What are the implications for the speed of evolutionary change?
Some of the problems in evolutionary theory concern the speed of speciation. It is clear from the geological record that this has not always been smooth. One of the reasons why Cuvier disagreed with Lamarck in the early nineteenth century is that he was able to discredit Lamarck’s idea of the transformation of species by pointing to the fact that the fossil evidence, as it was known then, was also consistent with multiple periods of creation. The record was that patchy, and it was also clear that some species remained essentially unchanged for very long periods of time (stasis). In the twentieth century, with much more evidence to consider, Eldredge and Gould in 1971 proposed the theory of punctuated equilibrium to account for the fact that most fossil species show long periods of stasis, and that rapid (on a geological time scale) changes occurred more rarely and were important periods of speciation. The difference between their theory and that of Cuvier was that Eldredge and Gould were simply proposing that evolutionary change from a common ancestor does not happen at a constant speed, whereas Cuvier interpreted the evidence to show that there had been multiple creations.
Darwin also realised that evolutionary change could not always have been smooth. In the fourth edition of The Origin of Species he wrote “the periods during which species have undergone modification, though long as measured in years, have probably been short in comparison with the periods during which they retain the same form.” This is, in essence, Eldredge and Gould’s idea of punctuated equilibrium.
There has been much argument about what precisely is meant by ‘punctuated’. A gradual change over a hundred thousand or a million or even a few million years will appear rapid on a geological time scale of hundreds of millions of years. The Cambrian explosion that occurred over 500 million years ago is a good example. Within just 20 million years all the phyla in existence today had developed. The standard response to these kinds of theories has therefore been that they are entirely consistent with neo-darwinism. Changes in selection pressure due to environmental changes or geographic distribution, and the occasional catastrophic environmental change, might account for the observed variations in speed of change without supposing additional mechanisms of change.
The new evidence from work on symbiogenesis, the various forms of inheritance of acquired characteristics, genetic assimilation, natural genetic engineering, including genome change and reorganisation over and above the accumulation of chance mutations, changes the situation in more fundamental ways that require either extensions of or replacement of the modern synthesis. These mechanisms resemble punctuated equilibrium theories in proposing that evolution can occur in jumps. Since these can be very sudden indeed it is best to use a word different from ‘punctuated’ to avoid confusion with Eldredge and Gould’s theory. ‘Saltatory’ means jumping. The new mechanisms produce saltations of various kinds:
Symbiogenesis is the fusion of two species. The best established example of this is the bacterial origin of mitochondria and chloroplasts and, perhaps, other organelles. Clearly, this process is the ultimate in ‘saltation’. It depends on processes of cellular ingestion that are natural in the feeding activity of unicellular organisms and, on an evolutionary timescale, it is therefore very rapid indeed. Of course, subsequent changes can then also occur more slowly. We know, for example, that some of the ingested DNA in what became organelles eventually moved to the nucleus in eukaryotes.
See the film by Lynn Margulis on
Endosymbiosis: Homage to Lynn Margulis, a painting by Shoshanah Dubineer, occupies a hallway in the Morrill Science Center at the University of Massachusetts, Amherst, where Margulis was a professor until her death in 2011. Margulis maintained that genetic variation emerges primarily through symbiosis, not through competition.
Image courtesy of the artist, http://www.cybermuse.com
See also Evolution’s Other Narrative
Natural genetic engineering could also occur within a single generation. Reorganisations of genomes involving duplications, deletions and insertions of long sequences, would be essentially instantaneous on a geological timescale. Defenders of the modern synthesis have argued that speciation due to such changes, and symbiogenesis, should not be classified as punctuated equilibrium. That is correct in the sense that it was not what Eldridge and Gould had in mind. But so far as timescale is concerned such changes would be saltatory in the ordinary sense of the word. They would be even more sudden than the punctuations proposed by Eldredge and Gould.
Genetic assimilation can also occur rapidly. Waddington’s mid-twentieth century experiments showed that an induced acquired characteristic in fruit flies could become permanent (assimilated) within fourteen generations. This must have represented the time required for selection for an induced characteristic to bring together in a single genome all the relevant alleles for that characteristic to be passed on to subsequent generations without the inducing environmental stimulus. Waddington coined the term ‘epigenetics’ to describe his discovery. Today, epigenetics usually refers to genome and chromatin marking.
Inheritance of acquired characteristics through the persistence of epigenetic effects through successive generations can also speed up the evolutionary process. These transgenerational environmental influences should spread through a population much more rapidly since it is possible for a large fraction of the population to be subject to the same changes at the same time. There is no need to wait for a single DNA change to spread slowly through a population. The orthodox response to this mechanism is to dismiss it as transient, which it certainly is in some cases. But there are now examples of such transmission over many generations and which show the same degree of strength as standard genetic transmission. Since such effects would not need to occur very frequently, the difficulty in identifying them experimentally would be perfectly understandable. Speciation itself is a rare event.
While there can be considerable uncertainty about the relative contributions of the different mechanisms to evolutionary change, two conclusions seem clear. The first is that, with a variety of mechanisms open to the evolutionary process, the speed of evolution should be faster. The second is that it is probable that the relative contributions varied at different stages in evolution.
|The MUSIC of Life: Biology Beyond the Genome ©Denis Noble|