Review of: New Scientists, 13th of January, 2007, edition number 2586, “The Ancestor within” by Michael Le Page et al.
This New Scientists articles explores the concept of atavism, forefather in Latin, but in biology the concept that evolution can have throwbacks to earlier species on a particular organisms evolutionary lineage. This is in defiance to the late 19th century “Dollo’s Law” which states “an organism is unable to return, even partially to a previous evolutionary stage”. Throughout the article it explores different modern species and their lineages, and phenotype expression, and the most likely genetic mechanisms that could facilitate that particular form of atavism.
The first mechanism explored details how the Axolotl (Ambystoma mexicanum) which can not undergo metamorphose, unlike other present-day mole salamander species. However fossils show that both evolved from the same ancestor that couldn’t undergo metamorphose like the axolotl. Yet even early ancestors show the ability to undergo metamorphism. The general process of evolution is based on mutations, with positive mutations in a species DNA that serve as an advantage propagates throughout the species population. In the case of the original genes, which inturn code for a particular polypeptide can be completely lost, or initially more likely mutations to the operator or to regulatory systems of expression, can prevent transcription of the gene and subsequently making the gene “silent” and an non-coding intron.
Work done by Rudolf Raff in 1994 in calculating the rate of mutations in a species concluded that a silent non-functioning gene will have a near zero probability of reappearing in a population of a eukaryotic species after 6 to 10 million years in going silent. This is due to the rate of more destructive mutations such as causing the gene to code for a different polypeptide chain, be it good or bad, is far more likely then the correction of the expression of the coding exons or other regulatory region from previous mutations. This timeframe fit perfectly with the salamander with the atavism of metamorphism reappearing and disappearing in fossils numerous times in the past 10 million years due to selective pressures favouring a particular form. Similar recently Gunter Wagne has validated this idea and time scale, is discovery of the Bachia. This South American lizard with small legs and toes, had evolved from a lizard with no leg, and this inturn evolved from one with legs and toes, in the past few million years, supporting the idea of reactivation of silent genes.
However this view of atavism is vastly complicated by stick insects population lost their wings 300 million years ago only to regain this phenotype 200 million years later, much longer than the gene could have survived silenced. Other examples include that of recent case in Japan of a dolphin AO-4 which has a pair of hind flippers similar to those to its ancestors 40 million years ago. The article stresses that as silent genes have no selective pressure from stopping them become increasingly mutated, and at a point, e.g. 10 million years in becoming silent, they are beyond the chance of mutations repairing them. Subsequently this article details two processes in the way the genes can remain.
The article points out the idea of parallel evolution, in the way two distant organisms can have the same phenotype. That slimily selective pressure forces the creation from scratch of a loss phenotype. However the process of evolution is a slow process and uncertain, and would have a nearly zero chance of a loss complex phenotype like pentadactyl limbs just reappearing in a few generations. Thus the genetic information must still be present in the organism.
The article than explores the concept, that in the development from fertilization of an organisms in the early stages the embryonic development shows characteristics of ancestral species, which serves important initial growth, such as hind limbs in snakes, or the development of tail buds in humans to form a neural tube. Subsequently the biologist Jonathan Slack of the University of Bath in the UK was quoted in the article in stating “It has vital embryonic function even though it has no adult function”. Thus the genes for these primitive phenotypes are maintained as any negative mutations would lead to the destruction of the embryo and its mutated genes, preventing its wider spread. One of many cases of atavisms detailed is the 100 plus recorded reports of humans born with rudimentary tails. The article than postulates from comments from Brehard Hermann of Max Plank Institute that the genes used in the development and cellular differentiation to form ectoderm tissue and subsequent vertebra has remained relatively constant genetically when compared to our evolutionary close primate species. Variation particular to a species such as in humans is due to early cell death (5th week of development) due to proteins and growth factors from differently differentiated tissue causes the embryonic tail to fuse to form the coccyx sooner than other primates with a tail, as phenotypes are specified by environmental conditions and genotypes. (“Biology , 8th Edition”, Campbell et. Al) However due to wrong cell regulation by protein secretion this process of vertebra growth can continue, to in extreme cases an additional vertebra in the form of a tail is present. (“Brain Facts Book” 2006, Society of Neurological Sciences) Yet this is not caused by some mutation but an error in the process of differentiation, thus the organisms offspring would have a near nil chances of also showing this atavism. This would illustrate the case of Ao4, which was unique to that individual.
The article then points out the actual role of a gene. No gene codes for an entire complex phenotype such as a leg. However specific genes are responsible for the production of particular underlying pattern or tissues, which due to differentiation caused by growth factors regulate their expression. The cumulative effect is that of a particular phenotype.
Subsequently the genes needed for the development of a particular ancient tissue or phenotype may still be in the production of other tissues, only the need of the presence of a growth factors in differentiation and development prevents this phenotype from appearing. The evidence for this claim was from experiments conducted by Edward Kollar in 1980 (Science, vol 207, p993) when embryonic chicken cells (where bird lost the ability to growth teeth 70 million years ago) where coaxed into growing rudimentary teeth tissue by grafting them onto mouse jaw tissue which would be producing the required growth factors to promote the differentiation into teeth tissue. While recently John Fallow in the University of Wisconsin provoked a mutation to an expressed growth factor in a chicken embryo to develop crocodile like teeth. (Current Biology vol 16 p 371). If true, this would illustrate that the eukaryotic genes are utilized in various tissues, and the presence of other factors like proteins/growth factors in differentiation coupled with the genotype has a crucial role in the development of a phenotype. Then mutations in growth factors gene can lead to fundamental changes in the expression of genes and subsequent phenotype, with little changes to the genes expressed.
However the article concludes that far more genetic research must be undertaken in the expression of tissues/phenotypes and the role of differentiation to better understand the mechanisms of atavism. Yet this article highlights and illustrates the ability of atavism and that not all is forgotten with genetics, and the great ability of a population of a species through selective mutations to adapt to the rapidly changing world, with previous adaptations.
(New Scientists, 13th of January, 2007, edition number 2586, “The Ancestor within” by Michael Le Page et al.)
(“Brain Facts Book” 2006, Society of Neurological Sciences)
(“Biology , 8th Edition” Pearson Press 2008, Campbell et. Al)
Matthew Heidecker, Student Number, 042009049, 2nd of April 2009
