But at the other extreme, were offspring identical to their parents, the mechanisms of heredity would be irrelevant to the evolutionary process. Features of transmission such as segregation, recombination, mutation, and so forth, give certain regularities to the differences between parents and offpring and take a central position in population genetics models of evolution. Differences between parents and offspring are necessary not only to make the mechanisms of heredity important to evolution, but are the sine qua non for the science of genetics itself, from Mendel's experiments with peas to the transformation of Drosophila. The way I would like to characterize population genetics models of evolution will not even refer to genes specifically. An individual's type is determined by, but not necessarily the same as, the types of its parents.
Transformation processes determine the nature of differences between parents and offspring.
Selection determines the relative proportion of the next generation that the offspring of a given individual comprise, and this proportion will be determined by the individual's type. But processes in transmission that cause transformations do not simply introduce variation and then disappear to let selection act alone on the population. They interact continually with selection to affect the composition of the population. For example, consider a classical case of transformation, the production of overdominant or underdominant heterozygotes from different homozygous parents. The offspring can all be quite different, with respect to selection, from either of their two parents.