Over time, the larger-beaked birds may tend to associate more with each other than with the medium-beaked birds. They may begin mating only with other large beaked birds and producing offspring with large beaks. When the large beaked birds mate and reproduce only with each other and not with medium-beaked birds, they would be considered a new species Species B. Species B can no longer reproduce with Species A. When two organisms can no longer produce offspring that can also reproduce, they are considered different species.