Frustration: I’m determined to learn about what seems to be renewed interest in whether or not the “principle” called Dollo’s law, is as absolute as stated, or if it is about to fall to new research. The very suggestion of “atavisms” has drawn scorn as pseudoscience, due to widespread wild speculation, including in Creationism, and other anti-evolution conspiracies. It’s a question of “cause” – for a perceived atavism.
Atavisms: Medical, Genetic, and Evolutionary Implications
Abstract (PDF available at ResearchGate)
Traits expected to be lost in the evolutionary history of a species occasionally reappear apparently out of the blue. Such traits as extra nipples or tails in humans, hind limbs in whales, teeth in birds, or wings in wingless stick insects remind us that certain genetic information is not completely lost, but can be reactivated. Atavisms seem to violate one of the central evolutionary principles, known as Dollo’s law, that “an organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors.” Although it is still not clear what triggers and controls the reactivation of dormant traits, atavisms are a challenge to evolutionary biologists and geneticists. This article presents some of the more striking examples of atavisms, discusses some of the currently controversial issues like human quadrupedalism, and reviews the progress made in explaining some of the mechanisms that can lead to atavistic features.
Annals of Morphology. Atavisms: Phylogenetic Lazarus?
The concept “atavism,” reversion, throwback, Rückschlag remains an epistemological challenge in biology; unwise or implausible over-interpretation of a given structure as such has led some to almost total skepticism as to its existence. Originating in botany in the 18th century it became applied to zoology (and humans) with increasing frequency over the last two centuries such that the very concept became widely discredited. Presently, atavisms have acquired a new life and reconsideration given certain reasonable criteria, including: Homology of structure of the postulated atavism to that of ancestral fossils or collateral species with plausible soft tissue reconstructions taking into account relationships of parts, obvious sites of origin and insertion of muscles, vascular channels, etc. Most parsimonious, plausible phylogenetic assumptions. Evident rudimentary or vestigial anatomical state in prior generations or in morphogenesis of a given organism. Developmental instability in prior generations, that is, some closely related species facultatively with or without the trait. Genetic identity or phylogenomic similarity inferred in ancestors and corroborated in more or less closely related species. Fluctuating asymmetry may be the basis for the striking evolutionary diversification and common atavisms in limbs; however, strong selection and developmental constraints would make atavisms in, for example, cardiac or CNS development less likely. Thus, purported atavisms must be examined critically in light of the above criteria. © 2013 Wiley Periodicals
For a simple review of what’s involved: Evidence of Evolution: Homology
Sometimes a discussion of evolution needs to consider how evolution does NOT happen.
A derived character is one that is relatively modified from its original, ancestral form. Example: the modified shapes of various vertebrate tails. An extreme example: The loss of the external tail in great apes (including humans). The baboon shows the primitive condition, whereas the gorilla and Very Handsome Man show the derived condition of the post-anal tail. We great apes still have a tail; it’s just very reduced and tucked where it doesn’t show. Though there is the occasional atavistic reminder of our ancestry.