dezembro 22, 2011

Selecção Natural

Uma citação de Richard Dawkins retirada do livro "The God Delusion" que resume o essencial da selecção natural, do ponto de vista genético:

In its most general form, natural selection must choose between alternative replicators. A replicator is a piece of coded information that makes exact copies of itself, along with occasional inexact copies or 'mutations'. The point about this is the Darwinian one. Those varieties of replicator that happen to be good at getting copied become more numerous at the expense of alternative replicators that are bad at getting copied. That, at its most rudimentary, is natural selection. The archetypal replicator is a gene, a stretch of DNA that is duplicated, nearly always with extreme accuracy, through an indefinite number of generations. [...] In the world of genes, the occasional flaws in replication (mutations) see to it that the gene pool contains alternative variants of any given gene - 'alleles' - which may therefore be seen as competing with each other. Competing for what? For the particular chromosomal slot or 'locus' that belongs to that set of alleles. And how do they compete? Not by direct molecule-to-molecule combat but by proxy. The proxies are their 'phenotypic traits' - things like length or fur colour: manifestations of genes fleshed out as anatomy, physiology, biochemistry or behaviour. A gene's fate is normally bound up with the bodies in which it successively sits. To the extent that it influences those bodies, it affects its own chances of surviving in the gene pool. As the generations go by, genes increase or decrease in frequency in the gene pool by virtue of their phenotypic proxies.


For didactic purposes, I treated genes as though they were isolated units, acting independently. But of course they are not independent of one another, and this fact shows itself in two ways .First, genes are linearly strung along chromosomes, and so tend to travel through generations in the company of particular other genes that occupy neighbouring chromosomal loci. [...] The other respect in which genes are not independent [...] concerns embryology which - the fact is often mis-understood - is completely distinct from genetics. Bodies are not jigsawed together as mosaics of phenotypic pieces, each one contributed by a different gene. There is no one-to-one mapping between genes and units of anatomy or behaviour. Genes 'collaborate' with hundreds of other genes in programming the developmental processes that culminate in a body, in the same kind of way as the words of a recipe collaborate in a cookery process that culminates in a dish. It is not the case that each word of the recipe corresponds to a different morsel of the dish. Genes, then, co-operate in cartels to build bodies, and that is one of the important principles of embryology. It is tempting to say that natural selection favours cartels of genes in a kind of group selection between alternative cartels. That is confusion. What really happens is that the other genes of the gene pool constitute a major part of the environment in which each gene is selected versus its alleles. Because each is selected to be successful in the presence of the others - which are also being selected in a similar way - cartels of co-operating genes emerge.


Different kinds of gene cartel emerge in different gene pools. Carnivore gene pools have genes that program prey-detecting sense organs, prey-catching claws, carnassial teeth, meat-digesting enzymes and many other genes, all fine-tuned to co-operate with each other. At the same time, in herbivore gene pools, different sets of mutually compatible genes are favoured for their co-operation with each other. We are familiar with the idea that a gene is favoured for the compatibility of its phenotype with the external environment of the species: desert, woodland or whatever it is. The point I am now making is that it is also favoured for its compatibility with the other genes of its particular gene pool. A carnivore gene would not survive in a herbivore gene pool, and vice versa. In the long gene's-eye-view, the gene pool of the species - the set of genes that are shuffled and reshuffled by sexual reproduction - constitutes the genetic environment in which each gene is selected for its capacity to co-operate.

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