about Hamilton's argument but is eventually "converted" to his way of thinking. As Wilson describes in his autobiography:

"By the time we reached Miami [...] I gave up. I was a convert, and put myself in Hamilton's hands. I had undergone what historians of science call a paradigm shift." (Edward O. Wilson, Naturalist, p. 320)

Note: If you are curious to know why Wilson was going to Florida; why he was taking the train rather than flying; and why he found it so difficult at first to accept Hamilton's reasoning, you can find the answers in his autobiographical book Naturalist.

The gist of Hamilton's theory is that to understand the evolution of behavior one must consider the effect of that behavior on the inclusive fitness of the individual organism:

Inclusive fitness of individual organism = own reproductive success + reproductive success of kin multiplied by degree of relatedness

Altruistic behavior is behavior that is beneficial to others at a cost in fitness (reduced reproductive success) for the organism engaging in the behavior, such as the call of a bird warning of the presence of a predator.
Altruistic behavior can evolve by natural selection if it increases inclusive fitness of the individual by increasing the survival and reproduction of relatives who share a proportion of genes by common descent.

Hamilton stated mathematically that an altruistic gene is selected (i.e., will spread in the gene pool) if

c < rb

where:
c = cost of the behavior in terms of individual fitness
b = fitness benefit to relative
r = coefficient of relatedness ( = probability that relative also carries the altruistic gene by common descent)

For example, a gene for producing a warning call when sighting a predator represents a cost c for the bird emitting the call, as the calling bird will be more likely to be spotted and attacked by the predator. The call represents a fitness benefit b for other birds who are warned by the call and able to take cover from the predator. To the extent that beneficiaries of the call include relatives (siblings, parents, offspring,...) of the calling bird, the inclusive fitness of the calling bird is increased; the altruistic behavior will be selected to the extent that the overall fitness benefit for relatives (weighted by the degree of relatedness) is greater than the individual fitness cost c.

Hamilton's theory explains the evolution of nepotistic altruism (= altruism benefiting relatives; from Latin nepos = "nephew")

Coefficients of relatedness r ( = fraction of genes identical by descent) for various relatives in a random-mating diploid population:

Parent-offspring 1/2
Full siblings 1/2
Half siblings 1/4
Uncle/Aunt - Nephew/Niece 1/4
First cousins 1/8

2. SOME CONSEQUENCES OF HAMILTON'S THEORY

There are many examples of nepotistic altruism aka kin selection in nature.

"Maternal instinct", the propensity of mothers and parents in general to defend their young is the most obvious consequence of nepotistic altruism. (It is so conspicuous and pervasive as to be taken for granted.)

The evolution of nepotistic altruism is manifested by the evolution of kin-recognition mechanisms in many species. Kin-recognition mechanisms allow organisms to evaluate the degree of relatedness of strangers. Kin-recognition mechanisms have been found in mammals as well as insects:

Exhibit: Territorial aggression and relatedness in ground squirrels (SEB Figure 3-5 p. 47)
Exhibit: Kin recognition in sweat bees (SEB Figure 3-8 p. 54)

One triumph of Hamilton's theory was his explanation of the evolution of eusociality in hymenoptera (the group of insects that includes bees and ants). Eusociality (or "good sociality") refers to animal societies such as bees in which the workers are sterile; only the queen reproduces. Hamilton argued that the basis of eusociality in bees is their special haplodiploid genetic system. One consequence of haplodiploidy is that a female bee shares 75% of her genes with her sister but only 50% of her genes with her own offspring (daughter or son). Thus a female can reproduce more of her genes by being a sterile worker helping to produce sisters, i.e. daughters of the queen, than by having daughters on her own!

Exhibit: Haplodiploid reproduction in hymenoptera (SEB Figure 3-6 p. 49)

Q - Under what conditions can altruistic behavior (i. e., a behavior that reduces the fitness of the individual but benefits others) evolve under natural selection?

Q - What are the components of Hamilton's inclusive fitness?

Q - What does "nepotism" mean?

3. HUMAN KINSHIP

What is the importance of kin selection and relatedness in the human species?

Humans share the general propensity to nepotistic altruism, or tendency to favor kin. In modern industrial societies laws and regulations have been developed to combat this tendency. For example, job hiring forms often inquire about the presence of relatives in the organization who might improperly favor a prospective employee.
The curious case of the avuncular relationship. In this custom the maternal uncle assumes a greater role in raising a child than the "father" (defined as the husband of the mother of the child). It turns out the avuncular relationship is more prevalent in societies that have a high incidence of adultery. The reduced certainty of paternity in these societies may induce men to devote more resources to raising the children of their sister (nephews or nieces) than their own (putative) children.
Is ethnocentrism (and racism, patriotism, sports team loyalty, etc.) rooted in nepotistic altruism? Sociologist Pierre van den Berghe ahs argued in his book, The Ethnic Phenomenon, that all these forms of group identification are manifestations of the same propensity and represent extensions to non-kin of tendencies evolved because they benefited relatives during human evolution.
Children living with an adult who is not biologically related to them (such as a stepparent) are at MUCH greater risk of abuse and infanticide (Martin Daly & Margo Wilson, Homicide). The interpretation is that a non-related parent is less interested in the welfare of the child, compared to a biological parent.

4. INCEST TABOO

Avoidance of incest (= prohibition of sexual relations between close relatives) is a universal feature of human societies.

The nature of incest was discussed in the media in the wake of the relationship between Woody Allen and Soon-Yi Previn. Some elements of that situation are:

Woody Allen was born 1 Dec 1935 and Soon-Yi Previn was born 8 Oct 1970
Soon-Yi was adopted in Korea when she was 7 years old by Mia Farrow and then husband André Previn
Mia Farrow and Woody Allen had a decade-long relationship starting about 1979, although Allen and Farrow never lived together
The relationship between Woody Allen and Soon-Yi came to light in 1992 after Mia Farrow discovered nude pictures of Soon-Yi (made by Allen) in his apartment
Woody Allen and Soon-Yi were married in December 1997 in Venice.
Exhibit: Picture of Woody Allen and Soon-Yi

The following article about the relationship of Woody Allen with Soon-Yi illustrates traditional theories on the origin of incest avoidance and misconceptions about biological consequences of incest.

Exhibit: What Is Incest? (Time, 31 August 1992)

Traditional cultural theories of the incest taboo explain incest avoidance as

preserving family harmony by preventing confusion of roles and disrupting jealousies caused by incestuous unions
creating a network of reciprocal ties among families by forcing individuals to find spouses outside the immediate kin group

The sociobiological perspective proposes instead that

incest avoidance is based on a spontaneous aversion to sexual activity among adults who have been raised together in early childhood called the Westermark effect (after Westermark 1891)
the predisposition to incest avoidance evolved because is helps prevent the inbreeding depression (= the accumulation of deleterious recessive genes in inbred individuals)

There are three known "natural experiments" suggesting a biological (non-cultural) foundation of incest avoidance between brothers and sisters

the absence of sexual activity among adults who have been raised together in Israeli kibbutzim (despite the absence of cultural opposition to these unions)
the high failure rate of minor marriages in Taiwan
the high divorce rate for preferential marriages between cousins raised in close proximity in Lebanon

Sociologist Pierre van den Berghe concludes his discussion of incest by stating

"Incest taboos are the cultural expression of a biological predisposition not to breed with close relatives [...] The cultural significance of incest taboos has been much exaggerated in the anthropological literature" (Pierre van den Berghe, Human Family Systems, p. 117)

Q - What is a kibbutz?

Q - The finding that unrelated children raised together in Israeli kibbutzim do not have sexual relations as adults is an instance of what phenomenon?

Last modified 7 Sep 2006