Sociobiology 1

Module 3 - LEVELS OF SELECTION & THE PARADOX OF ALTRUISM

For Modules 3, 4, 5 and 6 the main readings are in

Ridley, Matt. The Red Queen (RQ) (See syllabus for chapters assigned.)
Dawkins, Richard. The Selfish Gene (SG)

0. OBJECTIVES

In this module you will learn

how evolution by natural selection operates
the reasons why the gene (rather than the individual organism) is viewed as the basic unit of selection
the reasons for the demise of group selectionism
the reasons why altruism and cooperation are paradoxical
sociobiology's 2 main answers to the paradox of altruism and cooperation

1. EVOLUTION BY NATURAL SELECTION

Charles Darwin in The Origins of Species (1859) proposed that all existing organisms evolved by natural selection from simple ancestors that arose on earth in the distant past.

Natural selection is based on 3 processes:

organisms have a tendency to multiply indefinitely
they vary with respect to characteristics that affect their survival and reproduction
these traits are heritable (traits of parents are passed on to their offspring)

EX: the long neck of giraffes evolved as it helps to reach leaves high up in the trees

The following exhibits illustrates important points about evolution by natural selection:

Exhibit: The basic process of natural selection is differential reproduction (SEB Figure 1-3 p. 6)

Exhibit: Sometimes we have a fairly complete fossil record of the evolution of a species, as in the case of horses (SEB Figure 1-2 p. 4)

Exhibit: Evolution can be artificially simulated in the laboratory (Maynard Smith 1989, Figure 6.12 p. 116)

Exhibit: Some traits are more heritable than others: heritability of egg number and egg size (SEB Figure 2-3 p. 34)

Insofar as they are influenced by genes (i.e., are heritable) and affect survival and reproduction, behaviors are also subject to evolution.

2. THE GENE-CENTERED VIEW OF EVOLUTION

Sociobiology is a new perspective on the evolution of behavior that arose in the biological literature in the late 1960s and became prominent in the mid-1970s. Principal early contributions are George C. Williams (1966) Adaptation and Natural Selection; Edward O. Wilson (1975) Sociobiology: The Emerging Synthesis; and Richard Dawkins (1976) The Selfish Gene.

A major principle of sociobiology is that natural selection operates at the level of the gene (rather than the level of the individual, the group, or the species). The main argument of this gene-centered view of evolution is summarized using quotations from Richard Dawkins, The Selfish Gene:

"I am using the word gene to mean a genetic unit [segment of DNA] that is small enough to last for a large number of generations and to be distributed around in the form of many copies." (p. 32)
"Natural selection in its most general form means the differential survival of entities." (p. 33)
"Evolution is the process by which some genes become more numerous and others less numerous in the gene pool." (p. 45)

Q - Why "small enough to last for a large number of generations"?
A - Because of crossing over, smaller genetic units (=shorter segments of DNA) are more likely to remain intact over many generations.

Exhibit: Crossing over during meiosis (SEB Figure 1-8 p. 14)

In sexual species, the individual organism is not the primary level of selection, because the organism does not produce identical copies of itself. The individual organism is the result of a unique combination of genes that is reshuffled at each generation.

The gene-centered view of evolution implies that the organism is but a "survival machine" for the genes!

In explaining the evolution of most traits, reasoning in terms of selection at the level of the individual organism and selection at the level of the gene makes no difference, because a trait that promotes the survival of the organism also promotes the spread of the organism's genes.

But in explaining altruistic behavior (defined later), the level of selection makes a big difference (see later)!

3. THE DEMISE OF GROUP SELECTIONISM

Modern biologists have rejected the once-popular notion that a trait can evolve because it is "good for the species"; this view is called group selectionism. An example of group selectionism is the view of biologist Wynne-Edwards, who interpreted the apparent reproductive self-restraint of birds with a fixed clutch size as a instance of altruistic behavior that had evolved "for the good of the group" to prevent overpopulation.

Group selectionism was rejected by biologists on the basis of both field work and theoretical models of group selection:

Field work (e.g., by ornithologist D. Lack) showed that it is not necessary to assume group selection to explain fixed clutch size of birds such as the great tit (Parus major). Lack showed that the apparent altruism of the great tits in keeping a low clutch size can be explained more simply as the result of "selfish" individual-level selection as the typical clutch size is also the optimal one in terms of survival of the offspring.
Exhibit: The Lack effect (SEB Figure 2-1 p. 26)
Theoretical models (using mathematics and computer simulation) showed that group selection is very unlikely in nature, except in rare cases (such as the evolution of low virulence in the myxoma virus of rabbits in Australia):
Exhibit: Theoretical difficulty of group selection (Maynard-Smith 1989, Figure 9.7 p. 178)

The basic difficulty of group selection is that a behavior that benefits the group at the expense of the individual or its genes will not be favored by natural selection, so that the gene(s) causing that behavior should be eliminated in the course of evolution.

Levels of selection (summary):

Gene: the basic unit of selection; evolution is the differential reproduction of genes
Individual: viewed as a "survival machine" designed by its genes for their survival and reproduction, or as a "strategist" in pursuit of reproductive success
Group or species: almost never a unit of selection (except in rare cases), so that the evolution of behavior cannot be explained by selection "for the good of the group"

Q - In the modern view of evolution, the basic unit of natural selection is _____?

4. THE PARADOXES OF ALTRUISM & COOPERATION

Nature shows many instances of altruism and cooperation among animals.

Altruism refers to a behavior that benefits others at a cost (in reproductive fitness) to the individual organism engaging in the behavior. Examples of altruistic behaviors are

warning calls in birds
suicide committed by a bee to protect the hive

The puzzle with altruism is that since group selection does not work, how does one explain the evolution of instances of altruism found in nature?

Examples of cooperation are

pack hunting in wolves, hyenas, humans
mutual grooming for vermin (in primates)
male coalitions in lions
food sharing among vampire bats
the "live and let live" system that developed spontaneously among enemy soldiers fighting in trenches during World War I

Cooperation also represents a puzzle, since natural selection should produce purely selfish behavior by which the organism tries to receive the benefits of cooperation while avoiding doing its share of the collective effort. Module 5 explains in details why cooperation is paradoxical with respect to the theory of evolution.

THE PARADOX - How can a behavior evolve by natural selection if it lowers the reproductive fitness of the individual who engages in it?

Sociobiology has two principal answers to the paradoxes of altruism and cooperation:

the mechanism of relatedness (or "kin selection") with the notion of inclusive fitness can explain the evolution of altruistic behavior
the mechanism of reciprocity can explain the evolution of cooperation

We discuss these two mechanisms in turn in the next 2 modules:

relatedness and inclusive fitness in Module 4
reciprocity in Module 5

Q - What is the paradox of altruism?

Q - What are the 2 answers of sociobiology/evolutionary psychology to the paradoxes of altruism and cooperation?

Last modified 2 Sep 2004