Chapter 12: Patterns in Life: Diversity and Classification (Biology)

Answer: (ii) Body with jointed legs

Jointed legs are the defining feature of arthropods, the group to which insects belong. An earthworm (an annelid) has no legs at all. The other options do not distinguish them: both insects and earthworms are bilaterally symmetrical; an earthworm is cylindrical; and an earthworm is in fact clearly segmented (ring-like segments), so (iii) and (iv) would not confirm an insect.

(ii)

Answer: (iii)

Animal cells are bounded only by a cell membrane; they have no cell wall. Sponge cells have only a cell membrane, like other animal cells, so sponges are placed in Animalia.

• (i) is wrong: sponge cells, like all eukaryotic cells, have mitochondria.
• (ii) is wrong: sponges cannot photosynthesise; they are heterotrophic (they filter food from water).
• (iv) is wrong: a cell wall is a feature of plants, fungi, many protists and bacteria, not of animals.

(iii) Presence of a cell membrane (with no cell wall)

Example: a house lizard and a cockroach.

These features are used in classification. The presence of a backbone places the lizard among the vertebrates (phylum Chordata), and its dry scaly skin and egg laying on land place it in class Reptilia. The cockroach, with no backbone, jointed legs and exoskeleton, is an invertebrate in phylum Arthropoda (class Insecta: three body parts and six legs). Features that are basic to the body plan are used first to place animals in big groups, and finer features place them in smaller groups.

For example, a lizard (backbone, internal skeleton, 4 limbs, scaly skin) and a cockroach (no backbone, exoskeleton, 6 jointed legs, 3 body parts). Backbone places the lizard among vertebrates (Reptilia); jointed legs and exoskeleton place the cockroach in Arthropoda (Insecta).

• It applies to all living organisms. Every organism is made of cells (prokaryotic or eukaryotic; unicellular or multicellular), so cellular organisation can be used to compare and divide all of life. Xylem and phloem are found only in some plants (pteridophytes, gymnosperms and angiosperms); they are absent in bacteria, protists, fungi, animals, and even in algae and mosses. So they can be used only to divide plants into smaller groups.
• It reflects a more basic and older difference. The type of cell and the level of organisation arose early in evolution and decide many other features of an organism. Vascular tissues developed much later, as an adaptation of plants to life on land.
• So, in the hierarchy, a broad and basic character like cellular organisation is used at the highest level (kingdoms), while a later, narrower character like vascular tissue is used at lower levels (within Plantae).

Cellular organisation is present in every organism and represents a basic, early difference, so it can classify all life at the highest level; xylem and phloem occur only in some plants and can be used only to sub-divide the plant kingdom.

It most likely belongs to Kingdom Protista (it could be Paramecium).

• Single-celled: so it is not a plant, animal or (mostly multicellular) fungus.
• Well-defined nucleus: so it is a eukaryote, not a moneran (bacteria have no true nucleus).
• Cilia for movement: many protists move using cilia (like Paramecium), flagella (like Euglena) or pseudopodia (like Amoeba).

Unicellular eukaryotes are placed in Protista.

Protista (e.g. Paramecium): it is unicellular and eukaryotic (true nucleus), and uses cilia to move.

• Many food chains link into a food web. If one kind of food becomes scarce, consumers can feed on others, so the ecosystem does not collapse.
• Each group plays a role: plants and algae produce food and oxygen; animals consume and keep populations in check; fungi and bacteria decompose dead matter and recycle nutrients; insects pollinate flowers.
• Natural control of populations: predators, parasites and competitors stop any one species from growing out of control.
• Resilience: a diverse ecosystem has many species doing similar jobs. If disease, drought or climate change removes some species, others can take over their roles, so the ecosystem recovers faster.
• Genetic diversity within species also helps some individuals survive changes.

Diverse organisms form complex food webs, carry out different roles (producing, consuming, decomposing, pollinating) and control each other's populations; if some species are lost, others can fill their roles, so the ecosystem stays balanced and recovers from disturbances.

• Prokaryotes and eukaryotes would be mixed. Bacteria (no true nucleus or membrane-bound organelles) would be grouped with Amoeba and Paramecium (eukaryotes with a nucleus). These are fundamentally different cell types; this was in fact the problem that led to Monera being separated from Protista.
• Very different modes of nutrition (autotrophic Euglena and cyanobacteria, heterotrophic Amoeba, decomposer bacteria) and cell walls of different types would be in one group.
• Yeast, a unicellular fungus with a chitin cell wall, would be separated from its close relatives, the multicellular fungi.
• The group would be too large and varied, so it would not show true relationships, and it would be difficult to study or identify organisms in it.

Basic differences would be ignored: prokaryotic bacteria and eukaryotic protists would be mixed, organisms with very different nutrition and cell walls would be grouped together, and unicellular fungi like yeast would be separated from other fungi, so the group would not reflect real relationships.

The five kingdoms are based on the features of cells, but viruses are not made of cells:

• They are acellular: just genetic material (DNA or RNA) inside a protein coat, with no cytoplasm, cell membrane or organelles.
• So they cannot be called prokaryotic or eukaryotic, unicellular or multicellular; none of the criteria can be applied.
• They have no nutrition, respiration or growth of their own.
• They reproduce only inside a living host cell, using the host's machinery; outside a host they are inactive (they can even be crystallised).

So they show features of both living (genetic material, reproduction inside hosts) and non-living things, and do not fit into any kingdom.

Viruses are acellular (only genetic material in a protein coat), have no metabolism of their own and multiply only inside host cells, so the cell-based criteria of the five kingdoms cannot be applied to them.

I would keep viruses outside the five kingdom system, in a separate category of acellular (non-cellular) entities.

• All five kingdoms are built on cellular features (cell type, cell wall, level of organisation, nutrition). Viruses have none of these, so putting them in a kingdom would break the logic of the system.
• They are not independent living organisms: they show life only inside a host cell. Yet they are very important (diseases, evolution, biotechnology), so they need a separate classification of their own, based on their genetic material (DNA or RNA), shape and the hosts they infect. (Scientists do classify viruses separately in this way.)

(Some may argue for a sixth group "Acellular life"; either answer is fine if justified.)

What this shows: classification is not fixed. It changes as we learn more: from two kingdoms to three, four, five, and now with even more groups (such as Archaea). Science revises its systems when new discoveries do not fit, so classification is a tool that keeps evolving.

Keep them outside the kingdoms in a separate acellular category, because all kingdoms are based on cell features that viruses lack. This shows that classification systems change as new knowledge appears.

Features that keep viruses out:

• No cells: so no cell type (prokaryotic or eukaryotic), no cell membrane, cytoplasm or organelles.
• No cell wall or cellular structure to compare.
• Not unicellular or multicellular, so no level of organisation.
• No mode of nutrition of their own: neither autotrophic nor heterotrophic.
• They cannot grow or reproduce on their own; they multiply only inside a host cell.

Limitations of classification: a classification system can only group what fits its chosen criteria. Nature does not always have sharp boundaries: some entities lie between living and non-living, or between groups. So a classification system is a human-made tool based on present knowledge; it is useful, but it cannot place every form, and it must be changed when new discoveries do not fit.

They lack cells, cell type, cell wall, level of organisation and their own nutrition, which are the very criteria of the five kingdoms. Classification systems are human-made, based on chosen criteria and current knowledge, so they cannot fit everything and must keep changing.

Both reproduce by spores and lack flowers and seeds, so they are not seed plants. But the presence of vascular tissue (xylem and phloem) and a body with true roots, stem and leaves make pteridophytes more advanced. This key difference places them in a separate group from bryophytes.

Pteridophytes have vascular tissue (xylem and phloem) and a body with true roots, stems and leaves; bryophytes have no vascular tissue and only a simple body, so they are placed in different groups.

Genus.

In the hierarchy Kingdom → Phylum → Class → Order → Family → Genus → Species, each lower level is a smaller subdivision of the one above. A class contains several orders, each order several families, and each family several genera. So a genus is a much smaller group than a class.

As we go down the hierarchy, the organisms in a group share more and more characteristics, because each step groups only those that are more closely related. For example, class Mammalia includes humans, cats, whales and bats (they share only a few features, such as hair and mammary glands), while genus Panthera includes only big cats such as the tiger, lion and leopard, which are very similar.

Genus: it is a lower level in the hierarchy, so it has fewer members, and those members are more closely related and share more common features than the members of a class.

Locomotion and autotrophic nutrition alone are not enough: many protists, but also some monerans, show these. The scientist should check:

1. Cell type: it must be eukaryotic, with a well-defined, membrane-bound nucleus (this rules out Monera such as cyanobacteria).
2. Level of organisation: it must be unicellular (this rules out Plantae and Animalia).
3. Cell wall: absent, or made of cellulose (not chitin, which would indicate fungi).

A single-celled eukaryote that moves and makes its own food, like Euglena (flagellum and chloroplasts), belongs to Protista.

It must be unicellular and eukaryotic (true nucleus), with no cell wall or a cellulose wall, like Euglena.

Use a step-by-step key:

The deciding character is the chitin cell wall together with heterotrophic, absorptive nutrition and no chlorophyll. This is why yeast, though unicellular, is placed in Fungi.

Unicellular eukaryote + cell wall made of chitin + no chlorophyll, heterotrophic nutrition by absorption → Fungi (like yeast).

(i) Q belongs to Fungi: it is multicellular and filamentous with a cell wall, has no chlorophyll and grows on dead organic matter (saprophytic), like a mould.

(ii) P belongs to Monera: it has no true nucleus (prokaryotic). Its rigid cell covering and survival in high salinity and temperature suggest an archaean or bacterium.

(iii) Both are eukaryotes, but they differ in other criteria:

So the level of organisation (unicellular vs multicellular) and the mode of nutrition (and cell structure) separate them.

(iv) S is heterotrophic, but heterotrophic nutrition is shared by animals, fungi, many protists and most bacteria. Nutrition alone would put S together with moulds and bacteria. S must be classified using its other features: multicellular, with well-differentiated tissues, no cell wall and a backbone (Animalia, phylum Chordata, a vertebrate). Its aquatic respiration in early life suggests an amphibian such as a frog.

(v) T lacks cellular organisation; it is acellular, so none of the criteria (cell type, cell wall, level of organisation, mode of nutrition) can be applied to it. This shows that classification systems are built on chosen criteria and present knowledge: they cannot place everything (there is no place for acellular entities such as viruses), and they need to be revised as new discoveries are made.

(vi) Pond: R (a protist) and S (a frog, which lives in water in its early stage) would be grouped together, though one is a unicellular protist and the other a vertebrate. Damp soil near decaying logs: Q (a fungus) would be grouped with soil bacteria such as P. Digestive tract: bacteria and protists would be grouped with each other. Also, the same organism can live in several habitats (e.g. frogs on land and in water, bacteria everywhere), so it would be put in more than one group. Consequences: unrelated organisms would be grouped together and related ones separated; the groups would not reflect structure, cell type or evolutionary relationships; and we could not predict an organism's features from its group, so the classification would be scientifically useless.

(vii) Fungi. It is multicellular and eukaryotic, which fits both kingdoms, and lacks chlorophyll, so it is heterotrophic, also true of both. The deciding criterion is how it obtains food: it absorbs nutrients from outside its body (from the host), which is the absorptive (saprophytic or parasitic) nutrition of fungi. Animals ingest food and digest it inside the body. To confirm, the scientist should check for a cell wall made of chitin (fungi) as against no cell wall (animals).

(i) Q (no chlorophyll, grows on dead matter, cell wall) (ii) P (no true nucleus) (iii) R is unicellular, motile and photosynthetic (Protista); Q is multicellular, non-motile and saprophytic (Fungi) (iv) Heterotrophy is shared by animals, fungi, protists and bacteria; S needs features like tissues and backbone (v) Cellular organisation; classification systems cannot place acellular entities (vi) e.g. R and S (pond), P and Q (soil): unrelated organisms grouped together (vii) Fungi: absorptive nutrition (check for a chitin cell wall).