The area that is used to define the population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals from other areas. Normally breeding is substantially more common within the area than across the border.
- Solomon, E. P., Berg, L. R., & Martin, D. W. (2002). Biology, sixth edition. (N. Rose, Ed.). Stamford, CT: Thomson Learning.
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