Stochasticity in Evolutionary Biology
*
?
*
*
*
(Stebbins 1971, modified by Kutschera & Niklas 1994)
“When we look at the
plants and bushes clothing
an entangled bank, we are
tempted to attribute their
proportional numbers and
kinds to what we call
chance. But how false a
view is this!”
“…the idea that chance
begets order…”
(Peirce 1893, quoted in Beatty 1984)
Random with respect to…
Beatty, J. 1984. Chance and natural selection. Philosophy of Science 51:183-211.
Key insight into evolution: Particulate inheritance
Deterministic effects: genes to genotypes to phenotypes
Fisher
Wright
Haldane
Particulate Inheritance (Mendel) + Evolution by Natural Selection (Darwin) =
The Modern Synthesis
Population genetics theory built from scratch
4 processes: Drift (N), Mutation (m), Gene Flow (m), Selection (s)
Consensus on theoretical possibilities, and the “right” model structure
Disagreement on the relative importance of different processes in Nature
*?
The Evolutionary
Modern Synthesis
*
*
*
(Stebbins 1971, modified by Kutschera
& Niklas 1994)
* Processes with stochastic elements
Consensus on theoretical possibilities, and the “right” model structure
Disagreement on the relative importance of different processes in Nature
The Molecular Revolution:
Evolution at the level of proteins/DNA
Why so much variation?
Shouldn’t it get edited out by natural selection?
Maybe much of this variation isn’t even
“seen” by natural selection
The neutral theory of molecular evolution:
Mutation, Drift, Gene flow
Mootoo Kimura
Many DNA changes do not alter
the amino-acid composition of a
protein
Some (many?, few?) amino acid
substitutions don’t alter the
function of a protein
The neutral theory of molecular evolution is operational
(in a way that the ecological theory is not)
Each one of these loci (some neutral, some not) has the
exact same demographic history (i.e., drift, migration)
The neutral theory of molecular evolution is operational
An example: Selective sweeps
Okay, so all these processes are happening in Nature…
but what is their relative importance?
Does the answer really matter?
On relative importance:
Beatty, J. 1984. Chance and natural selection. Philosophy of Science 51:183-211.
The
importance
(or not) of
debates
about
relative
importance
Beatty, J. (1997) Why do biologists argue like they do? 64:S432-S443.
The reason evolutionary biology seems to rest on more
solid theoretical footing than ecology, is because
evolutionary biologists agree on the theoretical framework,
not the relative importance of different processes.
…about ecological communities
Everything you need to know…
Global community
Speciation
Drift
Selection
Dispersal
Regional community
Dispersal
Speciation
Drift
Selection
Dispersal
Dispersal
Local
Community
Speciation
Drift
Selection
Note: Extinction results from drift & selection
What about macroevolution?
Darwin’s first phylogeny
The Tree of Life Project
http://tolweb.org/tree/
Stephen J. Gould and “contingency”
A critique of assuming determinism
A critique of assuming gradualism
Replaying Life’s Tape
“I call this experiment
“replaying life’s tape.” You
press the rewind button…go
back to any time and place in
the past…Then let the tape
run again and see if the
repetition looks at all like the
original”
Contingency
(= stochasticity?)
“any replay of the tape would
lead evolution down a
pathway radically different
from the road actually taken”
Are the shapes of phylogenies different from
random expectation?
Are the shapes of phylogenies different from
random expectation?
Looks like we got null
models from evolution
too (in addition to
neutral models)
Lessons for Community Ecology:
• Can neutral theory be “operationalized”?
• Perhaps we can at least agree on a conceptual
framework (mine, of course)
• Can we skip past the mind-numbingly obvious aspects
of the selection-neutrality debate?
• Where is the “relative importance” approach going to
get us?
•Building models and empirical studies to be more
comparable to one another.
• Getting drift into models of selection…to come next
week