Download Notes - UNC

Soci111 – Human Societies
Module 3 – Levels of Selection & Paradox of Altruism
François Nielsen
University of North Carolina
Chapel Hill
September 14, 2016
Outline
Main Themes
Evolution by Natural Selection
Gene-Centered View of Evolution
Paradoxes of Altruism & Coopertation
Main Themes
É
how evolution by natural selection operates:
É
É
É
É
genes (rather than individual organisms) are the basic units of
selection (the “replicators”)
group selection does not work
thus altruism & cooperation found in nature are paradoxical
sociobiology’s 2 main answers to the paradoxes of altruism &
cooperation are:
1. evolution of altruism through relatedness and kin selection
(inclusive fitness theory), and
2. evolution of cooperation through reciprocity (tit-for-tat
strategy)
Evolution by Natural Selection
Natural Selection
É
É
In The Origins of Species (1859) Charles Darwin 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 processses:
1. organisms have a tendency to multiply indefinitely
2. they vary with respect to characteristics that affect their
survival and reproduction
3. these traits are heritable (traits of parents are passed on to
their offspring)
É
natural selection may produce physical organs
É
É
e.g. long neck of giraffes
but also patterns of behavior
É
e.g. waggle dance of bees
Evolution by Natural Selection
Natural selection is differential reproduction
Evolution by Natural Selection
Natural selection sometimes visible in fossil record
Evolution by Natural Selection
Natural selection can be simulated in laboratory
Evolution by Natural Selection
Egg number: low heritability; egg size: high heritability
Gene-Centered View of Evolution
Sociobiology & gene-centered view
É
É
sociobiology is new perspective on evolution of behavior
path-breaking works
É
É
É
É
George C. Williams (1966) Adaptation and Natural Selection
Edward O. Wilson (1975) Sociobiology: The Emerging Synthesis
Richard Dawkins (1976) The Selfish Gene
major assumption
É
natural selection operates at level of the gene rather than
individual, group, or species
Gene-Centered View of Evolution
The gene-centered view
É
From Dawkins The Selfish Gene
É
É
É
“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)
É
Q – Why “small enough to last for a large number of
generations”?
É
A – Shorter DNA segments more likely to remain intact in
crossing-over over many generations
Gene-Centered View of Evolution
Small DNA segment remains intact in crossing-over
Gene-Centered View of Evolution
The gene-centered view
É
In a sexually-reproducing species (like us)
É
É
É
É
É
É
É
organism does not produce identical copies of itself
organism can only project into the next generation a sample of
50% of its genes
an individual represents unique combination of genes that is
reshuffled each generation
organism is but a “survival machine” for the genes!
thus organism is not the primary unit of selection
genes are the basic units of selection (the “replicators”)
this has very important consequences for understanding the
social behavior of organisms: a behavior can evolve because it
increases the fitness of the organism’s genes, even though it
decreases the survival of the organism itself (see Hamilton’s
theory later)
Gene-Centered View of Evolution
Demise of Group Selectionism
É
Old model of group selection:
É
É
É
traits can evolve because they are “good for the species”
e.g. biologist Wynne-Edwards argued that in some birds a
fixed clutch size (reflecting reproductive self-restraint) has
evolved for the good of the group, to prevent overpopulation
and depletion of resources
Modern view:
É
É
group selection does not work, because natural selection will
tend to eliminate genes for an “altruistic” trait that favors the
group at the expense of the individual
group selection rejected because:
É
É
one can explain fixed clutch size without group selection
(evidence of Great Tits by Lack)
theoretical models show that group selection does not work
except in very rare circumstances (John Maynard-Smith, next
slide)
Gene-Centered View of Evolution
Group selection usually defeated by selection on individuals
Gene-Centered View of Evolution
Levels of Selection
É
Modern view on levels of natural selection:
É
É
the gene is the basic unit of selection; evolution is differential
reproduction of genes
the individual organism:
É
É
É
viewed as “survival machine” designed by genes for their
survival & reproduction
viewed as “strategist” in pursuit of reproductive success
group or species almost never a unit of selection (except in rare
cases such as parasites)
Paradoxes of Altruism & Cooperation
Evidence for Altruism & Cooperation
É
Natural world provides many examples of
É
altruism = behavior benefiting others at a cost (in reproductive
success) to the individual
É
É
É
warning call of some birds
eusociality in bees (non-reproductive worker bee commit suicide
to defend the hive)
cooperation = reciprocal exchange of favors
É
É
É
É
É
pack hunting (wolves, humans, . . . )
mutual grooming
cleaning-fish & host
food sharing in vampire bats
“live & let live” system in trench warfare
Paradoxes of Altruism & Cooperation
Paradoxes & Solutions
É
The paradoxes:
1. How can altruism evolve by natural selection if it lowers the
reproductive success of the individual engaging in it?
2. How can cooperation evolve since natural selection would
favor selfish behavior (take the benefit, do not reciprocate)?
É
Sociobiology’s solutions:
1. relatedness & inclusive fitness (“kin selection”) can explain
evolution of altruism → discussed in Module 4
2. reciprocity can explain evolution of cooperation → discussed in
Module 5