For example, a tiger and a clam are classified as different species, because they cannot produce offspring. A German Shepherd and a Beagle, although different breeds, are classified as being the same species because they can produce fertile offspring.
However, the definition of "species" has changed over time and is sometimes problematic.
Origin of the term
Linnaeus, who introduced the classification system in the 18th century, originally intended "species" to refer to the biblical creation "kinds", but the modern definitions of species do not match the biblical kinds. Despite this, many people, both Christians and evolutionists, believe that the idea of new species developing is inconsistent with the Bible. However, creationary scientists accept that speciation has been observed, although they deny that speciation can lead to the development of completely new features as predicted by evolutionists.
There is not a single universally-agreed definition of "species." This is because different definitions of "species" are germane to different contexts. For living organisms, the biological species concept is considered the norm, but it is not always the easiest definition to apply.
Examples of species concepts include:
- the biological species concept
- A species is a group of organisms capable of interbreeding. This concept only applies to extant organisms which undergo a significant amount of sexual reproduction; moreover, successful reproduction (or, at least, the lack of barriers to it) must be observed. When several reproductively isolated populations are observed within an apparently similar group of organisms, they are often considered different species. There are several different ways to define this concept depending on which methods of reproductive isolation are being considered.
- the morphospecies concept
- A species is a group of organisms characterized by common morphological features. While it is easy to observe and categorize species this way, especially fossil species, such categorizations are not foolproof, and this concept often yields faulty results. For example, a two-legged goat born to a four-legged goat mother is not a member of a new species.
- the genetic species concept
- A species is a group of organisms defined by DNA sequence traits. The cutoffs between species defined this way are often arbitrary, but this species concept is often considered the best one for microorganisms like bacteria.
Common third species
There are cases where two species are not capable of interbreeding, but each is capable of interbreeding with a third species.
"Species" is sometimes defined in such a way that groups of creatures that do not naturally interbreed (perhaps because of geological isolation) are classified as separate species despite the fact that they can interbreed.
"Ring species" occur when a contiguous lineage of ancestors co-exist at the same time. Unlike most species whose direct ancestral species are now extinct, the ancestral species of a ring species still survive. The existence of ring species makes it more difficult to define what a species is. For example, A can breed with B can breed with C can breed with D... however D cannot breed with A. The most prominent example of a ring species is the Larus Gull which resides in the north Atlantic. Neighboring species around the arctic interbreed, however this ability to interbreed terminates near Britain with the Herring Gull and Lesser Black-Backed Gull.
Classifying extinct creatures
Many extinct species known only from fossils have been given species names, despite the impossibility of determining their capability of interbreeding.