Transcript Constraints&NatSel

Developmental Constraints,
Genetic Correlations & Natural
Selection
Genetic Constraints Arise
From Pleiotropy and Epistasis
 2w(x eq , y eq ) 2
 2w(x eq , y eq )
 2w(x eq , y eq ) 2
 eq (x)  2
Coveq (x, y) 
 eq (y)
2
2
x
xy
y
When the above equation is not zero and
contains a non-zero covariance term,
evolution at the multi-trait level is often nonoptimal in the sense that not every trait, or
even no traits, are at their optimal value.
In this sense, many regard constraints and
genetic correlations as interfering or limiting
adaptive evolution via natural selection.
Even when correlated traits are “optimized”, the genetic
correlations due to pleiotropy often can cause a “non-optimal”
trajectory (Guillaume, F., and M. C. Whitlock. 2007. Evol.
61:2398-2409):
Evolutionary trajectory: each dot = mean in successive generations
Do Constraints Overwhelm or
Inhibit Adaptive Evolution via
Natural Selection?
Wagner GP (1988) The influence of variation and of developmental
constraints on the rate of multivariate phenotypic evolution. Journal
of Evolutionary Biology 1, 45-66.
Wagner GP (1988) The influence of variation and of developmental
constraints on the rate of multivariate phenotypic evolution. Journal
of Evolutionary Biology 1, 45-66.
If high fitness depends
upon 3 or more
integrated traits, and all
traits are genetically
independent, the
integrated state is
unlikely to evolve.
There is a greater
chance of multi-trait
evolution even under
random patterns of
pleiotropy and epistasis!
Genetic Correlations Among Traits Are Also a Phenotype That Can
Be Influenced By Genetic Variation and Therefore Can Evolve
Hansen TF (2006) The evolution of genetic architecture. Annual
Review of Ecology Evolution and Systematics 37, 123-157.
Simulations showed that
epistasis can constrain
adaptive responses (green
lines), but in other cases
can greatly facilitate them
(red lines).
Recall, like in coalescence
theory, what we see today
are the successful
lineages, so Hansen
argues that constraints
due to epistasis have
facilitated adaptive
change.
Ciliberti S, Martin OC, Wagner A (2007) Innovation and robustness
in complex regulatory gene networks. PNAS 104, 13591-13596.
Interaction networks with redundancy create robustness, but can sometimes
lead to high innovation and sometimes lead to evolutionary stasis.
Genetic Redundancy & Innovation: Gene Duplication Followed
By Divergence Yields Families of Functionally Related Genes
Genetic Redundancy & Innovation: Developmental Modularity
Functions
Character
Complexes
Genes
Wagner GP, Altenberg L (1996) Perspective - Complex Adaptations and
the Evolution Of Evolvability. Evolution 50, 967-976.
Genetic Redundancy & Innovation: Developmental Modularity
Evolution of diverse functions from modular traits.
Genetic Redundancy and Developmental
Modularity Give Life the Potential for Robustness
and Innovation.
Can Sometimes Lead to Stasis and
Maladaptations, but In Some Cases Constraints
are Facilitators of Adaptive Evolution
Pigliucci M (2008) Is evolvability evolvable? Nat Rev Genet 9, 75-82.
Within
species
Within species, the right balance
of interaction, redundancy, and
developmental constraints
interacts with selection to open
new areas of phenotypic space
for further evolution
Example: Anolis lizard
species in the Caribbean
Numbers of Species on Various Islands
Anolis lizard species on
different islands show
similar morphological
adaptations associated
with similar habitats.
Grass/Bush
Habitat
Specialist
Trunk/Ground Habitat
Specialist
Trunk/Crown Habitat Specialist
Twig Habitat Specialist
Species Adapted to the Same Habitat On
Different Islands Look Similar Whereas
Species Adapted to Different Habitats On
the Same Island Look Very Different
Puerto Rico
Jamaica
Hypothesis:
The Transitions in Development
Associated With the Various
Habitat Specialists Are So Difficult
To Evolve, That They Probably
Only Evolved Once.
The Comparative Method
PROBLEM: When
Evolutionary Trees Were
Constructed From
Morphological Traits, The
Tree Could Only Be
Constructed By Making
Assumptions About the
Evolution of Morphological
Traits.
The Comparative Method
Solution: George Gaylord Simpson, 1945:
The most direct, but unfortunately not the most
useful, approach to the phylogeny of recent
animals is through their genetics. The stream of
heredity makes phylogeny; in a sense, it is
phylogeny. Complete genetic analysis would
provide the most priceless data for the mapping of
this stream, . . . and the advantage of genetics lies .
. . in the fact that the genes . . . are the immediate
physical continuants of phylogeny, while
morphology is less direct, a result of those
hereditary factors as modified by other influences.
Ancestral Reconstruction of
Habitat Specialist Evolution on
Two Islands
Twig
Generalist
Jamaica
Twig
Generalist
Puerto Rico
Ancestral Reconstruction of
Habitat Specialist Evolution on
Two Islands
Twig
Trunk/
Ground
Crown
Generalist
Jamaica
Trunk/
Ground
Crown/
Twig Giant
Generalist
Puerto Rico
Ancestral Reconstruction of
Habitat Specialist Evolution on
Two Islands
Twig
Trunk/ Crown/
Ground Giant
Trunk/
Crown
Crown
Generalist
Jamaica
Trunk/
Ground
Crown/ Trunk/
Twig Giant Crown
Trunk/Grnd
Generalist
Puerto Rico
Ancestral Reconstruction of
Habitat Specialist Evolution on
Two Islands
Twig
Trunk/ Crown/
Ground Giant
Trunk/
Crown
Crown
Generalist
Jamaica
Crown/ Trunk/
Twig Giant Crown
Grass/
Bush
Trunk/
Ground
Trunk/Grnd
Trunk/Grnd
Generalist
Puerto Rico
Ancestral Reconstruction Implies
Great Evolutionary Flexibility
Crown/ Trunk/
Twig Giant Crown
Grass/
Bush
Trunk/
Ground
Trunk/Grnd
Trunk/Grnd
Generalist
Puerto Rico
Reject Hypothesis of
Developmental Rigidity: The
Developmental System
Leading To Basic Body Shape
and Limb Morphology in Anolis
lizards Shows Great
Evolutionary Flexibility And
Evolves Repeatedly In
Response To Habitat
Availability Upon Each Island
Genes Can Influence the
Timing and Duration of
Processes, Which Can Lead to
Much Phenotypic Innovation In
A Developmentally
Constrained System.
The Human Brain
Humans achieve their large brains in
part by retaining the normal primate
brain growth processes, but start
them earlier and retain them longer.
The Human Brain
Human
Chimpanzee
Humans achieve their large brains in part by retaining the normal primate brain
growth processes, but start them earlier and retain them longer.
The Human Brain
Humans retain fetal
and newborn growth
processes over a longer
portion of their life.
Pleiotropy: Not All Traits That Evolve
Under Natural Selection Are Adaptive.
The Human Jaw Is
Too Small For Its
Teeth, Leading to
Complications in
Arrangement and
“Wisdom Teeth.”
Selection Operates Through the
Genetic System Upon
Developmental Processes (as judged
from the gamete’s average
perspective) and Not Upon Isolated
Traits.
Much of Evolution Is Therefore NonAdaptive or Even Mal-Adaptive Even
When Driven by Natural Selection
(Recall Sickle Cell Anemia).
When traits are neutral but
developmentally correlated to a
selected trait, we expect its evolution
to obey those correlations.
When two or more developmentally
correlated traits are separately
selected, we expect deviations from
the expected developmental
correlations in their joint evolution.
Example, human face and jaws
A Quantitative Genetic Overlay Upon Facial Morphology Using Modern
Humans, Chimps and Gorillas as Models Revealed A Relaxation of
Selection On The Face and Jaws in the Human Lineage, Indicating A
Significant Increase in Reliance Upon Culture & Tools (Ackermann &
Cheverud, PNAS 101: 17946, 2004)
Constraints Insure That Much
Phenotypic Evolution is
Neutral or Even Maladaptive;
but Population Genetic Theory
Indicates that Epistasis and
Pleiotropy Can Sometimes Be
Strong Facilitors of Adaptive
Innovation.