Developmental Constraints, Genetic Correlations & Natural Selection

1. Developmental Constraints, Genetic Correlations & Natural Selection

2. Genetic Constraints Arise From Pleiotropy and Epistasis

3. 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.

4. Evolutionary trajectory: each dot = mean in successive generations 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):

5. Do Constraints Overwhelm or Inhibit Adaptive Evolution via Natural Selection?

6. Wagner GP (1988) The influence of variation and of developmental constraints on the rate of multivariate phenotypic evolution. Journal of Evolutionary Biology1, 45-66.

7. Wagner GP (1988) The influence of variation and of developmental constraints on the rate of multivariate phenotypic evolution. Journal of Evolutionary Biology1, 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!

8. Genetic Correlations Among Traits Are Also a Phenotype That Can Be Influenced By Genetic Variation and Therefore Can Evolve

9. Hansen TF (2006) The evolution of genetic architecture. Annual Review of Ecology Evolution and Systematics37, 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.

10. 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.

11. Genetic Redundancy & Innovation: Gene Duplication Followed By Divergence Yields Families of Functionally Related Genes

12. Genetic Redundancy & Innovation: Developmental Modularity Functions Character Complexes Genes Wagner GP, Altenberg L (1996) Perspective - Complex Adaptations and the Evolution Of Evolvability. Evolution50, 967-976.

13. Genetic Redundancy & Innovation: Developmental Modularity Evolution of diverse functions from modular traits.

14. 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 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.

15. Example: Anolis lizard species in the Caribbean Numbers of Species on Various Islands

16. Anolis lizard species on different islands show similar morphological adaptations associated with similar habitats.

17. Grass/Bush Habitat Specialist

18. Trunk/Ground Habitat Specialist

19. Trunk/Crown Habitat Specialist

20. Twig Habitat Specialist

21. 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

22. Hypothesis:The Transitions in Development Associated With the Various Habitat Specialists Are So Difficult To Evolve, That They Probably Only Evolved Once.

24. 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.

25. 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.

27. Ancestral Reconstruction of Habitat Specialist Evolution on Two Islands Twig Generalist Generalist Twig Jamaica Puerto Rico

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

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

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

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

32. 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

33. Genes Can Influence the Timing and Duration of Processes, Which Can Lead to Much Phenotypic Innovation In A Developmentally Constrained System.

34. 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

35. Human Chimpanzee 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.

36. The Human Brain Humans retain fetal and newborn growth processes over a longer portion of their life.

38. 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.”

39. 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).

40. 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.

41. 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)

42. 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.