Species, such as the quetzal, ahuehuete (Montezuma cypress), jaguar, dahlia, crocodile, and the monarch butterfly are the units in which we categorize all living things, including human beings. A species is a group of organisms that can reproduce and create fertile offspring.
In general, individuals of a species are recognizable because they are similar in form and function. Often, however, individuals of a species are very different. For example, male and female birds of the same species are very different, tadpoles are very different to frogs, and caterpillars are very different to butterflies. The opposite can also be true: some species appear very similar and are difficult to distinguish at times, even with the most expert eye.
In the past, species were classified according to their shape. Carl Linnaeus (1707-1778), the Swedish botanist, naturalist and explorer proposed a method of classification known as the binomial system, which assigns a pair of names to each species: The name of the genus which it shares with other related species and the species name, which is unique. For example, the wolf (Canis lupus) and coyote (Canis latrans) share the generic name Canis and are close relatives, but each also has its unique specific name.