Download Lectures - Frozen Evolution

Postneodarwinistic Theories of
Evolution - From the Selfish Gene to
Frozen Evolution
Jaroslav Flegr
Department of philosophy and history
of science
Prague 2008
Outline
• Darwinisms, neodarwinisms and evolution of
adaptive traits by means of natural selection
• Model of selfish gene – a theory of evolution of
adaptive traits in sexual organisms
• Shortcomings of the theory of selfish gene
– Context-dependent phenotypic expression of gene
(epistasis)
– Context-dependent impact of biological trait on
biological fitness
– Frequency dependent selection (ESS)
• Theory of frozen plasticity
Charles Darwin
Darwinistic explanation of the
origin of adaptive traits
• An average number of offspring/parent > 1
• Populations are stable in a long term
• Therefore, only a fraction of the young survive and
reproduce
• Organisms in populations differ
• Probability of survival and reproduction (fitness)
depends on properties of an individual
• Offspring inherit properties and fitness of their parents
• Therefore, useful (adaptive) traits (structures and
behavioral patterns accumulate in populations
What is wrong with darwinistic
theory of origin of adaptations?
• An average number of offspring/parent > 1
• Populations are stable in a long term
• Therefore, only a fraction of the young survive and
reproduce
• Organisms in populations differ
• Probability of survival and reproduction (fitness)
depends on properties of an individual
• Offspring inherit properties and fitness of their parents
• Therefore, useful (adaptive) traits (structures and
behavioral patterns accumulate in populations
What is wrong with darwinistic
theory of origin of adaptations?
• An average number of offspring/parent > 1
• Populations are stable in a long term
• Therefore, only a fraction of the young survive and
reproduce
• Organisms in populations differ
• Probability of survival and reproduction (fitness)
depends on properties of an individual
• Offspring inherit properties and fitness of their parents
• Therefore, useful (adaptive) traits (structures and
behavioral patterns accumulate in populations
Vanishing nature of heritability.
genotype
phenotype
fitness
Richard Dawkins
Theory of selfish gene
• Individual genes are (usually) transmitted from
parents to offspring unchanged
• Variants of the same gene (alleles) differ in their
influence on phenotype and fitness of an organism
• Successful variants of a gene are transmitted to next
generation in more copies than unsuccessful variants
• Therefore, variants of the same gene compete for
ability to program their “vehicle” to help them to be
transferred in as many copies as possible
• Mostly, but not always, the variants try to increase
biological fitness of their “vehicle”
Necker’s cube
Blue beard model
♀ XX
♂ XY
♀ XX
♂ XY
W : WM
8 : 5 or 4 : 5
?????????????

Theory of selfish gene
• Explains origin of adaptations in sexual organisms
• Explains evolutionary origin and success of “blue
beard” genes
• Explains evolutionary origin and success of an
important category of altruistic traits
– Under certain conditions, an organism could transmit more
copies of its genes by helping in reproduction to its relatives
than by its own reproduction (helpers, bees, ants)
Theory of selfish gene is now the
mainstream evolutionary theory
• Darwin: How this trait increases fitness of an individual?
• Dawkins: How this trait increases a number of copies of
variant of gene that is responsible for its formation?
What is wrong with the selfish
gene theory?
• It disregards a phenomenon of frequency dependent
selection
• It disregards context-dependency of an impact of
biological trait on biological fitness
• It disregards context-dependency of phenotypic
expression of gene (epistasis)
Frequency dependent selection
A selective value of many traits
depends on frequency of
particular traits in population.
b/2
b/2
Therefore, not the highest
fitness but an evolutionarily
stable strategy (ESS) wins
What is a final frequency of hawks in a population (p)?
0
b
Profit of hawks: ZH = p(b – c)/2 + (1 – p)b
Profits of doves: ZD = 0 + (1 – p)b/2
in equilibrium: ZH = ZD
p(b – c)/2 + (1 – p) b = 0 + (1 – p)b/2
p = b/c
= benefit/cost
(b-c)/2
(b-c)/2
Evolutionary stable strategy: to
behave with probability b/c as a hawk
and with probability 1-b/c as a dove.
Frequency dependent selection
A selective value of many traits
depends on frequency of
particular traits in population.
b/2
b/2
Therefore, not the highest
fitness but an evolutionarily
stable strategy (ESS) wins
What is a final frequency of yellow chromosomes in a
population (p)?
0
(b-c)/2
b
(b-c)/2
Profit of red chromosomes : ZR = p(b – c)/2 + (1 – p)b
Profits of yellow chromosomes: ZY = 0 + (1 – p)b/2
in equilibrium: ZY = ZR
p(b – c)/2 + (1 – p) b = 0 + (1 – p)b/2
p = b/c
= benefit/cost
Evolutionary stable strategy: to behave with
probability b/c as a yellow chromosome and
with probability 1-b/c as a red chromosome.
Effect of selection on
morphological trait (body size)
body size
end of selection
time (generations)
Mayr E. 1964: Animal species and
evolution. Cambridge Press,
Cambridge.
Context-dependency of an impact of
a trait on biological fitness
Then, tell me, what am I to do with my colts hanging pretty damn
low, with my pretty damn short arms!
Gene-phenotype trait relationships
pleiotropy
epistasis
Gene 1
Trait 1
Gene 1
Trait 1
Gene 2
Trait 2
Gene 2
Trait 2
Gene 3
Trait 3
Gene 3
Trait 3
Gene 4
Trait 4
Gene 4
Trait 4
Gene-phenotype trait relationships
pleiotropy
epistasis
Gene 1
Trait 1
Gene 1
Trait 1
Gene 2
Trait 2
Gene 2
Trait 2
Gene 3
Trait 3
Gene 3
Trait 3
Gene 4
Trait 4
Gene 4
Trait 4
Gene-phenotype trait relationships
pleiotropy
epistasis
Gene 1
Trait 1
Gene 1
Trait 1
Gene 2
Trait 2
Gene 2
Trait 2
Gene 3
Trait 3
Gene 3
Trait 3
Gene 4
Trait 4
Gene 4
Trait 4
Problem with heritabilaty of fitness
Then, tell me, how can help me that an allele is transmitted from
generation to generation unchanged when it has in each subject
different impact on phenotype and different impact on biological
fitness!
What is wrong with the selfish
gene theory?
• It disregards context-dependency of an impact of
biological trait on biological fitness
• It disregards context-dependency of phenotypic
expression of gene (epistasis)
• It disregards a phenomenon of frequency dependent
selection
Selfish gene theory is better than previous evolutionary
theories (it explains broader spectrum of biological
phenomena), however, it fails in its major purpose, i.e. in
explaining origin of adaptations in sexual organisms.
Neither Darwin nor Dawkins,
mama mia, what to do next?
creationist
Origin of new species - Speciation
sympatric
dichopatric
peripatric
Role of peripatric speciation in evolution
colonization
homogenization by genetic
drift
population
growth
freezing by accumalation of
genetic variability
adaptation by natural
selection
Theory of frozen plasticity
•
•
•
•
•
•
•
•
•
An average number of offspring/parent > 1
Populations are stable in a long term
Therefore, only a fraction of the young survive and reproduce
Organisms in populations differ
Probability of survival and reproduction (fitness) depends on
properties of an individual
Offspring inherit properties and fitness of their parents
Therefore, useful (adaptive) traits (structures and behavioral
patterns accumulate in populations
All these holds only in asexual species or in large genetically
uniform (plastic) population, ie. just after birth of species by
peripatric speciation
For most of time (98-99% of life of a species), the sexual species
are evolutionary frozen.
What genetic experiments say
body size
end of selection
time (generations)
Mayr E. 1964: Animal species and
evolution. Cambridge Press,
Cambridge.
What a paleontology says
time (5 milions years)
Eldredge,N. 1971: Allopatric
model and phylogeny in paleozoic
invertebrates. Evolution, 26, 156167.
What a biogeography says
Ricklefs,R.E. Cladogenesis and morphological diversification in passerine birds. Nature 430, 338-341, 2004.
What molecular phylogeny says
Mark Pagel,* Chris Venditti, Andrew
Meade: Large Punctuational
Contribution of Speciation to
Evolutionary Divergence at the
Molecular Level Science 314, 119-
Differences between classical and frozen
plasticity theory
clasical theory
anagenesis and cladogenesis ** are independent
genetic polymorphism
accelerates evolution
species respond to selection **
plastically (as plasticine)
species are adapted to
its current environment
abundance of species *
is independent on its age
species on islands are derived ** as much on continents
asexual species*
less adapted to their environment
cross-polinating species *
as stable as self-pollinating
divergence of species
does not correlate with taxon richness
invasive species
express average heritabilaty
domesticated species
express average heritabilaty
domesticated species
express average age
successful selection
has no influence on fitness
local and global abundance **
correlate for any species
group selection
is rare
rate of anagenesis with a clade * is (in average) constant
two species in the same niche
usually cannot coexist
slow long-term trends
are hardly possible
Flegr J.: Frozen evolution. Charles University, Prague 2008.
Amazon, or http://natur.cuni.cz/flegr/frozen/index.php
frozen plasticity theory
are coupled
decelerates evolution
elastically (as ruber)
its original environment
decreases with age of species
more than species on continents
more adapted to their environment
more stable than self-pollinating
correlates with taxon richness
express higher heritabilaty
express higher heritabilaty
are evolutionary younger
decreases fitness
does not correlate for old species
is frequent
always decreases
frequently could coexist
are quite possible
Conclusions
• Neither darwinism, nor neodarwinism can explain adaptive
evolution in sexual species
• Selfish gene model could operate only in systems without contextdependent gene expression and context dependent fitness
• Theory of evolutionarily stable strategies shows that sexual species
respond to selection elastically
• After peripatric speciation, a new species turns plastic
• After restoration of genetic polymorphism, the specie freezes and
rest of its existence just passively waits for its extinction
• Frozen plasticity theory could explain broader spectrum of
biological phenomena than classical evolutionary theory