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A species is a group of organisms that can interbreed and produce fertile offspring. They also tend to share similar characteristics because their DNA is relatively similar.

On the other hand, some organisms may look similar even though they are different species, like the Monarch and Viceroy butterflies. Each is a butterfly with orange-and-black patterns, but they are from two different genera.

Some individuals from different species can interbreed and produce offspring called hybrids, but these offspring are biologically incapable of further reproduction. For example, the resulting hybrid of a zebra and a donkey, called a zonkey, is usually infertile, preventing further gene flow between the parent species. This natural phenomenon is an example of a post-zygotic reproductive barrier, a mechanism that occurs after fertilization, preventing hybrid zygotes from developing into fertile adults, keeping species separate.

The other reproductive barrier, called the pre-zygotic barrier, prevents mating between two species. For example, two frog species may live in the same rainforest habitat. But one is active only during the day and the other only at night, preventing contact between the species.