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Members of one species do not, in general, mate with members of another species, though there are many exceptions and variations to this.
The answer given by Dobzhansky and Ernst Mayr was that species split mainly because they lived in different areas (geographical speciation). They developed featues which, if they later lived together, would make crossing not work properly.
The idea of the biological species concept was that a species was defined as a group of interbreeding individuals. They would be a natural population which is reproductively isolated from other such groups.284
A list was drawn up of isolating mechanisms: 
Factors which cause individuals to mate with their own species.
- Temporal isolation: Individuals do not mate because they are active at different times.
- Ecological isolation: Individuals only mate in their preferred habitat. They do not meet those with different ecological preferences.
- Behavioral isolation: Individuals of different species may meet, but choose members of their own species. They may not recognize sexual cues given by other species. The individuals may be incompatible due to size or morphology.
- Gametes not compatible.
- Zygote mortality: The egg is fertilized, but the zygote does not develop.
- Hybrid inviability: Hybrid embryo forms, but dies.
- Hybrid sterility: Hybrid is viable, but the resulting adult is sterile.
- Hybrid breakdown: First generation hybrids are viable and fertile, but further hybrid generations and backcrosses are inviable or sterile.
There is still much debate as to whether the Dobzhansky/Mayr account is satisfactory. Modern researchers tends to avoid the general term 'isolating mechanisms' in favour of the more specific terms 'mate choice', 'hybrid incompatibility' and other forms of 'reproductive isolation'.
- Mayr, Ernst 2001. What evolution is. Weidenfeld and Nicolson, London.
- King, David (2003-04-12). "The concept of species". Zoology 304, Evolution. http://www.science.siu.edu/zoology/King/304/species.htm#15.2.3. Retrieved 2007-07-18.
- Mayr E. 1970. Populations, species, and evolution. Harvard University Press, Cambridge, Mass.
- Mallet, J.L.B. (1998). "Isolating mechanisms". http://abacus.gene.ucl.ac.uk/jim/Sp/isolmech.html. Retrieved 2007-07-18.