Werner Arber (2009) - Molecular Darwinism

Werner Arber (2009)

Molecular Darwinism

Werner Arber (2009)

Molecular Darwinism

Abstract

Summary

Prof. Arber starts his lecture with some remarks on the history of evolution and genetics beginning with Darwin and Mendel. Due to the advancement of molecular genetics it is time now for a synthesis of molecular genetics and evolutionary biology which is called molecular evolution or molecular Darwinism. Mechanisms of genetic variation are known today due to extensive studies in microbial genetics which established DNA as the carrier of genetic information. In 1953 Watson and Crick showed that DNA has a double helix structure based on the pairing of nucleotides and these nucleotides were later identified as the letters encoding the genetic message. Alterations of the nucleotide sequence are called mutations and they may lead to favorable advantages and/or unfavorable disadvantages. Genetic variations are the driving forces of evolution, natural selection directs evolution, and isolation (like on the Galapagos Islands) modulates evolution. Today, nucleotide sequences from particular genes or parts of them can be compared and genetic variances can be studied with simple living beings such as bacteria or viruses. Many genetic changes are just replication infidelities caused by local sequence changes. ”All studied living beings have enzymatic repair systems” to correct these infidelities. In addition, there is also the possibility of a horizontal gene transfer called DNA acquisition from other organisms. DNA can be transferred from one kind of bacterium to another. “Practically all bacteria strains have one or a few restriction modification systems” which can distinguish foreign DNA from their own DNA. Identification and cleavage of defined sequences is based on restriction enzymes. The evolutionary tree is shown to illustrate the horizontal and vertical gene transfers. “This is a summary of the three strategies: Local sequence change – DNA rearrangement – DNA acquisition.” At this point of the lecture Arber also makes an important philosophical statement: “Charles Darwin explained to us that living beings have common ancestors, and in view of the horizontal transfer in which gene functions are transferred horizontally we have also a common future.” Many genes are responsible for the sustainment of the individual. However, there are also “genes which are responsible for further evolutionary developments, for the expansion of life, and these are the sources of biodiversity.” Finally, symbiosis may also play an important role in the evolution of life, as happened with the endosymbionts which gave rise to cellular organelles. And, in conclusion, more research on higher organisms is needed but the basis of all knowledge is deeply anchored in microbial genetics.

Abstract

Evolutionary biology and genetics have their roots some 150 years ago, but they were developed largely independently until about 1940 when it came to the modern evolutionary synthesis. Still at that time, the postulated gene was an abstract concept without known material basis. This should change, when microbial genetics was introduced in the 1940's. Very rapidly, it was discovered that DNA rather than any other biological molecules is the carrier of genetic information. It was a good coincidence that a few years later the structure of DNA could be identified as long filaments of double-helical molecules. It then became clear that genetic information could be stored in the linear sequences of nucleotides of DNA. While phenotypic variations, defined as mutations in classical genetics, could be shown to be caused by DNA sequence alterations, it became also clear that, and why, by far not all DNA sequence alterations cause a phenotypic change.

According to the neo-Darwinian theory of evolution, phenotypic variants are, together with their parental forms, the substrates for steadily exerted natural selection. The availability of genetic variants drives evolution, while natural selection, together with the available forms of life, directs evolution, and geographic and reproductive isolations modulate the process of evolution.

Efficient approaches are now available to study the molecular mechanisms that generate genetic variations. In microbial genetics, individual cases of spontaneous mutagenesis can be analysed experimentally. It has thereby become obvious that a number of different specific mechanisms are at work independently. In knowledge of these identified molecular mechanisms, one can compare DNA sequences from organisms that are more or less closely related. This allows one to conclude on their evolutionary history. This approach can be applied for functional domains, single genes, groups of genes and entire genomes of any kinds of living organisms.

The identified molecular mechanisms of genetic variation can be classified into three qualitatively distinct natural strategies, namely: (1) local sequence changes affecting one or a few adjacent nucleotides, (2) recombinational rearrangements of DNA segments within the genome, and (3) acquisition of a foreign DNA segment by horizontal gene transfer. Selected examples of these spontaneously occurring alterations in nucleotide sequences and in the genome structure will be discussed as well as their possible functional consequences.

The theory of molecular evolution that we also call "Molecular Darwinism" is based on the acquired knowledge on genetic variation. In genetic variation, products of evolution genes are involved as variation generators and/or as modulators of the rates of genetic variation. These evolution gene products act together with several non-genetic elements that can be assigned to intrinsic properties of matter, to environmental mutagens and to random encounter. We conclude that natural reality takes actively care of biological evolution. The evolution genes must have been fine-tuned for their functions by second-order selection, so that spontaneous genetic variation with different evolutionary qualities occurs at quite low rates. This ensures a relatively high genetic stability to individuals, as well as an evolutionary progress at the level of populations.

The presence of evolution genes points to a duality of the genome: while many genes act to the benefit of the individuals for the fulfillment of their lives, the evolution genes act to the benefit of an evolutionary development, for a slow, but steady expansion of life and biodiversity.

References for more detailed information:
W. Arber, Elements for a theory of molecular evolution, Gene, 317, 3-11 (2003).
W. Arber, Genetic variation and molecular evolution, In: R.A. Meyers (ed.), Genomics and Genetics, Wiley-VCH, Weinheim, Vol. 1, 385-406 (2007).

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