Classifying living beings: general principles

Authors : Guillaume Lecointre(plus d'infos)
Summary :
Proposal of pedagogic tips for the realization, on the basis of a small sample, of a tree that illustrates the ties existing between living beings. Activities are specifically described in the sequences "Classify the animals of the temperate forest" (children 7-9 years old), and "Classify the animals of the fish market" (children 9-11 years old).
Publication : 1 August 2002
Note :
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By assigning to the students the task of classifying a collection of species, the teacher can achieve a number of conceptual acquisitions, first of all: classify on the basis of what organisms have, and not on what they don't have/what they do/where they live/what they are used for or what they allegedly are. We thus remove from a classification that purports to be based in science practical ("seafood"), anthropocentric ("invertebrates"), ecological ("burrowing", "fish") classifications.

Initial questioning


Knowing a living being is first of all answering the question: "What characterises it?" - and this before answering questions like: "How does it function?" Answering the question "What characterises it?”  means, in order, (1) knowing what it is made of and what it has in common with others, (2) where it comes from and (3) which category it belongs to. One answers the last two questions thank to the first one. Thus, one can only define, group living beings and discuss their origin by focussing on what they are in practical terms. Grouping them on the basis of what they don't have would be senseless: what they don't have does not in any way give their specificity and could not attest to their origins. To justify this approach to the child, we can start from the following principle: knowing oneself is partly knowing what one is made of and where one comes from. The child might be asked to try to describe a classmate by saying what he doesn't have, then to make groups of classmates on the basis of what they don't have. Then, he can compare this approach to that of describing and grouping classmates on the basis of what they have. It is easy to conclude that the second approach makes sense while the former does not.


Objectives, for multiple sessions:
- describe the species: establish the level of description and generate a vocabulary of anatomy;

- distinguish, arrange, sort and classify;    

- generate classification criteria that allow to classify  on the grounds of what living beings do have;

- stimulate a debate during which observations made and knowledge will be contrasted;  

 - generate nested sets;   

- bring out the causality underlying the fact of sharing certain traits (what they have);   

- draw a tree diagram on the basis of the sets;   

-  classify supernumerary species, including fossils, in a classification already set.

1. Observe and describe


The first stage is observation/description work. For children unfamiliar with living beings, an initial stage of description of animals and plants from pictures is necessary in order to generate description phrases, then at the end, an abstraction. In fact, one can look at a depicted animal as an individual or as a representative of a taxon already defined. For instance, one can look at a robin as something unknown with a red throat or as a bird. In the description of what animals are, we can conclude that a certain cat is ginger while another one has a ball of wool. To make the abstraction possible, once the descriptions are made, one can try to sort attributes that can be generalised from attributes resulting from particular situations by:
- limiting size effects;

- limiting colours - by using black and white photographs;   

- differentiating what is peculiar to the physical structure of the organisms from what belongs to the contingent situation as portrayed in the photo.
Multiply photographs of individuals of the same species in order to, at the end, allow to notice what is peculiar to a species and then remove any duplicates.
One can try to provide a new animal and to link it to others, previously described.
Ultimately, by eliminating size, colours and situations, one obtains an adjusted level of species’ description and the vocabulary of anatomy (head, eyes, wings, hair, feathers, limbs, legs, fins, antennas).  The next stage will consist in carrying out the same exercise and in classifying a thoughtfully composed sample.

Note: this session would look radically different if working on a true sample of species collected in  nature. The objectives, however, are identical.

2. Distinguishing between sort, arrange, classify


A collection of species (figure 1: a moth, two butterflies, a ladybug, a cockhafer, a beetle, a long-nose gar, a john dory, a man, a rabbit, a cat, a bat, a pigeon, a chicken) is provided in the form of photocopied drawings on coloured paper. Students, in small groups, are asked to sort, arrange or classify species according to the colour of the paper: those having pink papers should sort, those having blue papers should arrange, those having white papers should classify. With scissors and glue, each group reorganises drawings according to what they have to do.
Immediately, the notion of criterion emerges. We are going to sort on "having hairs" or not, on "living in a farmhouse" or not. We are going to arrange from the biggest to the smallest or from the most beautiful to the ugliest one.
All the representations are collected and displayed. It will appear that the three operations are mixed: some sorted, others arranged, believing that they were classifying and vice versa, etc. The papers are then redistributed according to three categories:
-Those who sorted:
Sorting is done according to the presence/absence of the criterion or criteria. For instance, those that have fur (cat, bat, rabbit, man) and those who don't have (the others). This activity is used in species identification keys but does not constitute in any circumstances a classification.
-Those who arranged:
The criterion is used in a continuous way. For instance, species are arranged from the biggest to the smallest one, or from the "nicest" to the most "vicious".
-Those who classified:
Groups are constituted on the basis of what the species have (and not on what they don't have) and those attributes are the arguments of the classification. Attributes are multiple and are nested. For instance, among those presenting four legs, there is the sub-group of those having hair (we then say that the arguments are naturally hierarchical). Nested sets can emerge.


3. Classification: criteria and arguments based on attributes


The sorting and arranging activities completed, the truly classification session can start. At first,, we let classification criteria emerge; after that, we provide the instruction to classify only on the basis of what animals have. Actually, if we only let
 classification criteria emerge, we obtain a heterogeneous mixture of criteria. Animals will be classified together because:

- they do the same thing ("they fly", or "they eat meat");

 - they live in the same place ("they live in the farmhouse");  

 - they are  allegedly this or that ("they are insects"); 

- they are used for the same purpose ("we can eat them");  

 - they do not have this or that ("they do not have a backbone", or "they do not have legs");

 - they have this or that ("they have six legs").

One can let these criteria emerge and then sort them collectively.
If the objective is to obtain a classification in connection with the evolutionary history of organisms, the instruction should be eventually provided to classify on the basis of what animal have (the latter of the six criteria listed above).

Without instruction, mostly separated, not-inclusive sets will emerge . The “young classifiers” can be encouraged to especially make nested sets. This amounts to organising attributes into a hierarchy: all those that have hairs also have four limbs, etc. One way to encourage the nesting without giving explicit instruction is to ask “what all animals have”. For instance, they all have a head. From the argument of the head, the most inclusive group emerges. The other groups will be thus necessarily nested in the first one.

Pragmatically, children are divided into small groups; they cut and paste the pictures of the animals and group them in sets according to their attributes. In each group, the children discuss about what animals have. This discussion urges them to observe images and to compare their knowledge on those species. Each group of children thus produces sets according to arguments. For instance, the set (rabbit, cat, man, bat) combines the attributes "hair" and "breast". Each group of children is asked to draw the sets (balloons, with different colours) and to write the attributes beside the set.  They are encouraged to produce sets on the basis of multiple arguments.
If necessary, this exercise will lead children to reformulate their arguments when choosing the wrong criterion, to bring them back to anatomy (that is, what animals have). For instance:

One cannot say: I put them together because they swim (I classify them on the basis of what they do);

- One says: I put them together because they have fins (on anatomy) ;

One cannot say: I put them together because they are mammals (I classify them on the basis of what they allegedly are (or: what I presume they are)

- One says: I put them together because they do not have breasts (on anatomy) ;

Later, all the arguments from all groups in the classroom will be read, collectively commented and written on the board.  For the teacher, enlarged pictures of animals may have been previously cut out on cardboard and sticked up on the board. The sets are redone using all the arguments produced and validated by the class (figure 2).

Those arguments are:

- (Cockhafer, beetle, ladybug): elytra

- (Butterfly 1, Butterfly 2, moth): four wings

The previous two sets: six legs, antennas, external skeleton.

- (Pigeon, chicken) : feathers

- (Cat, man, rabbit, bat): breasts, hair

The previous set, plus the pigeon and the chicken: four limbs

- (John dory,  long-nose gar): rays in the fins

The previous two sets: Internal skeleton

All: head, eyes.

Pigeon, chicken and bat: wings.

We notice the nesting.

Note 1:
The bat is a problem. Indeed, some children will soon be disturbed by the fact that the bat shares with the cat hair and breasts, and wings with the pigeon and chicken. Three solutions: a. the bat is removed from the sample since the beginning; b. we propose to classify it immediately, as shown here; c. we show it only at the end of the session, in order to introduce the existence of more complicated cases. In the first two cases, it should be explained how to choose the position of the bat. This can be done by evoking an idea which is close to the principle of  parsimony.  We notice that the class has used two arguments to place the bat with the cat, the man and the rabbit, while only one argument is used for placing it with the bird. We will thus choose to place the bat in the group with which it shares the greatest number of attributes.


Note 2:
In a first version of this sample, there was only one bird: the pigeon. The fact the bird is the only feathered animal of the sample is not especially problematic - one can consider that it forms a set on its own. If this is a problem, a chicken can be added to the sample, as suggested here.

Note no 3:
The main point of the session is to avoid classifying animals on the basis of what they do not have.  Experience says that even without a particular instruction, spontaneously, children do not classify on the basis of what animals do not have. Privative groups without scientific value – such as “invertebrates" or "agnatha" are "cultural pollutions", a bigger problem for adults than for children. 

4. Underlying causality (the classification says something about the world)


We can ask to the class why species have things in common. They may answer, for instance:

- Because God made them like that (one can take the opportunity for defining what is specific to scientific claims).

- Because they are found in the same environment.

- Because they make babies.

- Because they are of the same family.

- Because they are cousins.

- Because they come from the womb of the same mother - but did the mother live in prehistoric times?

We can make the children recall that the unfolding of the story of a family over great periods of time is called “genealogy”. The word can also be given by the class. What’s in a genealogy? Ancestors.
Why is it that what they have, they have it in common? Because they inherited it from their ancestors, and specifically from common ancestors.
As soon as the word "genealogy" is uttered, children spontaneously talk about the tree. Some children even go on by saying that species are transformed. Behind nested sets, there is transformation of animals in the course of their genealogy.
Once the key words "ancestors", "cousins", genealogy", “transformation” or "evolution", and even "tree" are generated, we are ready to explain that what they have in common (and that others do not have), they inherited it from common ancestors (that are not others’ ancestors). For instance, six legs were inherited from an animal that was the ancestor of the cockhafer, the ladybug, the beetle, the two butterflies and the moth, but that is not the ancestor of the others (cat, rabbit, etc.), otherwise the others would also have six legs.

5. Form the classification to the tree


Nested sets represent a tree seen from above. Each set is a branch. The more the set is inclusive, the more the corresponding branch is deep. Two sets of the same hierarchical level are brother groups (figure 3). A series of sets, by projection in the third dimension, becomes a tree.

From a practical point of view, we can:
1. Either allow the teacher to draw the tree behind the sets
2. Or use a movable one
3. Or create a special session to allow the children drawing the tree themselves. In this case, a different colour per set and its corresponding branch will help locate branches visually. The student who has drawn the tree correctly can be asked to explain it, then check that no information has been lost when passing from the sets to the tree
4. Or, from the start, draw the tree corresponding to the sets on the board and allow the children to place animals at the end of the branches, in accordance with the sets - an exercise that necessarily requires understanding the relationship existing between branches and sets.
We can then place the attributes on the branches of the tree (figure 4). Here, we can see that birds and the bat have learned how to fly independently.

6. Placing a new specie in the classification


We can propose to place a mammoth in the final tree. Children can just check the attributes that the mammoth presents and classify it based on the fact that: it has a head, an internal skeleton, four limbs, breasts and hair. What is shown here is that the place of the fossil is not at a node of the tree but at the end of a branch, as with the other animals.

7. Learning new words


The aim here is not for children to learn the correct scientific terms, but to ensure that they can define groups of animals arranged into classes:

- Head, eyes: animals (or metazoans)

- Six legs, antenna, external skeleton: insects

- Four wings: butterflies

 - Elytra: beetles

-  Internal skeleton: vertebrates

- Rays in fins: finned fishes (or actinopterygians)

- Four limbs: four-legged animals (or tetrapods)

- Feathers: birds.

- Breasts, hair: mammals. 

8. Conclusions and precautions


Through this sequence, a number of objectives have been achieved:

-  We have talked about phylogenetic classification without saying it;

-  We have removed restricted anthropocentric groups (invertebrates, agnatha, etc.);

- We have developed a classification approach starting from observation and leading to classification, and not the other way around using presumptions  (as when projecting what we think we know based on our perception of the living to strengthen a mental classification often reassuring but false from a phylogenetic perspective);

- We have set the idea that a classification says something about the world (evolution in the genealogy), and the metaphor of the tree contributes to it;

- We have provided results consistent with what science produces today.

Tens of different sessions may be invented on this model, depending on the sample of animals to classify. We can collect species by the sea, in the forest, at the edge of ponds, etc., and create  sessions with a limited number of samples.  However, the main difficulties for the teacher will arise from the balance between similarities due to evolutionary convergence, similarities that are too widely shared in the living to serve for a grouping within the sample considered, and those that demonstrate a real relationship. In other words, the risk is that the approach can be successful and understood in class, while leading to a false classification from a phylogenetic point of view. For pedagogical reasons, the sample with the species to be classified is formed beforehand . The preparation of such a session thus needs a lot of preliminary documentary work: collections of species to be classified should be controlled, to asses the difficulty resulting from potentially conflicting arguments, and ensure that those that are most likely to emerge from the class are actually arguments leading to a phylogenetic classification.

Link to other activities


- Classifying animals of the forest. From the web site La main à la pate, this activity is meant for level 2 students.

- Classifying animals of the fish market. From the web site La main à la pate, this activity  is meant for level 3 students (and teachers) who have already carried out this kind of activity. 

- African savannah, Temperate mountains, The Arctic. These activities are proposed by the web site Ecole des sciences – Bergerac.

Bibliography (from the same author)
CHANET B., LUSIGNAN F. (2007). - Classer les animaux au quotidien : Cycles 2 et 3. Rennes: CRDP de Bretagne.