Some species will hybridize, while variants within the same species will interbreed. Some clearly defined cycad species can interbreed and produce fertile offspring. The four species can interbreed and produce fertile offspring. In the wild, subspecies do not interbreed due to geographic isolation or sexual selection. Hybrid zones are regions where diverged populations meet and interbreed. The Bactrian camel and the dromedary often interbreed to produce fertile offspring. The dromedary and the Bactrian camel often interbreed to produce fertile offspring. The two types do not readily interbreed, but fertile offspring can occur. Although zebra species may have overlapping ranges, they do not interbreed. Koi and common carp may also interbreed with goldfish to produce sterile hybrids. The terminal forms do not interbreed, though they coexist in the same localities. The abundant species can interbreed with the rare species, swamping its gene pool. The two species can interbreed, producing offspring capable of reproduction. It has 76 chromosomes, and therefore cannot interbreed with other canids. The three stocks are allopatric and don't interbreed.