| | Generation of Evolutionary Variation | Life is extraordinarily varied. The differences between a tiny archebacterium living in a superheated sulphur vent at the bottom of the ocean and a two-ton polar bear roaming the arctic circle span orders of magnitude in many dimensions. Many organisms consist of a single cell; a Sperm Whale has more than 1015 cells. Although very acidic, very alkaline or very salty environments are generally deadly, living things can be found in all of them. Hot or cold, wet or dry, oxygen-rich or anaerobic, nearly every niche on the planet has been invaded by life. The diversity of approaches to gathering nutrients, detecting danger, moving around, finding mates (or other forms of reproduction), raising offspring and dozens of other activities of living creatures is truly awesome. Although our understanding of the molecular level of life is less detailed, it appears that this diversity is echoed there. For example, proteins with very similar shapes and identical functions can have radically different chemical compositions. And organisms that look quite similar to each other may have very different genetic blueprints. All of the genetic material in an organism is called its genome. Genetic material is discrete and hence has a particular size, although the size of the genome is not directly related to the complexity of the organism. The size of genomes varies from about 5,000 elements in a very simple organism (e.g. the viruses SV40 or φx) to more than 1011 elements in some higher plants; people have about 3x109 elements in their genome. Despite this incredible diversity, nearly all of the same basic mechanisms are present in all organisms. All living things are made of cells: membrane-enclosed sacks of chemicals carrying out finely tuned sequences of reactions. The thousand or so substances that make up the basic reactions going on inside the cell (the core metabolic pathways) are remarkably similar across all living things. Every species has some variations, but the same basic materials are found from bacteria to human. The genetic material that codes for all of these substances is written in more or less the same molecular language in every organism. The developmental pathways for nearly all multicellular organisms unfold in very similar ways… It is the process of evolution that is responsible both for the diversity of living things and for their underlying similarities. The unity arises through inheritance from common ancestors; the diversity from the power of variation and selection to search a vast space of possible living forms. … In order to get a rough idea of the degrees of relatedness among creatures, it is helpful to know the basic timeline of life on Earth. The oldest known fossils, stromalites found in Australia, indicate that life began at least 3.8 billion years ago. Geological evidence indicates that a major meteor impact about 4 billion years ago vaporized all of the oceans, effectively destroying any life that may have existed before that. In effect, life on earth began almost as soon as it could have. Early life forms probably resembled modern bacteria in some important ways. They were simple, single celled organisms, without nuclei or other organelles. Life remained like that for nearly 2 billion years. Then, about halfway through the history of life, a radical change occurred: Eucarya came into being. There is evidence that eucarya began as symbiotic collections of simpler cells which were eventually assimilated and became organelles. The advantages of these specialized cellular organelles made early eucarya very successful. Single-celled Eucarya become very complex, for example, developing mechanisms for moving around, detecting prey, paralyzing it and engulfing it. The next major change in the history of life was the invention of sex. Evolution… is a mechanism based on the inheritance of variation. Where do these variations come from? Before the advent of sex, variations arose solely through individual, random changes in genetic material. A mutation might arise, changing one element in the genome, or a longer piece of a genome might be duplicated or moved. If the changed organism had an advantage, the change would propagate itself through the population. Most mutations are neutral or deleterious, and evolutionary change by mutation is a very slow, random search of a vast space. The ability of two successful organisms to combine bits of their genomes into an offspring produced variants with a much higher probability of success. Those moves in the search space are more likely to produce an advantageous variation than random ones. Although you wouldn’t necessarily recognize it as sex when looking under a microscope, even some Bacteria exchange genetic material. How and when sexual recombination first evolved is not clear, but it is quite ancient. Some have argued that sexual reproduction was a necessary precursor to the development of multicellular organisms with specialized cells. The advent of sex dramatically changed the course of evolution. The new mechanism for the generation of variation focused nature’s search through the space of possible genomes, leading to an increase in the proportion of advantageous variations, and an increase in the rate of evolutionary change. | — Lawrence Hunter, Molecular Biology for Computer Scientists in Artificial Intelligence and Molecular Biology, ed. Lawrence Hunter | Indexes/24 |
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