MPSC 2022 Prelims Paper-I · Question 46 of 100

Correct — option (4). The classification of animals runs through seven obligatory ranks, and in descending order — from the widest grouping to the narrowest — they are Kingdom, Phylum, Class, Order, Family, Genus and Species. Option (4) is the only one of the four that gives this sequence without a break. The logic of the hierarchy is worth holding rather than the string of names alone, because it makes the order self-correcting. Each rank is a box that contains the rank below it: a kingdom contains several phyla, a phylum contains several classes, a class contains several orders, an order contains several families, a family contains several genera, and a genus contains several species. Two consequences follow, and either of them will settle the order in the examination hall. The first is that the number of organisms in a group falls as one moves down the list, so the kingdom is the most populous category and the species the least. The second is that the number of characters shared by the members of a group rises as one moves down, so members of a kingdom have only a few very general features in common while members of a species share almost everything, including the ability to interbreed and produce fertile offspring. Applied to a familiar animal the sequence reads: human beings belong to the kingdom Animalia, the phylum Chordata, the class Mammalia, the order Primata, the family Hominidae, the genus Homo and the species sapiens; and the housefly to Animalia, Arthropoda, Insecta, Diptera, Muscidae, Musca and domestica. Working the sequence through one such example is the safest way to reproduce it, since the memory has something concrete to hang on. Two refinements complete the picture. In the classification of plants the rank Division is used where animals use Phylum, so the plant sequence reads Kingdom, Division, Class, Order, Family, Genus, Species. And the lowest two ranks are the ones that supply the scientific name of an organism under the binomial system introduced by Carl Linnaeus, the genus written first with a capital letter and the species second in lower case, both italicised in print or underlined separately in handwriting, as in Homo sapiens and Musca domestica. Option (4) is the answer.

• (1)Species → Genus → Order → Class → Family → Phylum → Kingdom — This option is the closest of the three wrong ones and it will catch anyone who checks only the two ends of the sequence. Running from species up to kingdom is a perfectly legitimate way to state the hierarchy — it is the same ladder climbed in the other direction — so a candidate who sees Species at the start and Kingdom at the finish may accept it without reading the middle. The middle, however, is wrong. The correct ascending sequence is Species, Genus, Family, Order, Class, Phylum, Kingdom, whereas this option places Order and Class before Family, which puts a family inside an order in the wrong direction and makes the ladder cross over itself. The lesson is procedural rather than factual: in a sequence question, check every step against its neighbour rather than checking the beginning and the end, because an examiner who wants to build a plausible wrong answer will keep the extremities correct and disturb the middle.
• (2)Kingdom → Genus → Order → Species → Phylum → Class → Family — This option begins correctly with Kingdom and then abandons the hierarchy altogether: Genus, which is the second narrowest rank, is placed immediately below the widest one, Species appears in the middle of the list instead of at its end, and Phylum and Class turn up after them. It can be rejected by a single test that is worth applying to any option in this family — Species must stand at one end of the sequence and Kingdom at the other, because the species is the smallest and most specific group and the kingdom the largest and most general, and no arrangement in which either of them sits in the middle can be a hierarchy of nested boxes. Applying that test alone eliminates this option and option (3) at once, which leaves only two to examine in detail and turns a seven-step comparison into a two-step one.
• (3)Genus → Species → Phylum → Class → Order → Family → Kingdom — This option puts Genus before Species, which reverses the two lowest ranks, and it ends with Kingdom after Family, which places the largest category at the bottom of a descending list. Both faults come from the same misunderstanding, that the sequence is a list of names to be recalled rather than a set of containers nested one inside another. A genus contains several species, so Species cannot be the wider of the two; and the kingdom contains everything else, so it cannot come last in a list that begins with a genus. The option does contain a correct run in the middle, Class then Order then Family, which is enough to make it look familiar to a candidate scanning quickly, and that is exactly why the whole chain has to be checked. Note also that the option would be wrong even read from the other end, since reading it backwards gives Kingdom, Family, Order, Class, Phylum, Species, Genus.

Taxonomy is the science of classifying living organisms, and it works by placing every organism in a series of nested groups called taxa, each group at a rank called a taxonomic category. Seven categories are obligatory — Kingdom, Phylum or Division, Class, Order, Family, Genus and Species — and intermediate ranks such as subphylum, superclass and subfamily are added where a group needs finer division. The species is the basic unit, a group of organisms fundamentally alike and, in the usual biological definition, capable of interbreeding to produce fertile offspring. Closely related species are grouped into a genus, related genera into a family, related families into an order, related orders into a class, related classes into a phylum, and related phyla into a kingdom. Moving up the ladder the groups get larger and the characters they share get fewer and more general; moving down, the groups get smaller and the shared characters more numerous and more specific. The system of naming that goes with the hierarchy is binomial nomenclature, formalised by Carl Linnaeus in the eighteenth century, under which every organism receives a two-word Latin name consisting of the genus followed by the species, printed in italics and written with the genus capitalised. The rules are enforced by international codes of nomenclature, separate ones for animals and for plants, so that a single organism carries one accepted name everywhere in the world regardless of the local names it may also have.

Classification is standard higher secondary biology and MPSC asks it in predictable ways, of which the sequence question is the commonest. Such questions are answered fastest by a structural test rather than by recall: since the hierarchy is a set of nested boxes, Species must stand at one end and Kingdom at the other, and any option that places either of them in the middle can be struck out without further reading. That single test removes two of the four options here. The remaining pair then has to be checked step by step, because the standard way of building a near-miss option is to keep the two ends correct and to transpose two ranks in the middle, which is precisely what option (1) does with Order and Family. Beyond the sequence itself, the Commission asks which category a named group belongs to, what the scientific name of a common organism is, who introduced binomial nomenclature, and what the rules for writing a scientific name are. All of these come from the same small block of material, and the most economical preparation is to memorise the seven ranks together with two worked examples, one animal and one plant, so that the abstract ladder always has something concrete attached to it.

• The seven obligatory taxonomic categories, in descending order for animals, are Kingdom, Phylum, Class, Order, Family, Genus and Species; in the classification of plants the rank Division takes the place of Phylum.
• Each category contains the one below it, so the number of organisms in a group decreases as one moves down the hierarchy while the number of characters its members share increases.
• Human beings are classified as Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Primata, Family Hominidae, Genus Homo and Species sapiens.
• Binomial nomenclature, formalised by Carl Linnaeus, gives every organism a two-word Latin name made of its genus and its species, printed in italics with the genus capitalised and the species in lower case.
• The species is the basic unit of classification and is usually defined as a group of organisms capable of interbreeding among themselves to produce fertile offspring.

• Checking only the first and last ranks of a sequence, which is how option (1) passes inspection despite transposing Order and Family in the middle
• Reversing Genus and Species, when a genus is the wider group containing several species and the species is the basic unit of classification
• Forgetting that plants use Division where animals use Phylum, which matters when the question is set on a plant rather than an animal
• Believing that the number of shared characters increases as one moves up the hierarchy, when it is the number of organisms that increases and the shared characters that grow fewer and more general

MPSC sets classification as a sequence question of the kind asked here, as a placement question naming an organism and asking for its class, order or family, as a nomenclature question on how a scientific name is written and who devised the system, and as a definitional question on what a species or a genus is. The five-kingdom system is asked alongside, usually by naming a kingdom and requiring its criteria or an example. Because the material is compact and entirely factual, it is among the most reliably scoring parts of the biology section, and the return on memorising the seven ranks with two examples attached is high. In sequence questions generally, whatever the subject, the productive habit is to fix the two ends first from the logic of the thing being ordered, and only then to compare the middle terms of the surviving options.

No directly related past PYQ was found.

• practice — not a real PYQ

  As one moves from the species upwards to the kingdom in the taxonomic hierarchy, which of the following happens ?

  • (a)The number of organisms decreases and the number of shared characters increases
  • (b)The number of organisms increases and the number of shared characters decreases
  • (c)Both the number of organisms and the number of shared characters increase
  • (d)Both the number of organisms and the number of shared characters decrease

  Answer(b) The number of organisms increases and the number of shared characters decreases — each higher category contains all the categories below it, so a kingdom holds far more organisms than any one species within it, while the features common to every member of a kingdom are few and very general. Moving downwards the relationship reverses: the groups get smaller and the characters their members share get more numerous and more specific, until at the level of the species the members are alike in almost everything.

• practice — not a real PYQ

  In the scientific name Musca domestica, the two words stand respectively for

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

  Answer(b) The genus and the species — under the binomial system formalised by Carl Linnaeus, the first word of a scientific name is the genus and is written with a capital letter, and the second is the species and is written in lower case, the whole being printed in italics or underlined separately when handwritten. Musca domestica is the housefly, Musca being the genus to which it belongs and domestica the species within that genus.