Daarom evolutie #13

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The modern evolutionary, synthesis (often referred to simply as the new synthesis, the modern synthesis, the evolutionary synthesis, neo-Darwinian synthesis or neo-Darwinism), generally denotes the integration of Charles Darwin's theory of the evolution of species by natural selection, Gregor Mendel's theory of genetics as the basis , for biological inheritance, random genetic mutation as the source of variation, and mathematical population genetics. Major figures in the development of the modern synthesis include Thomas Hunt Morgan, R. A. Fisher, Theodosius Dobzhansky, J.B.S. Haldane, Sewall Wright, Julian Huxley, Ernst Mayr, Bernhard Rensch, George Gaylord Simpson, and G. Ledyard Stebbins.

Essentially, the modern synthesis introduced the connection between two important discoveries: the units of evolution (genes) and the mechanism of evolution (selection). It also represents a unification of several branches of biology that previously had little in common, particularly genetics, cytology, systematics, botany, and paleontology.

According to the modern synthesis as established in the 1930s and 1940s, genetic variation in populations arises by chance through mutation (this is now known to be sometimes caused by mistakes in DNA replication) and recombination (crossing over of homologous chromosomes during meiosis). Evolution consists primarily of changes in the frequencies of alleles between one generation and another as a result of genetic drift, gene flow, and natural selection. Speciation occurs gradually when populations are reproductively isolated, for example by geographic barriers.

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In Biology, epigenetics is the study of all heritable and potentially reversible changes in genome function that do not alter the nucleotide sequence within the DNA. When a cell undergoes an epigenetic change, it is the phenotype of the cell that is affected. Epigenetic events during embryo development lead to the differentiation of fetal cells. The combined processes of fetal development and cell differentiation are called epigenesis. The term is also sometimes used as a synonym for the closely related topic of chromatin remodeling.

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The philosophical implications of epigenetics have been discussed by scientists such as Eva Jablonka, Marion Lamb and Massimo Pigliucci, who cite epigenetic inheritance as one of a number of factors suggesting that the neo-darwinian synthesis of the early twentieth century is incomplete. Jablonka and Lamb suggest that the standard way of thinking about evolution, in terms of changes in the frequency of one or more isolated genes needs to be questioned, and that, contrary to long-held majority opinion, not all genetic variation is entirely random or blind - some may be regulated and partially directed and that the concept of heridity that is currently being used in evolutionary thinking is far too narrow. Pigliucci suggests, in Nature that If one accepts this bold, expanded version of heredity and evolution, it turns out that evolution can proceed very rapidly and phenotypic modification can precede genetic changes, and that changes at the genetic level will often simply stabilize adaptive modifications that are initiated through phenotypic plasticity, epigenetic control mechanisms, or behavioural and symbolic means; that this framework would greatly help to solve old problems in evolutionary biology, such as the origin of novel structures, and that the ultra-reductionist, gene-centred approach has (at least partially) failed.

The extent to which evolution operates at several different levels is the nub of Patrick Bateson's "friendly disagreement" with Richard Dawkins. In a critique of determinism Robert Winston suggests that epigenetic inheritance is an important factor in the inadequacy of the "selfish gene" and "DNA" metaphors.

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Op woensdag 23 mei 2007 14:06 schreef Invictus_ het volgende:
--> Evolutie. Deel XIV <--

ff daar opnieuw posten denk ik?