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A species is a group of organisms that can interbreed and produce viable, fertile offspring. Individuals within a species such as males and females can look different from one another, but as long as they can mate and produce offspring that are also fertile, then they are members of the same species. Members of a species cannot be reproductively isolated from one another; this would halt the gene flow between populations and cause the two populations to gradually diverge. If two populations of organisms are reproductively isolated from one another, this means they are unable to mate with one another and can no longer be part of the same species.