NDA (I) 2018 General Ability Test · Question 73 of 150

Correct — A, Phylum – Class – Order – Family – Genus. The full ladder of classification runs Kingdom, Phylum, Class, Order, Family, Genus, Species, from the broadest grouping down to the narrowest. Each step down holds fewer organisms that share more characters with one another. The option quotes five consecutive rungs of that ladder in their proper descending order, starting below Kingdom and stopping above Species, and it is the only option that does so without a swap.

• (b)Phylum – Class – Family – Order – Genus — This is the answer with two rungs exchanged. Order is broader than Family, so Order must come first; the option has put Family above Order and inverted that one step.
• (c)Family – Order – Class – Species – Genus — Two things are wrong here. The first three names are in ascending order rather than descending, since Class is broader than Order and Order broader than Family. And Species is placed above Genus, when Species is the narrowest rung of all.
• (d)Class – Family – Order – Species – Genus — Two swaps again — Family before Order, and Species before Genus. A useful check is that Species must always be last in any descending list, and no option ending in Genus after Species can be right.

Taxonomy arranges organisms in a nested hierarchy of ranks. From the broadest downwards these are Kingdom, Phylum in animals or Division in plants, Class, Order, Family, Genus and Species. As one descends, the number of organisms in each group falls while the number of characters they share rises, so a species is the smallest group and its members resemble one another most closely. Carl Linnaeus established both this ranking and binomial nomenclature, in which every organism carries a two-part scientific name made of its genus and its species.

Three of the four options are the correct sequence with one or two neighbouring rungs exchanged, so the item is testing order and nothing else. The safest defence is to be able to write the full ladder in one go before looking at the choices, and then simply to check each option against it. Two extra checks catch almost every wrong option quickly — Species must sit at the bottom of any descending list, and the pair Order before Family must never be reversed. It is worth knowing what the ranks look like in a real case: a human being is Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, Species sapiens. That one example makes the abstract ladder concrete.

• The descending order of ranks is Kingdom, Phylum, Class, Order, Family, Genus, Species.
• Plants use Division where animals use Phylum.
• Going down the hierarchy, the number of organisms falls and the number of shared characters rises.
• Binomial nomenclature gives each organism a two-part name of genus and species, a system due to Carl Linnaeus.
• Members of one species can interbreed to produce fertile offspring; members of different species generally cannot.

• Reversing Order and Family, which is the single commonest swap in this item type.
• Placing Species above Genus; Species is always the lowest rank.
• Reading the list in ascending order when the question asks for descending.

NDA asks for the sequence, as here, or by giving an organism and asking which rank a named group belongs to.

• practice — not a real PYQ

  Which of the following is the smallest unit of the taxonomic hierarchy?

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

  Answer(c) Species — the narrowest rank, whose members share the most characters.

• practice — not a real PYQ

  In the scientific name Panthera tigris, the word Panthera denotes the

  • (a)species
  • (b)genus
  • (c)family
  • (d)order

  Answer(b) genus — in binomial nomenclature the first word is the genus and the second the species.