Wright though getting to another peak might be possible. It might happen by genetic drift if the population is small enough. If a species was divided into small populations, some could find higher peaks. If there was some gene flow between the populations, these adaptations could spread to the rest of the species. This was Wright's "shifting balance theory" of evolution. There has been much skepticism among evolutionary biologists as to whether this happens in natural populations. Wright had a long standing and sometimes bitter debate about this with R.A. Fisher, who felt that most populations in nature were too large for the effects of genetic drift to be important. Research by Dobzhansky and E.B. Ford showed that natural selection in the field was a much stronger force that Wright had expected. Wright's own biographer now doubts the validity of Wright's idea.