Later On

A blog written for those whose interests more or less match mine.

Unified theory of evolution

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Michael Skinner,professor of biological science at Washington State University and the principal investigator at the Skinner laboratory with research interests that include environmental epigenetics and disease etiology, writes in Aeon:

The unifying theme for much of modern biology is based on Charles Darwin’s theory of evolution, the process of natural selection by which nature selects the fittest, best-adapted organisms to reproduce, multiply and survive. The process is also called adaptation, and traits most likely to help an individual survive are considered adaptive. As organisms change and new variants thrive, species emerge and evolve. In the 1850s, when Darwin described this engine of natural selection, the underlying molecular mechanisms were unknown. But over the past century, advances in genetics and molecular biology have outlined a modern, neo-Darwinian theory of how evolution works: DNA sequences randomly mutate, and organisms with the specific sequences best adapted to the environment multiply and prevail. Those are the species that dominate a niche, until the environment changes and the engine of evolution fires up again.

But this explanation for evolution turns out to be incomplete, suggesting that other molecular mechanisms also play a role in how species evolve. One problem with Darwin’s theory is that, while species do evolve more adaptive traits (called phenotypes by biologists), the rate of random DNA sequence mutation turns out to be too slow to explain many of the changes observed. Scientists, well-aware of the issue, have proposed a variety of genetic mechanisms to compensate: genetic drift, in which small groups of individuals undergo dramatic genetic change; or epistasis, in which one set of genes suppress another, to name just two.

Yet even with such mechanisms in play, genetic mutation rates for complex organisms such as humans are dramatically lower than the frequency of change for a host of traits, from adjustments in metabolism to resistance to disease. The rapid emergence of trait variety is difficult to explain just through classic genetics and neo-Darwinian theory. To quote the prominent evolutionary biologist Jonathan B L Bard, who was paraphrasing T S Eliot: ‘Between the phenotype and genotype falls the shadow.’

And the problems with Darwin’s theory extend out of evolutionary science into other areas of biology and biomedicine. For instance, if genetic inheritance determines our traits, then why do identical twins with the same genes generally have different types of diseases? And why do just a low percentage (often less than 1 per cent) of those with many specific diseases share a common genetic mutation? If the rate of mutation is random and steady, then why have many diseases increased more than 10-fold in frequency in only a couple decades? How is it that hundreds of environmental contaminants can alter disease onset, but not DNA sequences? In evolution and biomedicine, the rates of phenotypic trait divergence is far more rapid than the rate of genetic variation and mutation – but why?

Part of the explanation can be found in some concepts that Jean-Baptiste Lamarck proposed 50 years before Darwin published his work. Lamarck’s theory, long relegated to the dustbin of science, held, among other things, ‘that the environment can directly alter traits, which are then inherited by generations to come’. Lamarck, a professor of invertebrate zoology at the National Museum of Natural History in Paris, studied many organisms including insects and worms in the late 18th and early 19th centuries. He introduced the words ‘biology’ and ‘invertebrate’ into the scientific lexicon, and wrote books on biology, invertebrates and evolution. Despite this significant academic career, Lamarck antagonised many of his contemporaries and 200 years of scientists with his blasphemous evolutionary ideas.

At the start, Lamarck might have been pilloried as a religious heretic, but in modern times, it is the orthodoxy of science – and especially Darwin’s untouchable theory of evolution – that has caused his name to be treated as a joke. Yet by the end of his career, Darwin himself had come around; even without the benefit of molecular biology, he could see that random changes were not fast enough to support his theory in full.

The question is this: if natural selection isn’t acting on genetic mutations alone, then what molecular forces create the full suite of variation in traits required for natural selection to finish the job? One clue came almost a century after Darwin proposed his theory, in 1953, just as James Watson and Francis Crick were unravelling the mysteries of DNA and the double helix. In that year, the developmental biologist Conrad Waddington of the University of Edinburgh reported that fruit flies exposed to outside chemical stimulus or changes in temperature during embryonic development could be pushed to develop varying wing structures. The changes the scientists induced in that single generation would, thereafter, be inherited by progeny down the lineage. Waddington coined a modern term – ‘epigenetics’ – to describe this phenomenon of rapid change. Notably, before Watson and Crick had even revealed their DNA structure, Waddington recognised the potential impact his discovery could have on the theory of evolution: the single-generation change in the fruit-fly wings were supportive of the original ideas of the heretic Lamarck. It appeared that the environment could directly impact traits.

Although Waddington described the general role of epigenetics, he was no more aware of the molecular elements or mechanisms involved than Lamarck or Darwin. But the more molecular biology decodes the workings of life, the more Waddington’s concepts – and Lamarck’s – make sense. Indeed, although the vast majority of environmental factors cannot directly alter the molecular sequence of DNA, they do regulate a host of epigenetic mechanisms that regulate how DNA functions – turning the expression of genes up or down, or dictating how proteins, the products of our genes, are expressed in cells.

Today, that is the precise definition of epigenetics: the molecular factors that regulate how DNA functions and what genes are turned on or off, independent of the DNA sequence itself. Epigenetics involves . . .

Continue reading. There’s much more. Later in the article:

Environmentally induced epigenetic transgenerational inheritance has now been observed in plants, insects, fish, birds, rodents, pigs and humans. It is, therefore, a highly conserved phenomenon. The epigenetic transgenerational inheritance of phenotypic trait variation and disease has been shown to occur across a span of at least 10 generations in most organisms, with the most extensive studies done in plants for hundreds of generations. One example in plants, a heat-induced flowering trait first observed by Carl Linnaeus in the 18th century, was later found to be due to a DNA methylation modification that occurred in the initial plant, and has been maintained for 100 generations. In worms, . . .

For I the LORD your God am a jealous God, visiting the iniquity of the fathers upon the sons to the third and fourth generation of those who hate Me . . .”

Written by Leisureguy

11 April 2020 at 10:29 am

Posted in Evolution, Science

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