The Living World Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1Start from the size of the problem: there are roughly 1.7 to 1.8 million described species, and new ones are being added every year, so no one can hold the whole list in mind at once. Classification reduces that overwhelming list to a manageable set of groups.
2. 2(a) It simplifies complexity — a whole population is replaced by a single name, so a biologist can talk about a group instead of enumerating every member, and the effort of identification is done once rather than repeatedly.
3. 3(b) It reveals relationships — grouping is only meaningful if organisms are placed together because they share ancestry and features, so classification is a statement about evolutionary relationships and not just a filing convenience. It also reveals the origin and evolutionary history of an organism.
4. 4(c) It makes prediction possible — once a taxon is characterised, new organisms that share its characters can be identified and placed without being studied from scratch, which is exactly how unknown species get named.
5. 5(d) It helps in identification and in delimiting taxa — classification is both the tool for telling organisms apart and the way boundaries of a group are fixed, so the two uses are really the same use seen from opposite ends.
6. 6(e) It aids in study — because each group is now small enough to be studied thoroughly, biology can proceed taxon by taxon instead of organism by organism, and this is what makes field and laboratory study comparable between laboratories.
7. 7The chain to write in the exam is therefore: complexity is reduced, relationships are revealed, prediction becomes possible, and identification and study become practical.

Step-by-step solution

1. 1The reason is that biology is a science in which new evidence keeps arriving, and a classification is only ever a snapshot of the evidence available when it was made. Whenever the evidence changes, the system has to change with it.
2. 2The first reason is new data. As microscopy, sequencing and chemistry improved, whole groups had to be moved: the prokaryotes had to be split off from the rest, and the single kingdom of Monera no longer held five-kingdom organisms, which is why Whittaker's system was replaced by a two-kingdom and then a three-domain arrangement.
3. 3The second reason is that classification depends on the criteria chosen, and different experts weight those criteria differently. If the sequence of base pairs in nucleic acids is used, the relationships come out one way; if morphology and development are used, they can come out another.
4. 4The third reason is that the number of described species keeps growing. Every new organism has to be fitted into the existing scheme, and a scheme built for a few thousand kinds of organism does not stretch indefinitely without revision.
5. 5The fourth reason is that the basic aim itself has widened. Artificial grouping by habitat or by a single convenient character was acceptable when the aim was convenience of identification, but the modern aim is to express evolutionary relationships, and that aim demands a different set of criteria.
6. 6Two safeguards keep the changes in check and make the process scientific rather than arbitrary: the rules of nomenclature are fixed and universal, so a name once published cannot be changed merely because it is inconvenient; and the system is built only after extensive sampling, so a change is made on evidence rather than on preference.

Step-by-step solution

1. 1This is an open question and the answer earns marks by listing criteria with examples, so use the same categories the chapter has just introduced and apply them to a familiar group.
2. 2The criteria you could use are: the place or habitat you meet them in; the language or mother tongue they speak; the community or religion they belong to; the state or country they come from; their occupation; their age group; their sex; and any distinctive external feature such as height, complexion, hair or eye colour, the colour of clothing, or the vehicle they use.
3. 3The important qualification is that these are phenotypic, superficial and largely artificial criteria. A classification built this way is useful for finding a person quickly and is perfectly valid for a practical purpose, but it does not express any biological relationship.
4. 4That limitation can be stated sharply: two people classified together because they speak the same language or wear the same colour may be entirely unrelated, while a parent and child who look quite different share nearly all their genetic material. The criterion chosen decides the classification, and the same person can sit in different groups under different criteria.
5. 5So the honest answer distinguishes an artificial, need-based grouping, which is what everyday life uses, from a natural, phylogenetic grouping based on shared ancestry, which is what biology prefers.

Step-by-step solution

1. 1Separate the two levels first, because the answer is different at each. Identification of an individual is a taxonomic problem; identification of a population is a genetic one.
2. 2For an individual, identification means placing a particular specimen in the taxonomic hierarchy, so we learn its species, and with it its genus, family, order, class, phylum and kingdom. This is the practical payoff of the whole classification effort.
3. 3At the level of a population, identification means recognising that all the members of that population belong to one species. Doing this across the whole range of that species is what establishes the limits of the species, so species boundaries are set by observation of populations, not by inspecting one specimen.
4. 4Two further things come out of it. First, comparing individuals within a population exposes the variation that always exists inside a species, which shows why a single specimen can never define a species. Second, the geographic and seasonal range of a population tells us how widely the species is distributed and in what habitat, which is the raw material of ecology and biogeography.
5. 5The general conclusion is that the species is not a fixed type with identical members but a population whose members vary and interbreed, so recognising individuals and recognising populations are two steps towards defining what a species is.

Step-by-step solution

1. 1There is one correct answer: Mangifera indica.
2. 2The rules of binomial nomenclature decide it, and there are three of them. The name has two parts, the generic name and the specific epithet. The generic name is written with a capital letter, and the specific epithet is written with a small letter. Both parts are printed in italics, or if italics are not available they are underlined separately.
3. 3Mangifera Indica breaks the second rule, because indica is written with a capital I. Mangifera indica satisfies all three rules: genus capitalised, epithet lower case, both italicised.
4. 4The same rules apply everywhere, so a plant, an animal and a bacterium are all named this way, and the full form may be followed by the author's abbreviated name in Roman type, as in Mangifera indica Linn.

Step-by-step solution

1. 1A taxon is a formal, named group of organisms at any level of the hierarchy. The word comes from the same root as taxonomy and is defined as a group of one or more populations forming a unit, so the definition to write is that a taxon is a formally named group of organisms at any rank.
2. 2The important point for the rest of the answer is that taxa are named units rather than loose categories, and each rank has its own ending in the codes of nomenclature, so an example can be checked by its suffix.
3. 3Take the mango. Its species is a taxon, and its genus is a taxon. Move up and Mangifera is a genus, so a taxon; the family to which it belongs, Anacardiaceae, is a taxon; the order Sapindales is a taxon; the class Dicotyledonae is a taxon; the phylum or division Angiospermae is a taxon; and the kingdom Plantae is a taxon.
4. 4The same is true on the animal side, which shows that the ranks are general and not tied to one group: the species Panthera leo, the genus Panthera, the family Felidae, the order Carnivora, the class Mammalia, the phylum Chordata and the kingdom Animalia are each a taxon.
5. 5Two practical points close the definition. A taxon is a population, not an individual, and the same organism belongs to a taxon at every level simultaneously, so Homo sapiens is a member of the species taxon, the genus taxon Homo and the family taxon Hominidae at the same time.

Step-by-step solution

1. 1The correct sequence is option (c): Species, Genus, Order, Phylum.
2. 2The test is that the categories run from the smallest and most exclusive to the largest and most inclusive, with each category containing all the categories below it. The full order from largest down is Kingdom, Phylum, Class, Order, Family, Genus, Species, so the reverse of that is Species, Genus, Family, Order, Class, Phylum, Kingdom.
3. 3Option (a) is wrong because it jumps from Species straight to Order, skipping Genus and Family, and it then jumps from Order to Phylum, skipping Class.
4. 4Option (b) is wrong because it places Genus above Species, which inverts the most important relationship of all: the genus is the group of closely related species, so it must contain the species and cannot be contained by it. It then jumps from Order to Kingdom, skipping Class and Phylum.
5. 5Option (c) is correct because each term is larger than the one before it, so Species sits inside Genus, Genus inside Order, and Order inside Phylum.

Step-by-step solution

1. 1Collect the definitions first. The currently accepted meanings run from the very narrow to the broad: a species is the smallest unit of classification; it is a group of individuals capable of interbreeding and producing fertile offspring; it is a group of populations whose members can interbreed freely in nature to give a fertile offspring; it is a group of organisms that share the same gene pool and can interbreed under natural conditions to produce a viable, fertile offspring; a species is also the basic unit of classification, of evolution and of reproduction; and there is the related definition in which two organisms are of the same species if they can interbreed freely in nature and produce fertile offspring.
2. 2The definitions converge on one idea: a natural population whose members can interbreed freely in nature to produce viable, fertile offspring. In practice the concept is applied through three tests — the ability to interbreed, the production of fertile offspring, and the formation of a common gene pool under natural conditions.
3. 3For higher plants and animals the biological species concept works reasonably well, and the usual examples are given. Members of a plant or animal species can freely interbreed under natural conditions to produce a fertile offspring, and the offspring show no fundamental deficiency; a horse and a donkey are different species because their hybrid, the mule, is sterile; and the two sexes of a species are the female and the male, which are not separate species but two forms of the same one.
4. 4For bacteria the biological species concept fails completely, and the reason is worth stating precisely. Bacteria reproduce by binary fission and not by sexual reproduction, so there is no interbreeding and no fertilisation to test, and a sterile-offspring argument cannot even be set up. They also exchange genes horizontally through conjugation, transformation and transduction, so gene flow is widespread and not restricted to parent-offspring transfer within a pair of organisms.
5. 5Bacteria are therefore classified by other criteria — morphology, shape, cell wall, flagella, staining response, biochemical and metabolic characters, and above all sequence similarity of rRNA, which is what the three-domain system actually used. And a practical consequence of the failure is that the same morphologically defined bacterium can be several different species, and one species can be several different morphotypes, so a single criterion is not sufficient.

Step-by-step solution

1. 1Phylum, or division in plants: a major taxonomic category above class and below kingdom, containing organisms with a broadly similar body plan or organisation. Examples: Chordata and Arthropoda in animals, Angiospermae in plants.
2. 2Class: a taxonomic category above order and below phylum, grouping organisms that share fundamental features such as a similar body plan, and in animals also a similar developmental pattern and level of organisation. Examples: Mammalia and Reptilia; Dicotyledonae in plants.
3. 3Order: a taxonomic category above family and below class, grouping families whose members show a certain similarity in structure and behaviour, and in animals also a common pattern of organisation at the organ-system level. Examples: Primata, Carnivora, and the family names used at this level such as the order of the eagle.
4. 4Family: a taxonomic category above genus and below order, grouping genera whose members show a few common characters and, in animals, a common reproductive pattern; it is the category that ends in -idae in zoological names. Examples: Felidae, Hominidae, Solanaceae, Poaceae.
5. 5Genus: a taxonomic category above species and below family, grouping closely related species that share many characters and, in animals, a common pattern of reproduction; it ends in -a or -um in zoological names. Examples: Panthera, Homo, Mangifera, Solanum.
6. 6The single idea that makes the list memorable is that these are all ranks on one nested ladder, each containing all the ranks below it, and each of them is a taxon.

Step-by-step solution

1. 1The clearest way to answer is a single ranked chain for each organism, because a hierarchy is a nesting and a chain shows the nesting. The chain for both must be written in descending order, from kingdom to species.
2. 2Plant, the mango: Kingdom Plantae, Phylum or Division Angiospermae, Class Dicotyledonae, Order Sapindales, Family Anacardiaceae, Genus Mangifera, Species Mangifera indica.
3. 3Animal, the lion: Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Carnivora, Family Felidae, Genus Panthera, Species Panthera leo.
4. 4Two points of interpretation close the illustration. The species is the basic unit, so the two names at the end are the ones used in ordinary speech, and the two names must be written in italics with the genus capitalised.
5. 5The nesting means the same organism belongs to every one of these groups at once, so the mango is simultaneously a Plantae, an Angiospermae, a Dicotyledonae, a Sapindales, an Anacardiaceae, a Mangifera and a Mangifera indica. The hierarchy is a set of concentric groups of decreasing size and increasing similarity.