Richard Lewontin:


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It is not yet clear which percentage of evolutionary change is due to natural selection and which is due to random events. Modern evolutionary genetics stems from the merging of two traditions, the Darwinian and the Mendelian, both of which take variation as the crucial aspect of life. The Darwinian view can be summarized as "evolution is the conversion of variation between individuals into variation between populations and species in time and space". The paradox is that Mendelian theory dictates the frequencies of genotypes as the appropriate genetic description of a population, whereas variation is much more important. "What we can measure is uninteresting and what we are interested in is unmeasurable". Most theories of genetic variation in populations (allelic variation) are also theories of natural selection. Variation and selection turn out to be dual aspects of the same problem.
Even worse is the situation with respect to "the origin of species", i.e. theories of the genetic changes that occur in species formation. Geographic isolation (or, better, ecological divergence) is recognized as the preliminary stage, causing the appearance of genetic differences sufficient to restrict severely the genetic exchange with other populations (reproductive isolation). The second stage occurs when isolated populations come into contact and the third stage starts when the newly formed species continue to develop independently.
Lewontin reviews evidence in favor of each theory. His conclusion, in regarding the genome as the unit of selection, is that "context and interaction are of essence".

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