The Modern Synthesis of Evolution with Genetics (eBook, ePUB)
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In the beginning, there was only chemistry on the Earth. There were no minds, no creativity, and no intention. Nevertheless, once self-replicating chemicals had a chance to arise, there would have been an automatic tendency for more successful variants to increase in frequency at the expense of less successful variants. Success in chemical replicators is simply synonymous with frequency in circulation. A successful replicator molecule will be one that has what it takes to get duplicated. DNA, which is a self-replicating material present in nearly all living organisms, is so uniform that it con...
In the beginning, there was only chemistry on the Earth. There were no minds, no creativity, and no intention. Nevertheless, once self-replicating chemicals had a chance to arise, there would have been an automatic tendency for more successful variants to increase in frequency at the expense of less successful variants. Success in chemical replicators is simply synonymous with frequency in circulation. A successful replicator molecule will be one that has what it takes to get duplicated. DNA, which is a self-replicating material present in nearly all living organisms, is so uniform that it consists of variations in sequence of the same four proteins: A, T, C, G. Although the products developed by DNA sequences are almost infinitely variable (creating brains for mammals, wings for birds, and leaves for plants), the recipes for building these products are just permutations of A, T, C, G. With DNA, there arose a self-copying system in which there was a form of hereditary variation, with occasional random mistakes in copying. The consequence was that the planet Earth came to have a mixed population, in which variants of life competed for resources. Resources will be scarce, or will become scarce when the competition heats up. Some variant replicants will turn out to be relatively successful in competing for scarce resources. Others will be relatively unsuccessful. So now we have a basic form of natural selection. To begin with, success among rival replicators will be judged purely on the direct properties of the replicators themselves: for example, on how well their shape fits their template. But after many generations, replicators survive not by virtue of their own properties, but by virtue of causal effects on something else, called phenotype. Phenotypes are parts of animal and plant bodies that genes can influence. Phenotypes are the way replicators manipulate their way into the next generation. More generally, phenotypes may be defined as consequences of replicators that influence the replicators'' success, but are not themselves replicated. The chemical world in which a gene (which is the heritable unit in DNA) does its work is not the unaided chemistry of the external environment. The necessary chemical world in which the DNA replicator has its being is a much smaller, more complicated bag: the cell. The chemical microcosm that is the cell is put together by a consortium of thousands of genes. The simplest of autonomous DNA copying systems on the Earth are bacterial cells, and they need at least a couple hundred genes to make the components they need. Cells that are not bacteria are called eukaryotic cells. Our own cells, and those of all animals, plants, and fungi, are eukaryotic cells. They typically have tens of thousands of genes, all working as a team. It seems probable that the eukaryotic cell itself began as a team of a few bacterial cells that joined up together. All genes do their work in a chemical environment put together by a consortium of genes in the cell. The next threshold in life on Earth is an increase in the speed at which information is processed. In the animals, this is achieved by a special class of cells called neurons, or nerve cells. Predators can leap at their dinner and prey can dodge for their lives, using muscular and nervous organs that act at speeds hugely greater than the embryological machinations at which the genes put the apparatus together in the first place. Among the consequences of high-speed information processing may be the development of large aggregations of data-handling units, which we call brains. Brains are capable of processing complex patterns of data apprehended by the sense organs, and capable of storing records of them in memory. A more elaborate consequence of crossing the neuron threshold is a conscious awareness. Many philosophers believe that consciousness is crucially bound up with language.
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