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.

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 Evol. 61: ): 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,

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, 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, 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, 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, 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 Jamaica Twig Puerto Rico Generalist

Ancestral Reconstruction of Habitat Specialist Evolution on Two Islands Twig Trunk/ Ground Jamaica Twig Puerto Rico Crown/ Giant Trunk/ Ground Crown Generalist

Ancestral Reconstruction of Habitat Specialist Evolution on Two Islands Twig Trunk/ Ground Jamaica Twig Puerto Rico Crown/ Giant Trunk/ Ground Crown/ Giant Trunk/ Crown Trunk/ Crown Generalist Trunk/Grnd

Ancestral Reconstruction of Habitat Specialist Evolution on Two Islands Twig Trunk/ Ground Jamaica Twig Puerto Rico Crown/ Giant Trunk/ Ground Crown/ Giant Trunk/ Crown Trunk/ Crown Grass/ Bush Crown Generalist Trunk/Grnd

Ancestral Reconstruction Implies Great Evolutionary Flexibility Twig Puerto Rico Crown/ Giant Trunk/ Ground Trunk/ Crown Grass/ Bush Generalist 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.