Generally, outside this zone, the two forms retain distinct differences from one another but within the zone intermediate forms between the two occur as a result of interbreeding. For example, a kind of woodpecker, flickers, show a hybrid zone in the United States. In the middle of the country there is an area just a few hundred miles wide where the two forms both occur and where they reproduce with each other. There, some flickers have red shafts, some yellow, and some have some red and some yellow. There are also DNA markers that have been identified such that all eastern forms are fixed for one DNA form and all western forms are fixed for a different DNA form but in the hybrid zone both forms occur.
Hybrid zones raise some interesting evolutionary questions.
Since the two forms can reproduce with each other, we would expect gene flow to occur from one form to the other. If gene flow occurs, we would expect the traits from one form to spread into the other form. We observe, however, that the forms on the two sides of the hybrid zone retain fixed differences. If this is the case, then hybrids that move out of the zone will have low fitness and selection against them may counteract the effects of gene flow out of the hybrid zone. The environment where the hybrid zone occurs may be very poor quality habitat for either form or for hybrids.