developmental constraints, genetic correlations & natural selection
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Developmental Constraints, Genetic Correlations & Natural Selection. Genetic Constraints Arise From Pleiotropy and Epistasis. - PowerPoint PPT PresentationTRANSCRIPT
Developmental Constraints, Genetic Correlations & Natural
Selection
Genetic Constraints Arise From Pleiotropy and Epistasis
When the above equation is not zero and contains a non-zero covariance term, evolution at the multi-trait level is often non-optimal 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.
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∂2w(xeq ,yeq )
∂x 2σ eq
2 (x) + 2∂ 2w(xeq ,yeq )
∂x∂yCoveq (x,y) +
∂ 2w(xeq ,yeq )
∂y 2σ eq
2 (y)
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
Wagner GP, Altenberg L (1996) Perspective - Complex Adaptations and the Evolution Of Evolvability. Evolution 50, 967-976.
Genes
Character Complexes
Functions
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 MethodPROBLEM: 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 MethodSolution: 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
Jamaica
Twig
Puerto Rico
Generalist Generalist
Ancestral Reconstruction of Habitat Specialist Evolution on
Two Islands
TwigTrunk/Ground
Jamaica
Twig
Puerto Rico
Crown/Giant
Trunk/GroundCrown
GeneralistGeneralist
Ancestral Reconstruction of Habitat Specialist Evolution on
Two Islands
TwigTrunk/Ground
Jamaica
Twig
Puerto Rico
Crown/Giant
Trunk/Ground
Crown/Giant
Trunk/Crown
Trunk/Crown
Crown
GeneralistGeneralist
Trunk/Grnd
Ancestral Reconstruction of Habitat Specialist Evolution on
Two Islands
TwigTrunk/Ground
Jamaica
Twig
Puerto Rico
Crown/Giant
Trunk/Ground
Crown/Giant
Trunk/Crown
Trunk/Crown
Grass/Bush
Crown
GeneralistGeneralist
Trunk/Grnd
Trunk/Grnd
Ancestral Reconstruction Implies Great Evolutionary Flexibility
Twig
Puerto Rico
Crown/Giant
Trunk/Ground
Trunk/Crown
Grass/Bush
Generalist
Trunk/Grnd
Trunk/Grnd
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
Humans achieve their large brains in part by retaining the normal primate brain growth processes, but start them earlier and retain them longer.
Chimpanzee
Human
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 Non-Adaptive 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.