NDA (II) 2016 General Ability Test · Question 100 of 150

Correct — C, species. A species is defined biologically as a group of organisms that can interbreed among themselves to produce fertile offspring. The horse and the donkey are close enough to mate and to produce a healthy, vigorous animal, but the offspring cannot breed, and that failure is the standard test that the parents belong to two different species. The mechanism is chromosomal: a horse has 64 chromosomes and a donkey 62, so a mule inherits 63. At meiosis the chromosomes must pair off before being shared between gametes, and an odd number with two mismatched sets cannot pair properly, so viable eggs or sperm are almost never formed. The mule is therefore vigorous but sterile — an animal that exists only because its parents are separate species that happen to be closely related.

• (a)class — Both animals are mammals and so belong to the same class, Mammalia. Parents from different classes could not produce offspring at all — the question already tells you that a foal is born, which rules out any difference at so high a rank.
• (b)order — Horse and donkey are both odd-toed ungulates of the order Perissodactyla, along with rhinoceroses and tapirs. As with class, a difference at this level would make interbreeding impossible rather than merely unproductive.
• (d)genus — The main trap, because it sounds like the next rank up and therefore like a bigger difference. In fact horse and donkey are both in the genus Equus, and it is precisely that shared genus — the close relationship — that lets them mate successfully in the first place. The barrier to fertility lies one rank lower down, at the species level.

Classification proceeds through a nested hierarchy of ranks — kingdom, phylum, class, order, family, genus and species — with each rank containing the ones below it and the members becoming steadily more alike as you descend. The species is the basic unit, and under the biological species concept its test is reproductive: two populations belong to the same species if they can interbreed in nature and produce fertile offspring. Horse and donkey share every rank down to genus, both being Equus in the family Equidae, and separate only at species.

The item rewards a candidate who reads the ranks in the right direction. Many students assume that a bigger word means a bigger gap, and so pick genus over species; the hierarchy runs the other way, with genus being the wider grouping and species the narrower. The clue is embedded in the question itself. Because an animal was actually born, the two parents must be closely related, which excludes class, order and even genus; and because that animal cannot breed, they cannot be the same species either. That leaves exactly one answer. Two extra details are worth carrying. The reciprocal cross — a male horse with a female donkey — gives a hinny, which is likewise sterile. And the general principle applies well beyond horses: the liger and the tiger-lion crosses among the big cats, all within the genus Panthera, are infertile for the same chromosomal reason.

• A species is a group whose members can interbreed to produce fertile offspring; infertile hybrids indicate two separate species.
• Horse and donkey are both in the genus Equus, the family Equidae, the order Perissodactyla and the class Mammalia.
• A horse has 64 chromosomes and a donkey 62, so a mule has 63 — an odd number that prevents proper pairing at meiosis.
• A mule is the offspring of a male donkey and a female horse; the reciprocal cross produces a hinny, which is also sterile.

• Reading genus as the finer division and species as the broader one; the hierarchy runs the other way.
• Forgetting that the birth of a live hybrid itself rules out any difference above the genus level, which is a free elimination.

Taxonomy comes up in NDA as sequence-of-ranks items or as reasoning items like this one, so learn the ranks in order and attach the fertile-offspring test firmly to the species rank.

• practice — not a real PYQ

  The correct sequence of taxonomic ranks from the broadest to the narrowest is

  • (a)Class – Order – Family – Genus – Species
  • (b)Order – Class – Genus – Family – Species
  • (c)Class – Family – Order – Species – Genus
  • (d)Family – Class – Order – Genus – Species

  Answer(a) Class – Order – Family – Genus – Species — each rank is contained within the one before it.

• practice — not a real PYQ

  Under the biological species concept, two populations are regarded as belonging to the same species if they

  • (a)look alike
  • (b)live in the same habitat
  • (c)can interbreed to produce fertile offspring
  • (d)have the same number of chromosomes

  Answer(c) can interbreed to produce fertile offspring — appearance, habitat and chromosome number are all suggestive but none of them is the test.