The second mechanism of evolution

In Darwin s times heredity was a mystery, but this did not prevent him from concluding that natural selection works on heritable variations. All that he needed to know about heredity were the two facts that he learned from breeders, namely that (1) every individual in a population has unique characteristics, and (2) many distinctive traits are inherited. The discovery of the hereditary mechanism could not cancel these experimental facts, and could not therefore deny natural selection. That discovery, however, could reveal new mechanisms of evolution, and reduce the role that natural selection played in the history of life. This is why the study of heredity came to be seen as the testing ground for any evolutionary theory, and for almost a century, in fact, the debate on evolution has largely been a debate on genetics. 

In 1902, Walter Sutton and Theodor Boveri were able to show that the Mendelian characters (the genes) are physically carried by chromosomes (the chromosome theory of heredity), and the study of meiotic divisions and gametogenesis proved that Mendel s hypotheses were absolutely correct, so much so that they could be regarded no longer as hypotheses but as experimental realities. Mendel s laws gave a direct support to the conclusions that Darwin had obtained from the breeders. Every individual is indeed unique, because the recombination of its genes is a totally random process. 

The constant reshuffling of recombination, furthermore, means that the genetic variability of any population is virtually unlimited and continuously renewable. 

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