Heredity,Gene And Evolution

By the 1940 s, the marriage between Darwinian theory of evolution and Mendelian genetics was integrated into a general consensus known as the Modern Synthesis(MS), which has been regarded as a paradigm for evolutionary theory over the last sixty years. This synthesis, with its gene-centric view about heredity, regarded inheritance as the transmission of genes and evolution as the results of natural selection affecting on random genetic mutation. Not long after, the discovery of DNA structure provided a material foundation for the gene-centric view and this has resulted in treating genes as simply DNA molecules. However, recent studies on epigenetics has led serious doubts about the genecentric view, which causes a wide discussion on the question whether the MS needs a rethink. Epigenetic inheritance refers to the phenomena of the transmission of epigenetic modifications(e.g., DNA methylation) via cell division across generations. The existence of epigenetic inheritance reveals that DNA molecules may no longer be the sole heritable materials. Moreover, epigenetic inheritance combined with phenotypic plasticity provides a mechanism for biological evolution without involving the changes of genes. Because of the above two reasons, epigenetic inheritance is thought to overthrow the theoretical foundation of MS.This thesis suggests that the ambiguity surrounding the conception of the gene represents a background semantic issue in the debate. The genes in Mendelian genetics is defined soley by their effects on the phenotype and thus represent hypothetical or theoretical entities which are not physically restricted. However, since the structure of DNA was established in 1953, most biologists commonly refer to genes as DNA pieces in their verbal accounts and this has resulted in the assumption that genes must be made up of DNA. But this step was taken too hastily. If there is physical material, other than DNA pieces, that can affect the phenotype and be transmitted across generations, then there would be nothing to prevent this material from being included in the concept of gene in MS. When facing the challenges from epigenetics, the most important starting point is a clarification of the basic concepts of MS, and a thorough analysis of its possible theoretical space.The major goal of this thesis is to define senses of “gene”, “environment” and “phenotype” in a way that makes consistent with the gene-centric MS, especially to define a notion of “evolutionary gene”. When being materialized, the “evolutionary gene” does not need to be restricted to DNA molecules, but can encompass other heritable units such as heritable epigenetic modifications. The conceptions of “environment” and “phenotype” from an evolutionary gene-centric view are also different from the standard understanding among current biologists. Based on this set of conceptions, MS can respond to the challenges from epigenetic inheritance successfully. If the gene is understood as the “evolutionary gene”, epigenetic inheritance can still be seen as “genetic inheritance”. As for the second objection that the combined mechanism of epigenetic inheritance and phenotypic plasticity provides an alternative way for biological evolution which is excluded from the gene-centric MS, by demonstrating the roles that heritable epigenetic modifications play in different circumstances, it will turn out that this objection is not upheld, either. It follows the conclusion that the gene-centric MS will not require a radical conceptual change in order to incorporate the mechanisms of epigenetic inheritance.