Two organisms are part of the same species if they are able to interbreed. Two organisms that look similar but cannot breed together are not actually the same species. For example, Eastern and Western meadowlarks look nearly identical but are incapable of interbreeding; thus, they are two different species.
The interbreeding definition of species is referred to as the biological species concept in science. There are some organisms that look very different but are still the same species. Pheidole barbata ants may look different due to their roles within the colony, yet they are still capable of interbreeding as a species.
Some situations in nature pose difficulty for the biological species concept, such as asexual organisms and organisms that occasionally produce hybrids. The definition also poses problems for certain widely-mating plants such as orchids. Certain species of orchids are capable of cross-breeding with large numbers of non-orchid plants, blurring the distinction between species.
Though the biological species concept is the most prominent view held in science as of 2015, other definitions of species are still applicable in certain instances. The morphological species concept, for example, is used in paleontology to define pre-historic species by appearance alone. Because an extinct organism's breeding partners are unknown, appearance is the only way to categorize species from this period. A potential problem with the morphology species concept is accidentally assigning different species to male and female organisms actually capable of interbreeding, due to lack of information.