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A simple model for natural selection

Class practical

In this simple procedure, grains of rice are used to model small organisms, and students act as predators selecting food during limited time spans. Over a number of generations, the proportion of grains of rice of each colour in each population will change if students select one colour in preference to another. This is a simple model for natural selection resulting from predation.

Lesson organisation


Picking out the grains from each tub of vermiculite should only take a couple of minutes each generation. But counting the grains of rice to restore populations and analysing the results can be laborious. Collating group results on a central spreadsheet would remove the labour involved in calculations, but may run the risk of students not following the detail of the process.

Download the spreadsheet Calculating record sheet for model for selection (19 KB)

Apparatus and Chemicals


For each group of students:

Tub of stained horticultural vermiculite, to which about 50 grains of rice stained the same colour, and about 50 grains of rice stained with a different colour have been added

Forceps

Petri dish

Stop watch

For the class – set up by technician/ teacher:

Plastic containers (such as empty ‘spread’ tubs)

Horticultural vermiculite coloured with food colouring (Notes 1 and 2)

Long grain rice (dyed in assorted colours with food colouring – Note 2)

Health & Safety and Technical notes


1 Horticultural vermiculite is a granular mineral used as an additive to composts to improve drainage and nutrient retention. It is available from most garden centres or DIY stores.

2 Stain the vermiculite with a strong solution of food colouring; yellow, green, red and blue dyes usually take well. Put the dye and vermiculite in a polythene bag, seal with an elastic band or similar, and shake well. Spread the vermiculite on a newspaper to dry. Dye uncooked long grain rice in a similar way.

Ethical issues


Be sensitive to the fact that some students may find it harder to distinguish colour contrasts than others; they could be distressed by finding fewer grains of rice, or attaining slower rates of population change. Using a range of colour combinations limits the likelihood of this being noticed.

Procedure


SAFETY: If food colouring is used to dye the rice and vermiculite, there are no significant hazards with this procedure.

Preparation

a Add approximately 100 grains of stained rice to each container of stained vermiculite. Set up different combinations of rice colours, with half the grains matching the vermiculite each time.

Investigation

b Start the stopwatch/ stopclock and time one minute. During this time, use the forceps to pick out as many grains as possible. Place the grains in a Petri dish.

c Count how many grains of rice of each colour you have removed. If the total number of rice grains is an odd number, pick out one more grain.

d Work out how many grains of each colour are still in the container.

e Add more rice grains to the container to make it up to 100 again, by adding equal numbers of each colour. Make a note of how many grains there are of each colour now. The proportion of the two colours will now be different.

f Shake the container to mix the grains with the vermiculite.

g Time another minute and pick out as many grains as possible again.

h Repeat steps c to e again and note how many grains there are of each colour now.

i Record the results of the experiment in a table like this:

  Number of grains removed Number of grains replaces Perrcentage of grains in each colour
  Colour 1 Colour 2 Colour 1 Colour 2 Colour 1 Colour 2
Starting ................ ................ ................ ................    
After pick 1            
After pick 2            
After pick 3            
             

Teaching notes


In this model, the grains of rice represent a population of small organisms. Picking out the grains of rice represents predation. Adding more grains by adding equal numbers of each colour represents reproduction, keeping the size of the population constant. Preferential picking of one colour (probably contrasting with the vermiculite) over another (probably camouflaged against the vermiculite) represents natural selection. The change in relative proportions of grains of each colour represents adaptation of the population – the result of ‘survival’ of the ‘fittest’ grains.

It is important for students to understand that ‘fittest’ does not mean fastest or strongest or most able to run a long distance. It means ‘the ones that best fit the environmental situation’.

The spreadsheet below allows you to fill in the result of the picks, and will calculate the number of grains of each colour needed for the next generation.

An interesting variation to the method is to try again under coloured lights, or wearing coloured filters over your students’ eyes.

It is worth discussing the model and identifying its strengths and weaknesses as a model for real life situations. Identifying a weakness in the model does not negate its usefulness as an illustration of how selection can affect a population.

One significant divergence of this model from ‘real’ situations is that the colours of the new organisms in each generation are in the same proportion as the original population. In nature, it would often be the case that new organisms would have features in number proportional to the most recent parent generation. If your students can cope with the arithmetic of this (including rounding up and rounding down where necessary) you could add this feature to your method.

Another weakness of the model is that the prey is static and identified by the predator largely on the basis of colour, with form and movement making a relatively low contribution.

Health & Safety checked, June 2010

Downloads


Download the spreadsheet Calculating record sheet for model for selection (19 KB).
Download student sheet A simple model for natural selection (75 KB) with questions and answers.

Related experiments


A model for natural selection - spaghetti worms

Selection in action - banded snails

 

Page last updated on 29 November 2011