On this page
The word "species" appears on nearly every page of your biology books, and the NCERT Class 11 Living World chapter opens with a box on the man who gave it its most used definition: Ernst Mayr. This post explains where his definition came from, how to apply each phrase of it to an exam question and where it runs into trouble.
Who was Ernst Mayr?
Walter Bock's National Academy of Sciences memoir records that Mayr was born on 5 July 1904 in Kempten, Bavaria, and died on 3 February 2005, aged 100. The NCERT box gives his death as 2004. The NAS memoir and the Royal Society memoir both give February 2005, a reminder to check dates against primary sources.
He began in medicine, then switched to zoology at the University of Berlin under the ornithologist Erwin Stresemann. He finished his PhD in June 1926, just before his 22nd birthday, after only 16 months. Stresemann arranged an expedition to Dutch New Guinea in 1928, and Mayr then joined the Whitney South Sea Expedition of the American Museum of Natural History. Three experienced local museum assistants trained him to collect and survive in the forest.
He worked at the American Museum from 1931 to 1953, then became Alexander Agassiz Professor of Zoology at Harvard, where he directed the Museum of Comparative Zoology from 1961 to 1970. NCERT notes he won the Balzan Prize (1983), the International Prize for Biology (1994) and the Crafoord Prize (1999).
What exactly did Mayr define?
In his 1942 book Systematics and the Origin of Species, Mayr wrote, on page 120 as quoted in the NAS memoir: "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups."
The memoir explains that he separated two things that are often mixed up. Physical barriers, such as a sea between islands, are needed while a species is forming. Intrinsic isolating mechanisms, such as differences in courtship or sterile hybrids, belong to the species and stop genes flowing between species once they exist. He later added the "founder principle": a new species can arise from a small group that colonises an isolated place.
Bock writes that Mayr's work on the museum's vast bird collections from New Guinea, Australia and the South Pacific gave him a firm foundation for this theoretical work.
Worked reasoning: apply the definition
Take each case and test it against the three key phrases.
Two bird populations on islands 50 km apart that look slightly different. They cannot meet, so "actually interbreeding" fails. Ask whether they are "potentially" interbreeding: if birds from both islands breed and raise fertile young when they meet, Mayr would call them one species. Looks alone do not decide.
A horse and a donkey produce a mule. They can mate, but the mule is sterile, so genes cannot flow on. The two are reproductively isolated and are separate species.
Two species of big cat produce hybrids in a zoo. The definition says "natural populations". If the two do not interbreed where they live in the wild, captive hybrids do not make them one species.
A bacterium that reproduces only by dividing. There is no interbreeding at all, so the definition cannot be applied.
Two fossil shells from different rock layers. You cannot test breeding in fossils, so scientists must use form instead.
Common mistakes:
"Can produce offspring" as the test. The offspring must be fertile, and the populations natural.
Treating "looks different" as "different species". Mayr's point was that interbreeding, not appearance, decides.
Forgetting the limits. Cases 4 and 5 show the definition does not cover everything.
Is the biological species concept the only one?
No. Kevin de Queiroz's review Species concepts and species delimitation (2007) describes half a century of controversy over how to define species. He argues that most definitions agree that a species is a separately evolving lineage, and differ over which evidence to use, such as reproductive isolation, distinct features or shared ancestry. On that view, Mayr's reproductive isolation is one strong line of evidence among several.
For NEET and CBSE, use the NCERT wording. Our biological classification guide shows where the species sits in the taxonomic hierarchy, and our profile of Carl Linnaeus covers the naming system that came first.
What can a student take from his way of working?
Build theory on careful data. His ideas grew out of years of sorting museum bird collections.
Separate ideas that look alike. Barriers and isolating mechanisms are different, and he said so.
Write definitions you can test. Each phrase of his definition can be checked against a case.
Keep working. His career lasted nearly 80 years, according to NCERT.
Practising definition questions
Questions on species often turn on one word. The Fermi Concept Library has every concept in biology with its own lesson and its place in the map, so you can check how species, speciation and isolation connect.
Try it now: write Mayr's definition from memory, then underline the phrase that rules out zoo hybrids.
