Biological devolution

Devolution, de-evolution, or backward evolution is the notion that species can revert to supposedly more primitive forms over time.

The concept is related to the idea that evolution has a purpose (teleology) and is progressive (orthogenesis), for example that feet might be better than hooves or lungs than gills. However, evolutionary biology makes no such assumptions, and natural selection shapes adaptations with no foreknowledge of any kind. It is possible for small changes such as in the frequency of a single gene to be reversed by chance or selection, but this is no different from the normal course of evolution.

In the 19th century, when belief in orthogenesis was widespread, the idea of devolution was advocated by zoologists such as Ray Lankester and Anton Dohrn, and the palaeontologists Alpheus Hyatt and Carl H. Eigenmann. The idea was used in Kurt Vonnegut's 1999 novel Galápagos, where he portrays a society that has evolved backwards to have small brains.

Dollo's law of irreversibility, first stated in 1893 by the palaeontologist Louis Dollo, denies the possibility of devolution. The evolutionary biologist Richard Dawkins explains Dollo's law as being simply a statement about the improbability of evolution's following precisely the same path twice.

The idea of devolution is based at least partly on the presumption that evolution requires some sort of 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 and orthogenesis 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 neo-Darwinism. 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.

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