Species

In the syllabus and the exam

Species is defined using the biological species concept: a group of organisms that share similar structural and physiological features and, crucially, can interbreed naturally to produce fertile offspring capable of reproducing themselves. Candidates are frequently tested on this fertility criterion through examples such as horses and donkeys, whose offspring (a mule) is usually sterile, which is the key evidence used to classify horses and donkeys as separate species rather than one. A common misconception is judging species membership purely on physical similarity, assuming that any two similar-looking organisms must belong to the same species; examiners specifically test this by presenting look-alike organisms that cannot interbreed, or very different-looking members of the same species (such as a Great Dane and a Chihuahua, both the same dog species), to check whether candidates rely on the correct biological criterion rather than appearance alone. Species sits at the base of the classification hierarchy tested in this chapter (kingdom, phylum, class, order, family, genus, species), and candidates should be able to state that species is the most specific, narrowest level, containing organisms most closely related to one another. The concept of species is also the foundation for binomial nomenclature, since the second part of a scientific name identifies the specific species within its genus. Candidates should further note that geographic isolation, where two populations of the same original species become separated for long enough, is the mechanism most commonly cited for how a single ancestral species can eventually split into two genetically distinct species over time.

Example

Lions and tigers can mate to produce a liger in captivity, but ligers are almost always infertile, which is why lions and tigers are still classified as two separate species rather than one.

Related terms

ClassificationBinomial nomenclatureBiodiversity

Source:SRC-DSKP-EN