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Two Ways of Seeing Selection
B
A
Here, ai and bi give
the fitness of an A
type and B type,
respectively, in a
group with i A types
Selection occurs
a2=2
b0=2
b1=3
B
a1=1
a3=3
b2=4
A
Here, pi gives the
productivity of a
group with i A types
and fi gives the
fraction of that
productivity claimed
by the A type.
Selection occurs
p0=6
(f0=0)
p1=7
f1=1/7
p2=8
f2=1/2
p3=9
Individualistic or
contextual perspective
(f3=1)
Multi-level or
collective perspective
Contextual
b2=4
a2=2
Translation
p i  ia i  ( n  i ) b i
ia i
fi 
ia i  ( n  i ) b i
Contextual
Perspective
Collective
p2=8
f2=1/2
Collective
Perspective
ai 
bi 
fip i
i
(1  fi )p i
ni
Perspectives, Parameterizations and Pluralism
• The two crucial components to any trait-group
model are:
– The group frequency distribution
– The fitness structure
• The fitness structure can be represented in
different ways:
– Contextual perspective (assigns groupdependent fitnesses to individuals)
– Collective perspective (assigns fitnesses to
groups as well as how the productivity is
shared between individual types)
W.E. Hill, 1915
• These two parameterizations are simply different ways to package the same
information. They are mathematically interchangeable.
• There can be benefits from culturing an ability to switch perspectives:
“gestalt-switching” pluralism
– Analogies, visualization, understanding difficult terms, sorting causal
stories, and even practical benefits
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
Poultry Breeding:
The Individualistic Approach
• The standard approach to increasing
the egg yield of a hen is to select the
hen that produces the most eggs as a
breeder (this focuses on a and b, not p).
• The expectation is that over
generations, egg yield will increase.
• In some cases, using this selection
regime actually led to decreases in egg
production!
• What is going on?!
breeder
Hens Live in Groups
• In the current poultry industry hens are
raised in groups.
• It turns out that the hens with the highest productivity of eggs were
the hens that were the most aggressive (e.g., pecking at others,
stealing food, etc.).
• Thus, as you select for good egg-layers, you are indirectly
selecting for nasty hens.
• As more nasty hens fill a coup, every hen’s productivity decreases.
• How could we address this problem?
Poultry Breeding: The Group Approach
• Muir (1995) decided to select
instead breeders based on group
(not individual) productivity (this is
a p parameter, not an a or b).
• Annual egg production was
increased by 160 percent using
this group selection regime!
• “If this [group-evolved] strain
becomes widely used in the
poultry industry, the projected
annual savings will far exceed the
money spent by the U.S.
government for basic research in
evolutionary biology.” (Sober &
Wilson, 1998)
Coup 1
Coup 2
mostly nice hens
mostly mean hens
Artificial Ecosystem Selection
•
Swenson and colleagues performed artificial selection
on ecosystem phenotypes (e.g., biomass of a plant
growing in an evolving microbial community).
W. Swenson
Selection for High Biomass
Selection for Low Biomass
D.S. Wilson
R. Elias
No Selection for Biomass
•
•
•
In 2 of 3 experiments, the authors
reported that the ecosystem
phenotype responded to selection
in a sustained way.
Different lines had different
chemical profiles, suggesting that
the microbes evolving had affects
on abiotic character of the
environment.
Just as Darwin made a case for
natural selection through analogy
with artificial selection, the
authors propose that natural
ecosystem selection may occur.
Question:
What would be required for natural
ecosystem selection to occur?
•
There may be practical
applications (e.g., bioremediation,
agriculture, biofuels, etc.)
dry weight (mg)
Ecosystem Response to Selection
high
low
generation
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
The Origin of New Levels
• Emergence of a new level in the
biological hierarchy is termed a
“major transition”:
–
–
–
–
Genes to chromosomes
Prokaryotes to Eukaryotes
Single cells to multi-cellular
Solitary individuals to societies
prokaryote
eukaryote
solitary insect
social insect
• What are the common themes for
these transitions?
– Lower level units cooperate to
ensure the functioning of the
higher level unit
– There is often division of labor
between the lower level units
– There is a sense of “common
fate”: lower-level units “go down
together with the higher-level ship”
Potential for Major Transition in Bacteria?
• Rainey proposed an interesting hypothesis:
– Cheats might serve as a type of propagule in
a “multicellular mat life cycle” in his
Pseudomonas fluorescens system.
• The life cycle:
1. WS forms mat.
2. Mat grows.
3. And grows.
4. SM mutants
arise.
5. SM mutants
rise in frequency.
6. Mat collapses &
SM are liberated.
7. Back mutation
to WS occurs.
Question:
How does this life cycle
compare with cycles from
multicellular organisms?
Wrinkly
Spreader
(WS)
Smooth
Morph
(SM)
The Real-Time Evolution of Multicellularity
•
Ratcliff, Travisano, and colleagues
performed an elegantly simple
experiment with budding yeast:
1. Growth under shaken conditions
2. Culture sits undisturbed for 45 min.
3. Culture’s bottom 1% is propagated.
W. Ratcliff
evolved
ancestor
•
After a handful of transfers, they
found yeast that had evolved a
“snowflake” clustering phenotype.
•
These snowflakes:
M. Travisano
– Form by post-division adhesion.
– Have a selective advantage under
gravity selection (but not otherwise).
– Reproduce by fission.
fitness
evolved
ancestor
minutes of growth
gravity
no gravity
Selection at the Emerging Level
•
One snowflake isolate was subjected to varying degrees of selection:
selection
strength
Weak
25 min.
Int.
15 min.
Strong
5 min.
•
Larger clusters with a faster settling rate evolved under strong selection.
•
A higher rate of apoptosis (cell death) evolved under strong gravity selection.
•
These dead cells are found where
the cluster splits– this is the origin
of somatic tissue!
•
“We conclude that selection was
acting on the reproduction and
survival of individual clusters rather
than that of their component cells.”
early snowflake
late snowflake
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
Summary
• Few topics in evolutionary biology have been as contentious as
the debate over the units/levels of selection.
• Darwin invoked selection at levels above individuals; this
process was formalized by V. C. Wynne-Edwards and criticized
by George Williams.
• Higher level selection was resurrected by D. S. Wilson and
colleagues in the context of multi-level selection theory.
• There have been artificial selection experiments targeting units
above the level of the individual (e.g., chicken coups and
microbial ecosystems).
• There have been real-time experiments on the origins of new
levels of individuality (e.g., multicellularity in budding yeast).