Dollo’s Law Tuesday, May 29, 2007Posted by henry000 in anti-creationism, biology, Dawkins, Dollo's Law, evolution, genetics, Richard Dawkins, science, sea snails, Stephen Gould.
Dollo’s law is one interesting hypothesis in the theory of evolution. In its original form, it states that evolution is irreversible – that is, once an organism has lost or discarded a structural feature during the course of evolution, then it cannot regain that feature.
This law was proposed firstly in 1890 by a biologist named Louis Dollo. In its modern form, this law should be understood as one that hypothesises that evolution is highly unlikely reversible – that is, it is highly improbable that an organism would regain a lost feature.
This law kind of makes sense under the first impression – life evolves from simple to complex, and it seems that there is no known case of organisms regrowing a previously discarded feature. However, on a second thought, one must pause and examine the issue further. None of the evolution mechanisms – natural selection, sexual selection, genetic drift, genetic mutation, genetic recombination, symbiogenesis and so on – dictates that organisms cannot completely regain lost features. In fact, scientists have argued that at least theoretically there is no block to evolutionary reversion.
Richard Dawkins has suggested in his popular book The Blind Watchmaker that this law reflects the statistical improbability of going down the same evolution trajectory twice; where as the late Stephen Gould suggested that once a feature is discarded it is then being closed/blocked by other evolutionary pathways.
As it turns out, the explanation lies within genetics. When a gene is not needed in the blueprint for building a feature of an organism, it is called a silent gene. A silent gene is not required to be deleted from the genome. Either it continues to contribute to other features of the body, or it remains dormant. A feature may be regained because the silent gene is reactivated through back-mutation, gene recombination or gene conversion.
One such example is that of sea snails. Studies on the DNA of sea snails seem to indicate that several species have regained the ability to produce mobile larvae – a feature which has been lost in many species.
2. Dollo’s Law and the Death and Resurrection of Gene (downloadable PDF can be found on the page)