Patterns in Life: Diversity and Classification

Yes — very often they do. When many organisms share the same set of features, the simplest explanation is that they inherited those features from a common ancestor. The chapter itself says that similar features in organisms suggest that they have evolved from common ancestors.

Here is why. Features are passed on from parents to offspring through inherited (genetic) material. Over many generations, small differences pile up and one ancestral population slowly gives rise to several new forms. The new forms keep many of the old, inherited features and add a few new ones of their own. So organisms that share many features are usually close relatives on the ‘family tree’ of life. For example, the tiger and the lion share a similar skull structure and the ability to roar, which is why both are placed in the same genus, Panthera.

A word of caution. One or two similar features are not always enough. Sometimes unrelated organisms look alike simply because they live in the same way — a bird and an insect both have wings because both fly, yet they are not close relatives. That is why scientists do not rely on a single outward feature. They compare many features together — external form, internal structure, cell structure, mode of reproduction and, today, genetic (DNA) similarity. When many independent features agree, a common ancestry becomes very likely.

A single-celled organism such as Amoeba, Paramecium or Euglena (Kingdom Protista) manages because its one cell is a complete, self-sufficient living unit. Inside that one cell, different parts (organelles) share out the work that, in our body, is done by whole organs:

Why one cell is enough for them but not for us. A single cell is tiny, so every part of it is very close to the outside. Its surface is large compared with its small volume, so gases, food and wastes can move in and out quickly by diffusion. These organisms also live in water or moist places, which keeps the cell surface wet and supplied.

Our body is huge by comparison. Most of our cells lie deep inside, far from the air and far from food, so diffusion alone would be far too slow. That is why a multicellular body needs division of labour: groups of specialised cells form tissues and organs — lungs for gas exchange, a gut for digestion, blood to transport, kidneys to excrete. Billions of cells are needed not because each life process is harder, but because a large body has to be supplied and coordinated.

These are the features of the plants that first moved from water to land — the bryophytes (mosses, Marchantia) and the pteridophytes (ferns).

In short: rhizoids, a differentiated body and (in ferns) true roots with xylem and phloem reduce dependence on water for living; but because fertilisation still needs water, these plants remain tied to moist conditions. Only seed plants (gymnosperms and angiosperms) are free of this need.

In a tall plant, water and minerals are absorbed by the roots at the bottom, while food is made by photosynthesis in the leaves high above. These materials must travel a long distance in both directions.

Simple diffusion from one cell to the next works only over very short distances. Over the length of a tall stem it would be far too slow, and the top of the plant would dry out and the roots would starve. A tall plant therefore needs pipe-like transport tissues:

This is exactly why bryophytes, which lack xylem and phloem, always stay small and close to a moist surface, while pteridophytes, gymnosperms and angiosperms, which have vascular tissue, can grow into tall plants.

Seeds and fruits are the main reason why gymnosperms and angiosperms have spread over almost every kind of land habitat.

How seeds help:

How fruits help (angiosperms only): fruits enclose and protect the seeds, and they help to disperse the seeds through insects, birds, other animals, wind or water. Dispersal carries seeds to new locations, away from the parent plant, so the young plants face less crowding and the species can colonise new areas.

So seeds decide how plants survive (protected, fed and independent of water for reproduction), and fruits decide where they can survive (by carrying seeds to many new places). This is why angiosperms are the most diverse plant group on the Earth.

The beetle’s hard external skeleton (exoskeleton) is a rigid outer covering. It helps the beetle survive in several ways:

Both animals are segmented, but the exoskeleton is what allows arthropods such as the beetle to survive in dry and exposed environments, and it is one reason why arthropods are found almost everywhere.

It is much wider than just the number of different kinds of organisms. Biodiversity covers variety at several levels:

Biodiversity also includes the interactions between organisms and their surroundings — food webs, pollination, decomposition and nutrient cycling — which keep ecosystems stable and functioning. So biodiversity is the variety of life together with the variety of its genes, habitats and relationships.

I would observe its features in the same order that classification uses — starting with the broadest features and moving to more specific ones, so that each step narrows down the group.

Why: these are exactly the criteria on which the five kingdom system is built — cell type, cell number, cell wall, mode of nutrition and body organisation. Checking them in order places the organism first in its kingdom, then in smaller and smaller groups. Comparing it with already known pond organisms would then show whether it is truly a new species.

Every living cell contains genetic material (DNA), which carries the instructions for the organism’s growth and functioning. Because DNA is passed from parents to offspring, it keeps a record of ancestry. This is why genetic studies go deeper than looking at outward features:

So genetic studies show not just what an organism looks like, but where it came from and how it is related to all other living beings.

Every species is adapted to a particular range of temperature, rainfall and seasons. When the climate changes, these conditions change faster than many species can adapt, and biodiversity falls. Some ways this happens:

Since every species plays a role in the food web, the loss of a few species can weaken the whole ecosystem. That is why climate change, along with pollution, deforestation and overuse of resources, is listed among the main threats to biodiversity.

Answer: (ii) Body with jointed legs.

Insects belong to the phylum Arthropoda (arthro = joint, poda = appendages), whose defining features are jointed appendages and a hard external skeleton. An earthworm (Annelida) has no legs at all, so jointed legs settle the question at once.

The other options do not help to decide:

Answer: (iii) Presence of a cell membrane.

In the five kingdom classification, animals are multicellular, heterotrophic eukaryotes whose cells have no cell wall — each cell is bounded only by its cell membrane. Sponge cells are like this, which places sponges in Animalia even though they have no true tissues.

Why the other options are wrong:

This question asks you to observe animals around you, so your own pair may differ. Here is the method, worked out for one common pair.

Method: compare the two animals feature by feature — body covering, presence of a backbone, limbs or legs (and whether they are jointed), segmentation, body symmetry, where they live, how they breathe and how they feed. Then use these features in order, from broad to specific, to place each animal in its group.

Worked example — an ant and a house sparrow:

How these features place them in groups:

So each feature acts like a sorting question: broad features (backbone or not) separate large groups, and finer features (jointed legs, feathers) place the animal in a smaller group.

A good basic characteristic must apply to every organism and must split life at its deepest differences. Cellular organisation does both; xylem and phloem do neither.

So the scientist would argue that cellular organisation is universal and fundamental, which makes it suitable for the highest levels of classification. Xylem and phloem are useful only lower down — for dividing Kingdom Plantae into groups such as bryophytes (no vascular tissue) and pteridophytes (with vascular tissue).

It most likely belongs to Kingdom Protista. It is probably something like Paramecium, a common protist covered with cilia.

Reasons:

A unicellular eukaryote that moves with cilia therefore fits the definition of Protista.

In an ecosystem, every species plays a role, and the different roles fit together. The more varied the organisms, the more stable the system.

When biodiversity falls, these links break. The loss of one species can cause others that depend on it to decline, and the ecosystem becomes unbalanced and less able to recover from change.

Such a kingdom would put together organisms that differ in their most basic features. The main problems are:

In short, ‘unicellular’ is one useful feature, but grouping by that single feature alone would create a huge, confused kingdom of unrelated organisms. That is why the five kingdom system also uses cell type, cell wall and mode of nutrition.

All five kingdoms are built on cellular organisation: Monera are prokaryotic cells, and Protista, Fungi, Plantae and Animalia are made of eukaryotic cells. Viruses do not fit anywhere because:

Because the questions that sort organisms into kingdoms — cell type, number of cells, cell wall, mode of nutrition — cannot be answered for a virus, viruses are kept outside the five kingdom classification.

This asks for your own reasoned choice; either position is acceptable if it is justified. A well-argued answer is given below.

My choice: keep viruses outside the five kingdoms, but list them in a separate non-cellular category alongside the system.

Justification:

What this shows about classification: classification is not fixed. It has changed from Aristotle’s grouping by habitat, to the two, three, four and five kingdom systems, and then to the three domain system based on DNA. Each change happened because new tools (microscopes, genetic studies) revealed organisms or differences that the older system could not handle. The difficulty of placing viruses shows that classification is an ongoing process of reasoning and change, revised as our knowledge grows.

Features that keep viruses out of the five kingdoms:

What this tells us about the limitations of classification systems:

They share two features — no flowers and no seeds, and dependence on water for reproduction. But they differ in their body organisation and, above all, in the presence of vascular (transport) tissue. That is why they are placed in separate groups.

Explanation: bryophytes were the first steps onto land; without vascular tissue they stay small and need moisture all the time, so they are called the ‘amphibians’ of the plant kingdom. Pteridophytes are a step further: their true roots, stems and leaves with xylem and phloem transport water and food through the whole plant and adapt them better to land. The presence or absence of a transport system is a major difference in body organisation, so the two are classified separately even though neither produces flowers or seeds.

The genus has fewer members but more features in common.

The classification hierarchy runs Kingdom → Phylum → Class → Order → Family → Genus → Species. It is like a pyramid: each lower group is a part of the group above it. As we move down:

Genus lies three levels below class, so it is much smaller and much more uniform. Example (tiger): the class Mammalia includes very different animals such as tigers, dogs, cows, whales and humans, which share only general features (hair, mammary glands). The genus Panthera includes only a few big cats — the tiger (Panthera tigris), the lion (Panthera leo) and a few others — which share many detailed features, such as a similar skull structure and the ability to roar.

Locomotion and autotrophic nutrition alone are not enough, because they are found in more than one kingdom — some cyanobacteria (Monera) are autotrophic, and many protists both move and photosynthesise. The scientist must check the characters that define Protista:

If the organism is a single eukaryotic cell that moves (for example with a flagellum) and photosynthesises, it is a protist — like Euglena or Chlamydomonas.

Normally a unicellular eukaryote is placed in Protista. To place it in Fungi instead, the key must look past the number of cells and check the cell wall material and the mode of nutrition — exactly the reason the book gives for keeping yeast in Fungi: although yeast is unicellular, its cell wall is made of chitin.

Suggested identification key (answer each step, then go where it points):

1. 1. Does the cell have a true, membrane-bound nucleus?
   No → Monera.  Yes (eukaryote) → go to 2.
2. 2. Does the cell have a cell wall?
   No → Protista if unicellular, Animalia if multicellular.  Yes → go to 3.
3. 3. What is the cell wall made of?
   Cellulose → Plantae if multicellular, Protista if unicellular.  Chitin → go to 4.
4. 4. Does it lack chlorophyll and absorb its food from dead or living matter (heterotrophic by absorption)?
   Yes → Fungi, whether unicellular (yeast) or multicellular (mushroom, Aspergillus).

So the deciding characters for keeping a unicellular organism in Fungi are: eukaryotic cell + cell wall made of chitin + no chlorophyll, absorptive heterotrophic nutrition. This matches the five kingdom concept map, where ‘with cell wall → chitin → heterotrophic decomposers’ leads to Fungi.

(i) Organism Q belongs to Kingdom Fungi. Supporting observation: it has no chlorophyll and grows on dead organic matter, i.e. it absorbs food from decaying material like a saprophyte. Its multicellular, filamentous body with a cell wall also matches fungi, whose fine filaments form a mycelium (like bread mould or Aspergillus).

(ii) Organism P belongs to Kingdom Monera. Justifying characteristic: it has no true nucleus, so it is a prokaryote. Its ability to survive high salinity and temperature fits monerans such as bacteria and archaea, which live even in extreme environments like hot springs.

(iii) Criteria that separate R and Q:

Being eukaryotic only places both above Monera. The five kingdom system then separates them by level of organisation (one cell versus many), cell structure (cell wall) and mode of nutrition (photosynthesis possible versus purely absorptive). A single, motile eukaryotic cell that can photosynthesise is a protist; a multicellular, walled, non-green decomposer is a fungus.

(iv) Why S cannot be classified by mode of nutrition alone. S is an animal — it has a backbone (vertebrate), well-differentiated tissues, and breathes in water during its early life stage, as a frog tadpole does with gills, so it is probably an amphibian. But its mode of nutrition is simply heterotrophic, and heterotrophy is shared with Q (fungi), many protists and most bacteria. Nutrition alone would therefore put S in the same group as a mould. To place S correctly, we need other criteria: multicellularity with tissues, no cell wall, food taken in (ingested) rather than absorbed, and a backbone, which puts it in Animalia, Chordata, Vertebrata.

(v) T lacks cellular organisation. T is acellular — it has genetic material but no cell, and it is inactive outside a host cell, just like a virus. Since every kingdom is defined by cell type and cell structure, T cannot be placed anywhere. This reveals that classification systems are human-made frameworks based on the features known at the time; borderline forms that lack those features do not fit, and the systems must keep being revised as new knowledge appears.

(vi) Classification by habitat only. Grouping by the three environments would give:

So completely unrelated organisms — a single-celled protist and a vertebrate, a fungus and a worm — would be grouped together. Scientific consequences: such groups would not reflect cell structure, body organisation or common ancestry, so they would tell us nothing about how organisms are related or how they function. Closely related organisms living in different places would be split apart, and the same organism could even change groups during its life — S breathes in water early in life but may live on land later. This is exactly the limitation of Aristotle’s habitat-based system, which is why scientists moved to classification based on structure and, later, genetic similarity.

(vii) The new organism should be placed under Fungi.

Absorptive nutrition therefore places it in Fungi. To confirm, scientists would check its cells for a cell wall made of chitin; animal cells have no cell wall.