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Levels of Selection
E3: Lecture 7
The Hierarchical Structure of Life
The “Unit” of Selection
According to Lewontin (1970), selection acts at a specific level when:
1) There is variation at that level
2) There is heritability across generations
3) There is differential extinction/reproduction
Question: What is
?
A gene? An organism? A group? A species? A community?
Can a Group of Organisms be Selected?
“For everything concerning population dynamics,
[group selection is] much more important than
selection at the individual level.”
(Wynne-Edwards, 1962)
V. C. WynneEdwards
“Group-level adaptations do not, in fact, exist.”
(Williams, 1966)
“Group selection…has soaked up more
theoretical ingenuity than its biological
interest warrants.” (Dawkins, 1982)
George
Williams
Richard
Dawkins
“Group selection is a significant evolutionary
force and its products have been amply
documented in nature.” (Sober & Wilson, 1998)
Elliott Sober
David Sloan
Wilson
Levels of Selection
Lecture Outline
• History of the levels of selection debate
• Pluralism in the group selection debate
• Artificial selection at higher levels
• Origin of new levels
• Summary
Levels of Selection
Lecture Outline
• History of the levels of selection debate
• Pluralism in the group selection debate
• Artificial selection at higher levels
• Origin of new levels
• Summary
Darwin “Takes it Up a Level”
•
Darwin is mostly associated with
arguments where individuals are the units
of selection (e.g., the “struggle for
existence” occurs between individuals).
•
As we saw in the last lecture, altruism
(behavior increasing the fitness of others
at personal cost) seems to be the
opposite of what an individual should do.
•
Darwin’s solution?
– He invoked selection at a higher level.
On bees…
“In social animals [natural selection] will adapt
the structure of each individual for the benefit
of the community; if the community in
consequence profits by the selected change”
-Origin of Species (6th Ed.)
A
B
altruistic
type
selfish
type
either
type
+
On humans…
“It must not be forgotten that although a
high standard of morality gives but a slight
or no advantage to each individual man and
his children over the other men of the same
tribe, yet … there can be no doubt that a
tribe including many members who, from
possessing … patriotism, fidelity, obedience,
courage, and sympathy, were always ready …
to sacrifice themselves for the common
good, would be victorious over other tribes;
and this would be natural selection.”
-Descent of Man
Reasoning by Analogy
• Individuals are comprised of
multiple units (connected by a
living matrix), whereas groups
of social individuals are
connected by a non-living
matrix in which “stimulusresponse” activities play out.
• Division of labor and
specialization occurs in multicellular bodies as well as in
some groups of organisms
(e.g., the eusocial insects).
≈
• Cells within an organism exhibit
reproductive restraint and so
too may organisms within
groups
≈
Formalizing Group Selection
“Some [groups] prove to be better adapted socially and individually than
others, and tend to outlive them, and sooner or later to spread and multiply
by colonising the ground vacated by less successful neighboring communities
… Evolution at this level can be ascribed to…group selection.”
V. C. Wynne-Edwards
•
Wynne-Edwards is associated with the “for the good of the group” arguments.
•
He focused on behaviors that he interpreted as “population regulators”
– Territoriality is established to prevent food overexploitation
– Bird song in the morning is to assess population density (and adjust
appropriately)
•
Such socially advantageous (but individually disadvantageous) traits evolve
because of their effect on preserving the group – i.e., groups overrun by
cheaters go extinct.
Take 5 minutes to talk to your neighbor about the following:
What do you think about Wynne-Edwards process? Can you think of
alternative explanations for the evolution of territoriality and bird song?
1966: The Death of Group Selection
• Williams granted that group-level adaptations required the
process of group selection. However, he claimed that such
group-level adaptations rarely, if ever, existed.
• His argument rested on the principle of parsimony: often
social behaviors could be explained by either group-level
or individual-level evolutionary stories. As the individuallevel story is often simpler, it should be the first choice.
• Williams thought that the conditions favoring WynneEdwards vision were rare (e.g., small semi-isolated
populations) and within group defection would outpace
between group selection. Thus, truly altruistic individuals
would be suckers for a selfish mutant (or invader).
George Williams
The Resurrection of Group Selection
• In the 1970’s, several mathematical models
investigating the evolution of groupbeneficial, but individually disadvantageous
behaviors were explored.
• Not all of these studies explicitly mentioned
“group selection”.
• The leader of the new “group selection”
movement was D.S. Wilson. Other major
contributors were philosopher Elliott Sober
and biologist Michael Wade.
• Wilson introduced a life cycle that he argued
was more conducive to the operation of
group selection: the trait-group framework.
Wade then tested this framework in the
laboratory with the flour beetle.
David Sloan
Wilson
Elliott Sober
Michael Wade
The Trait-Group
• Wilson reasoned that many types of
organisms interact in small groups between
dispersal episodes.
• Collectively, these groups (the “trait-groups”)
constitute the population.
The Evolution of Altruism by
B
Group Selection
Multi-level Selection
(New)
A
50%
1
20%
80%
2
50%
Groups form
20%
80%
Selection occurs
-1
Before selection
+2
base
fitness
47%
53%
After selection
6%
6%
94%
94%
Individual selection: within
each group, A is selected
against
26%
26%
72%
72%
Group selection: groups
with more A types have
higher productivity
An Example: The Myxoma Virus
• This virus was introduced by the Australian
government to control the exploding rabbit population.
• The virulence of the disease (i.e., what fraction of
hosts were killed) was tracked over time and it was
noticed that virulence decreased over time!
A
B
• Imagine again two types of virus particles, type A
exercises reproductive restraint, while type B
reproduces at a maximal rate.
• Within rabbits with both types, B increases in
frequency at A’s expense.
AA
B
B B
B
• However, B infected rabbits are more likely to die.
• Again, multi-level theory suggests we have group
selection for avirulence working against individual
selection for virulence.
A
A
B
B B
B
Levels of Selection
Lecture Outline
• History of the levels of selection debate
• Pluralism in the group selection debate
• Artificial selection at higher levels
• Origin of new levels
• Summary