Speciation

Originally, species were defined based on morphological similarities. This became a problem when scientists discovered that some organisms have many different forms, such as queens and workers in insect societies.

The modern definition is based on the ability to reproduce.

The modern definition of species is based on the ability to reproduce. However, this ability is not always clean-cut. Some populations are able to reproduce but do not in nature because they never meet. In other cases, two populations that cannot mate with each other can both mate successfully with a third intermediate population.