Natural Selection and Phenotypic Variation

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Long standing arguments against the theory of natural selection stem from the occurrence of incipient structures and complex traits in organisms despite the seemingly stochastic nature of mutations. Many complex adaptations observed in nature today are thought to have arisen from less complex ones with simpler functions, therefore these characters are thought to have been “pre-adapted.” In order to go from a simple to a complex structures there must have been a transitional phase, where the two structures function simultaneously or where the new function is assumed without interfering with the old function. These structures are termed incipient or incomplete, and given what we know to be true of natural selection and the theory of evolution it becomes hard to reconcile the idea that natural selection continued to favor these structures despite the lack of selective value. Incipient structures are thought to be neither sufficiently large enough not elaborate enough to perform an adaptive function and thus it also becomes difficult to understand how larger complex characters arise. A discussion of morphological and developmental genetics explains that these structures have been performing useful functions since their simple origins, therefore being selectively favored while at the same time evolving to become large enough to accumulate new more complex functions. Modification of pre-existing genes and regulatory circuits in early development has been extensively studied in metazoans, Hox genes and the development of complex structures such as eyes, limbs and appendages. Phenotypic variation is therefore generated via the modification of existing genes, regulatory processes and developmental processes and this variation is acted o... ... middle of paper ... ...7. Gould, S.J., & Vrba, E.S. 1982. Exaptation- a missing term in the science of form. Paleobiology, 8(1): 4-15. Kirschner, M., & Gerhart, J. 1998. Evolvability. National Academy of Science, 95 (1): 8429-8427. Liubicich, D.M., et al. 2009. Knockdown of Parhyale Ultrabithorax recapitulates evolutionary changes in crustacean appendage morphology. PNAS 106 (33): 13892-13896 Oster G., & Alberch, P. 1982. Evolution and Bifurcation of Developmental programs. Society for the study of evolution, 36 (3): 444-459. Shubin, N.H., & Marshall, C.R. 2000. Fossils, genes and the origin of novelty. Paleobiology, 26(4): 324-340. Shubin, N.H, Tabin, C., & Caroll, S. 2009. Deep homology and the origins of evolutionary novelty. Nature, 457: 818-823. West-Eberhard, M.J. 1998. Evolution in the light of developmental and cell biology, and vice versa. PNAS, 95: 8417-8419.

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