These data show us that genetic change is associated with speciation. We want to be able to describe the genetics of speciation and the genetics of species differences . To do so we need to distinguish genetic changes that cause speciation from those that accompany speciation . These will differ a lot from one group of organisms to the next and will depend on the genetic architecture of speciation. Postzygotic or postmating isolation can be measured as in the following example.
Isolation index is the average score for the four cases.
In any particular case one could choose to score isolation in terms of the presence or absence of either isolation or sterility. Hence an index based on sterility would have higher values than an index based only on evidence for inviable hybrid offspring. Coyne and Orr extracted these two types of data from the literature and tested some important ideas about the genetics of speciation. When individuals are crossed between these divergent populations, there will deleterious pleitropic interaction effects between these new alleles on the X and other genes throughout the genome. The new mutations certainly were not deleterious when they arose within each separated population, but when paired with autosomes from a diverged population these mutations do not function properly, thus one would only see the effect in a hybrid cross.