The idea of devolution is based on the presumption of orthogenesis, the view that evolution has a purposeful direction towards increasing complexity. Modern evolutionary theory, beginning with Darwin at least, poses no such presumption. and the concept of evolutionary change is independent of either any increase in complexity of organisms sharing a gene pool, or any decrease, such as in vestigiality or in loss of genes. Earlier views that species are subject to "cultural decay", "drives to perfection", or "devolution" are practically meaningless in terms of current (neo-)Darwinian theory. Early scientific theories of transmutation of species such as Lamarckism perceived species diversity as a result of a purposeful internal drive or tendency to form improved adaptations to the environment. In contrast, Darwinian evolution and its elaboration in the light of subsequent advances in biological research, have shown that adaptation through natural selection comes about when particular heritable attributes in a population happen to give a better chance of successful reproduction in the reigning environment than rival attributes do. By the same process less advantageous attributes are less "successful"; they decrease in frequency or are lost completely. Since Darwin's time it has been shown how these changes in the frequencies of attributes occur according to the mechanisms of genetics and the laws of inheritance originally investigated by Gregor Mendel. Combined with Darwin's original insights, genetic advances led to what has variously been called the modern evolutionary synthesis or the neo-Darwinism of the 20th century. In these terms evolutionary adaptation may occur most obviously through the natural selection of particular alleles. Such alleles may be long established, or they may be new mutations. Selection also might arise from more complex epigenetic or other chromosomal changes, but the fundamental requirement is that any adaptive effect must be heritable.