What can interfere with the free crossing of individuals within a population?

Panmexia (free crossing) can be hindered by the following factors:
1. In order for species to interbreed, there should be no differences between them affecting the reproductive system. The fact is that due to the emergence of a geographic isolate in a certain period of time, individuals could acquire new qualities with a radical change in the old ones. This is caused by different environmental conditions, different food resources. And when the isolate disappears and the population reunites, the old species may reject the new one due to reproductive incompatibility.
2. There is a concept “average value of a feature”. Individuals have traits that have their minimum and maximum. The average is the necessary indicator for individuals to cross freely. For example, if a peacock has a tail that is too large or too small, the females will not interbreed with it.
3. Mutations. Basically, they are completely dominant, which displace the necessary trait and replace it with a new one, which is rejected by individuals within the population.