What Does It Mean to Be Social?
The Selfish Gene and an Imaginary Social Group
Sociobiology, the study of the biological bases for social behavior, has as its primary tenet the expectation that phenotypes should behave in ways that promote the continuation of their genotypes. Sometimes it is a much maligned and misunderstood topic, partly because people often do not like the idea that their activities are influenced by their genes, and partly because the vocabulary of sociobiological theory is sometimes used by bigots to promulgate personal prejudices. Sociobiology presumes that genetic components in social behaviors are subject to the same selection processes as any other gene influenced characteristic. The fundamental problem of sociobiology is that the heritability of behavior is poorly known. Consequently this topic is strong on conceptual models and weak on documentable examples whose mechanisms of inheritance are understood.
Thus, we think that if an organism is going to reproduce and continue its existence, it must behave in ways that benefit its genotype. At the very least, if it is to remain successful (extant), it should not consistently act in ways that endanger the existence of the genotype it represents. The idea that behavior should make evolutionary sense is the basis for sociobiology theory.
The primary concept in sociobiology is kin selection, the idea that we share our genotype with our kin, and if we behave in ways that promote their success then we benefit our own genotype. Parental investment in offspring is a common and conspicuous form of kin selection. In a more general sense, kin selection is cooperation between closely related individuals that results in leaving more of their gene alleles in the population's gene pool (Hamilton, 1964). Social behavior is a familiar form of this type of kin selection, since members of the same breeding population are likely to share most of their genotype. Altruism, behaving in a manner that produces a net loss while neighbors gain, seems to be an unprofitable long term strategy. However, reciprocal altruism, an act that incurs relatively minor risk but produces a large gain from reciprocation by a neighbor, is an important and frequent feature in biological communities. For example, sentinel or alarm behaviors exchanged between different species that habitually forage near each other produces substantial mutual gain at low risk.
Fitness refers to the reproductive
success of a genotype. If a genotype does not reproduce, its fitness
is zero. The genotype in the population with the greatest average
number of surviving offspring is described as the most fit genotype
in the population. W.D. Hamilton (1964) proposed the idea that
an individual attempts to maximize its inclusive fitness, a dimension
that includes both its own reproductive fitness plus that of close
relatives. In this model, an individual's behavior should be influenced
by the degree of relatedness. In diploid species, the proportion
of identical chromosomes should be:
parent-child 1/2
full siblings 1/2
half siblings 1/4
first cousins 1/8
grandparents-grandchild 1/4
In the logic of this table, an individual who fails to reproduce, but whose behavior allows his or her sibling to produce three or more offspring, actually has gained genetic representation in the gene pool. This type of gain through kin selection is what is meant by the term inclusive fitness.
Societies that seem to reflect
this model of inclusive fitness are seen in the insect order Hymenoptera
(ants, bees, hornets, and wasps). These insects have an unusual
haploid-diploid genetic system. Diploid reproducing females produce
haploid ova with the usual independent segregation of chromosomes,
whereas haploid males produce haploid sperm all of which share
the same haploid complement of chromosomes. A female may store
one male's sperm after mating and produce progeny throughout her
life span from the same father. Fertilized ova become diploid
daughters, unfertilized ova become haploid sons. Consequently,
all sisters share all of their father's chromosomes and one-fourth
of the maternal chromosomes and all sons share one-half of the
complement of maternal chromosomes. Sons also have the unusual
situation of having no fathers, but they do have grandfathers
whose sperm fertilized the ova that became their mother. In these
haploid-diploid insects, the proportion of identical chromosomes
should be:
mother-daughter 1/2
sisters 3/4
brothers 1/2
brother-sister 1/4
Sisters, sharing 3/4 of their parental chromosomes, devote their lives to caring for the colony. Sisters are more related to each other than they are to their daughters. Sons, sharing only 1/4 of their sisters' chromosomes, do little for the colony but concentrate on attempting to mate (and produce daughters). Sisters may try to drive their brothers from the colony. This extraordinary social system seems to conform to what would be expected in a haploid-diploid species on the basis of inclusive fitness.
A model of inclusive fitness however, does not explain all social behavior or all social systems. Termite colonies (order Isoptera) for example, consist of reproducing males and females and non-reproducing workers of both genders. All have a normal diploid genetic system, yet worker castes appear to be instruments by which the reproducing individuals (kings and queens) generate large numbers of reproducing offspring (kings and queens). It is possible that the colony is analogous to a metazoan organism with worker castes being analogous to somatic body cells. Individual metazoan organisms are colonies of cells, in which many cells devote their energy toward sustenance of a small number of cells devoted to organismic reproduction. The phenotype coded by a genotype is larger than the cell. Indeed, the phenotype is often larger than the individual since it includes characteristics that produce societies. A social group is one logical extension of the machine geared to perpetuate the genotype. It is a group of individuals cooperating in some way to survive and reproduce their common genotype.
The numbers in the inclusive fitness models are somewhat misleading in primate populations. Due to mutations and cross-over events, the expectation of identical chromosomes is less than these values. Furthermore, computations beyond parent-child are statistical expectations. Since chance determines which chromosomes are passed in the second meiotic event, grandparents may have more than or less than the expected identical chromosomes. Since the probability of a particular chromosome surviving a meiotic event is 0.5, it is statistically unlikely for a chromosome to survive several generations unless its bearers have multiple offspring and disperse it into the population. After a distance of several meiotic events, an individual is related genetically to a population, but not to a distant ancestor. Since primate breeding populations are small, individuals are likely to be kin, that is they are likely to share chromosomes that were frequent in the parental populations. This kinship enhances the importance of inclusive fitness.
Richard Dawkins (1976) produced a "selfish gene" theoretical model that seems to help understand selection processes in complex organisms. In this model, he assumes that the universe tends to be populated by stable things and he proposed the idea of replicators, molecules that reproduce or make copies of themselves. Once replicating molecules evolved, selection began to discriminate between varying phenotypes of replicators. Individual molecules could have short lifetimes, but the copying process made the code effectively immortal. There exists a series of replicators that grade from a nucleotide to all of life (return to outline)).
Replicators
Level Replicator Effective unit for selection
1 - nucleotide
2 - codon
3 - small segment of chromosome
4 - gene (cistron) +
5 - larger segment of chromosome
6 - chromosome +
7 - cell (asexual) +
8 - cell (sexual) +
9 - individuals +
10 - group
11 - population (deme)
12 - race
13 - species
14 - community
15 - all planetary life
In theory, selection could operate anywhere between levels four and fourteen, the range of units that can be viewed as machines designed to improve the likelihood of effective replication. The most stable unit, the unit that comes closest to immortality, is the gene. In theory, selection could operate at any level above the gene and below planetary life, but changes in gene allele frequencies would be explained by models applied to the small replicating machine, the individual. Just as genes come in packets called chromosomes, genotypes come in packets called individuals. Just as we must study heredity through the behavior of chromosomes, we must study evolution through the behavior of individual organisms. The study of the group is as important to understanding phenotype as the study of populations is to understanding the genotype.
Why or how then, did evolution proceed past the asexually reproducing cell as its machine for replication develop metazoans? Perhaps because there were niches that could be filled. Asexual organisms are no less successful or "fit" than sexually reproducing organisms. Evolution does not optimize, so anything is possible as long as it is successful. Indeed, the general trend in the history of metazoans is a reduction in the number of progeny produced. Just as the phenotype of a metazoan consists of more than individual cells, the phenotypes of social animals consists of more than one individual.
Selection, based in part
on competition between phenotypes, may enhance conformity to a
niche, a process that resembles optimization. A greater amount
of genetic variability, through sexual reproduction, provides
a greater diversity of phenotypes and more material upon which
selection can act. At the same time, an increase in complexity
of sensory and behavioral abilities allows greater plasticity;
that is, greater accommodation to a wider range of environments
without genetic changes (return
to outline).
Probably the most powerful force that promotes social life is kin selection. Primate social groups are essentially associations between related animals, and their behavior promotes the livelihood and success of their genetic relatives. In groups of related animals, the phenotype (as coded into the genotype) can be larger than an individual in the same sense that among metazoans, the phenotype is larger than a single cell. Furthermore, phenotypes change as individuals experience growth, maturation, and aging. Generally among primates, kinship is recognized by association: the parent by the acts of caretaking, and group members on the basis of prolonged mutual associations.
Opposing this trend toward kin selected groups is an incest prohibition and resistance to inbreeding, a force that compels primate kin-selected groups to outbreed and thus maintain genetic heterozygosity. Much of primate social behavior can be better understood in the context of these two forces. These forces are not volitional, but are a consequence of natural selection.
The strongest and most
easily understood form of kin selection is protection and caretaking
of offspring by parents. Acts that promote offspring survival
are directed toward offspring of relatives in a small social group.
Since such relatives possess most of the same genes as the non-parent
caregiver due to common descent of the group, the effect of this
kin selection directed at relatives enhances the survivorship
of the shared genotype through enhanced inclusive fitness (return to outline).
Acts and emotions which are very beneficial when directed at kin sometimes produce illogical and unfortunate consequences when stimulated and performed outside of their adaptive context. What about a person who gets snake bitten trying to rescue a mouse from a rattlesnake or another who runs into a burning building to rescue a pet? These apparent "altruistic" acts are simply impulses or emotions that evolved in the context of kin selection and are sometimes seen in unusual non-kin circumstances. True altruism consists of self-destructive acts that are performed exclusively for non-relatives. One would expect truly altruistic genotypes to be rare. If caretaking or defensive acts occur more frequently in non-kin contexts, these genotypes become increasingly at risk. In such a context, selection might promote phenotypes that discriminate between kin and non-kin situations and thus reduce self-destructive behaviors. What about a phenotype that chooses to act against its own genotype (kin rejection)? If those behaviors are influenced by genes, one would predict, on the basis of natural selection, that those alleles would be at risk when competing with other alleles that produced behaviors that enhanced kin selection.
Caretaking behaviors play key roles in the domestication of plants and animals. Such acts can clearly have great advantages and we may soon face a time when environmental caretaking may be the most important of human attributes. Caretaking is an example of a behavior that could evolve in the context of kin selection and be adaptive outside the context in which it evolved.
Selection in a large and heterogeneous social group may promote distinctions between degrees of kinship. The group may be composed of subsets of competing relatives and appropriate distinctions are made by participants. However, in the context of an outside danger, distinctions involving degrees of relatedness usually are overridden by the adaptive value of inclusive fitness.
Emotional responses and
bonds that promote kin selection acts develop in the context of
a social group. Close association promotes these bonds, so to
a great extent, kinship is functionally defined for emotional
responses on the basis of association (return to outline).
The Selfish Gene and a Hypothetical Social Group
It is very important to remember that a set of genes that produces social behaviors can be subject to stabilizing selection. In a universe composed of relatively stable things, those characters that do not replicate adequately are transitory. A social group offers a potential for diversity of strategy and an increased complexity of phenotype. Degrees of relatedness and increased plasticity in behavior offer the opportunity for increased genetic variability.
There may be conflicts
of interest between different members of a group. For example,
consider the pressure to maintain heterozygosity in a hypothetical
multi-male, multi-female group. One manifestation is sexual
facilitation, the increased sexual attention that is aroused
by appearance of strangers. It should be no surprise that if the
approach of a strange male arouses sexual interest among resident
females, there is likely to be an aggressive response by group
males toward the newcomer. At the other extreme, if a male stays
in a group too long, he begins to be treated as close kin and
finds the number of consorts diminish as a consequence of incest
prohibition. In this manner, the behavior of females in our hypothetical
group selects for male personalities (phenotypes) who migrate
from group to group. Group tenure usually should be no longer
than the interval needed for a male to have sexually active daughters,
unless some other incest avoidance mechanism allows a male to
discriminate between appropriate and inappropriate consorts (return to outline).
In order to be tolerated
by the group, a newcomer (or an immature individual) may have
to conform to certain standards of expected behavior and perform
certain acts. Such acts are referred to as roles, and lack of
attention or incompetence can result in aggression or intolerance
directed toward the newcomer. These roles often take the form
of acts that promote inclusive fitness (predator alarms, taking
positions that shield other group members from potential danger,
supporting or defending the group or its resources,...). For example,
since an immigrating male finds himself investing his efforts
and taking risks to promote offspring sired by other males, one
would expect strategies to emerge that allow a male to maximize
the degree of relatedness between himself and the infants in which
he invests. One way would be to specialize in protecting his own
progeny if he can identify them. Females counter such strategies
by hiding paternity. If the male can not be absolutely sure an
infant is not his, he is likely to invest in it. Of course one
way to increase the likelihood is to kill all infants upon entry
into the group (infanticide) and vigorously participate in consorts
to produce new pregnancies. Another male strategy might be for
males to immigrate to groups in which older male relatives have
held tenure (return
to outline).
Animals who share space
or resources are bound to come into conflict with each other from
time to time. The most widespread device for efficient conflict
resolution among socially living animals is the maintenance of
status hierarchies. When one animal wins a conflict
another animal loses. Traditional patterns of win/loss relationships
provide powerful social devices for avoidance of aggression between
potential competitors. Foreknowledge of the outcome allows all
participants to anticipate events and plan effective strategies
without injury. Those individuals who habitually win are usually
described by behavioral scientists as being higher in rank than
individuals who lose conflicts. Status hierarchies are beneficial
to all group members (return
to outline).
Primates groups are tied to particular locations. They do not wander at random, but stay in a familiar area and utilize familiar resources. An area used by a group is called its home range. Within the home range there is often a subarea that is intensively used, a core area, that contains most frequently used resources such as a water source or preferred sleeping sites. Territoriality is the defense of the home range by aggression and display from intrusion by other groups of the same species. Territoriality is more likely (but not mandatory) in situations in which the core area approximates the home range.
Species often specialize
in particular niches, that is they exploit a particular habitat
and certain resources in that habitat. Thus several species of
primates may live in the same area but use different resources.
Species with overlapping ranges are said to be sympatric. Those
with non-overlapping ranges are called allopatric species (return to outline).
Primates generally live in groups that have clear boundaries for membership and complex organization. Relationships between individuals are controlled by dominance rank, political alliances, friendships, kinship, role, age, gender, and situation. Composition of the social group can be influenced by historical circumstance, but there is usually a typical or usual composition and social structure for each species. The nature of the social group is shaped by inherited characteristics of bonding, personality, and motivation. Elaborate rules usually control social interactions such as proximity, attention, mating, coordination of movements during foraging, and performance of roles.
Perhaps the simplest society is the monogamous pair, in which the typical adult social group consists of a single breeding adult pair and their immature offspring. This should not be confused with a monogamous family or a monogamous pattern of consort behaviors. In a monogamous society, the two adults have no allies or nonaggressive relationships with other adults outside of their pair bond. Such species are usually highly territorial and intolerant of outsiders.
Another simple society is the noyau social group characterized by individual adult females who establish separate ranges. The individual female and her immature offspring are the primary social group. A male tries to establish a range that includes the ranges of several females and consorts with them during the mating season.
A one-female society consists
of a single adult female, several adult males, and their progeny.
A one-male group includes a single adult male, several adult females,
and their progeny. Multi-male multi-female groups are composed
of several adults of both genders, and are often further divided
into subgroups based upon kinship, bonding, age, or rank (return to outline).
Table of Contents
15 Aug 2004
Department of
Department
of Anthropology, College
of Liberal Arts , UT Austin
Comments to cbramblett@mail.utexas.edu