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A model for natural selection – spaghetti worms

Class practical

This involves placing baits of different colours (the ‘worms’) in a place where birds are known to search for food. Uneaten ‘worms’ are collected (or counted) regularly and the ‘worm population’ is restored with more coloured bait in the same proportions as the bait that remains. After several cycles of predation and breeding, the proportions of each colour in the population will have changed – simulating directional selection.

The model uses real organisms (the predatory birds), is easy to prepare, and the consumables are low in cost for future repeats.

Practical adapted with permission from Survival Rivals activity “I’m a worm – get me out of here” produced by the Wellcome Trust for Darwin 200.

Lesson organisation

This is a long-term practical – taking one or two weeks to ‘set up’ the area and to introduce the concepts behind the investigation to the students. You can then collect data over several weeks. If timetabling allows, you might be able to collect information every day for 2 or 3 weeks by working with more than one group and pooling the results.

There is scope for students to take an active part in planning the details of the experimental design, and to develop their own tactics for analysis of the data collected. Students could also carry out preliminary investigations to determine the best position for the bird feeding area (Note 1), the kinds of birds likely to visit the area, and the types of bait that are likely to be taken.

Students could take on particular roles for the procedure such as ‘worm’ production, monitoring the feeding area, ‘worm’ replacement and data recording.

Apparatus and Chemicals

For each group of students:

Plastic bags, resealable, 10 cm x 14 cm, 10

Polythene gloves, disposable, 5 pairs

Golf tees, 5

Twine, biodegradable

Ruler, 150-300 mm

Plastic knife to cut spaghetti, 1

For the class – set up by technician/ teacher:

Webcam (optional) to observe bird feeding activity

The following items could be set up by the students or the technician/ teacher

Bird table/ feeding area (Note 1)

Wild bird food

Spaghetti, 500 g (Note 5)

Food colouring, red, black and green (up to 15 cm3 of any colour – see ‘Making spaghetti worms' part b for more detail)

Pasteur pipettes, 5 ml, 3

Health & Safety and Technical notes

Commercial food colourings, although they may stain the skin, are not harmful used in this way – either to the students or to the birds who eat the pasta.

Hand-washing: make sure students wash their hands thoroughly using soap and hot water after collecting ‘worms’. The feeding area will be contaminated with bird droppings, and may be contaminated with faeces from other visiting animals. Any student who is picking up ‘worms’ from the ground should wear disposable plastic gloves. Blunt plastic forceps could also be used.

1 For this project, you need an area where the ‘worms’ can be placed, undisturbed but observable by students, for the birds to prey upon. If, for example, the birds are scared away by human activity or by predators such as cats, the practical investigation will not be successful. A bird table with a webcam might be necessary. An ideal area will be near a hedge or trees (to provide cover for birds that do not like to feed in the open for long), and close to a window (for easy observation). If the ‘worms’ disappear overnight, they may be being taken by nocturnal animals such as foxes, cats or rats. An area of 1 m x 1 m is ideal – small enough to collect and count the worms from, but big enough for a significant number of birds to feed from. Some investigators have used smaller plots (500 mm x 500 mm).

2 The RSPB recommends carrying out this investigation between December and March, when wild food is scarce and birds will be more likely to take food put out for them. It also recommends putting out food for one day’s feeding at a time, and not leaving food out overnight. Adding chilli powder to the ‘worms’ will deter mammalian predators but not birds, because birds cannot taste chilli.

3 Acclimatise birds to the area where the practical is to be carried out, by placing food in the feeding area every day for one to two weeks.

4 Background colour of the feeding area: It is important that there is a difference in the rates at which the two types of prey are selected by the birds. You could prepare coloured or patterned boards on which to place the worms. Or you can use the ground (either grass or bare soil) of the feeding area. If so, it is mark out the area clearly so that it is easy to locate and search for remaining ‘worms’. Mark the corners with golf tees, and the edges of the area with twine wrapped around them.

5 Making spaghetti ‘worms’: Spaghetti is convenient, robust and inexpensive. 20 strands will make a hundred 50 mm ‘worms’. Add the food colouring as the spaghetti is cooking. You can prepare the ‘worms’ in bulk in advance, cut into 50 mm lengths and store in plastic bags or sealed boxes in a freezer until required, or in a fridge for a few days. If the strands stick together, loosen them with a little water. The dye does not readily wash out of the pasta, even in heavy rain, and it can be handled without excessive amounts of dye transferring to your hands. The ‘worms’ will disintegrate in the field after 24-48 hours, either by drying out or getting soaked. This is why you should replace them every day.


Wear disposable plastic gloves to pick up the ‘worms’ from the feeding area and wash your hands thoroughly after working outside.


a Begin by acclimatising the local bird population to a new source of food – by setting out bird food in the area selected for the practical (see Notes 1 to 4).

b Observe the birds visiting the feeding area; identify them using prior knowledge, bird identification books, or the key provided with the Survival Rivals materials.

c Make some spaghetti ‘worms’.

Making spaghetti ‘worms’

a Add food dye to the water in which dried pasta is cooked. Do not add any oil. Simmer for 10-12 minutes. Pour away the coloured water. Use fresh water and more food dye for further batches of pasta.

b Try different colours and intensities. 15 cm3 of green colouring in a litre of cooking water gives a bright grassy green colour. 5 cm3 of black and 10 cm3 of red colouring in a litre of water will give a dark red/brown colour.

c Cut the pasta into suitable lengths and store in plastic bags or sealed boxes, in a fridge (for a few days) or freezer.


a Select and mark out a feeding area in which to place your ‘worms’.

b Present equal numbers of two different colours of spaghetti ‘worms’ to the birds.

c Each day or at regular intervals, count the number of ‘worms’ remaining of each colour. The easiest way to do this is by collecting all the remaining ‘worms’. Wear disposable plastic gloves to collect the ‘worms’.

d Make a new population the same size as the original, but with fresh ‘worms’ of each colour in proportion to the numbers of each colour you collected (Note 5). (See 'Teaching notes' for details of the calculation required.)

e Wash your hands thoroughly when you come in from collecting ‘worms’ (Note 6).

f Record information about the number of ‘worms’ of each colour left each day, and calculate the number of ‘worms’ of each colour which been eaten each day.

g When you have collected data for a week or more, plot the data on a graph and interpret the results. A graph of the number of each type of ‘worm’ present against time might be a useful place to start.

Teaching notes

The method here is based on a general technique developed by John Allen and his colleagues and students between the late 1960s and early 1990s to simulate several aspects of evolution by natural selection. See references below.

This exercise demonstrates selection – a key principle of Darwinian evolution. From the simple effect on the ‘worm’ population, there is scope to develop ideas about how the bird population is also affected and how avian vision might evolve.

In variants on this procedure, you can use dyed bread in place of spaghetti for baits. Or you could use coloured wools, with students acting as predators and catching as many worms as possible in a limited time. In the papers listed below, Allen et al have suggested many other variations which allow exploration of different aspects of natural selection.

Prepare your students for the work by explaining the process of natural selection and providing an outline of the investigation. The attached student sheet (73 KB), or the Survival Rivals material might be useful here.

Plan the details with your students including:

  • selection of suitable ‘worm’ colours
  • selection of suitable background colours
  • location of the feeding area
  • size of the feeding area
  • how to observe bird feeding if possible
  • deciding how many ‘worms’ to put in the feeding area; a high density reduces the effect of sampling errors – try 50-250 per square metre
  • deciding how to distribute the ‘worms’ in the feeding area – spaced out or randomly scattered
  • deciding on the length of ‘worms’ to use; consistency means birds are not selecting the ‘worms’ on size, and also allows you to identify half-eaten ‘worms’
  • procedure for recording half-eaten or pecked ‘worms’, or ‘worms’ thrown out of the feeding area by the birds
  • what data to collect
  • how often data will be collected
  • how to record the data
  • what to do when there are gaps in the data, such as over weekends
  • method of counting ‘worms’, calculating proportions for replacement, and replacing them
  • how to present and analyse the results.

Calculating the numbers of each colour in the replacement worm populations:

Total number of ‘worms’ in starting population = N
Number of ‘worms’ of type A collected = a
Number of ‘worms’ of type B collected = b
Total number of ‘worms’ collected = (a + b) = n
Number of ‘worms’ of type A to be put out for next generation = a x N ÷ n
Number of ‘worms’ of type B to be put out for next generation = b x N ÷ n

Allen et al reported that the birds in their studies preferred brown over green baits in certain habitats. To humans, green ‘worms’ are camouflaged against a grassy background. However, birds’ vision is very different from ours. They may see some colours in the ultra-violet range. Students could carry out more research into bird vision, and test whether particular bait colours fluoresce under UV light. This only gives a crude indication of what the birds might be seeing – but it emphasises the point that these factors are important in the experimental design.

Health & Safety checked, May 2009


Download the student sheet A model for natural selection – spaghetti worms (73 KB) with questions and answers.

Web link

“I’m a worm, get me out of here” was one of the Wellcome Trust’s experiments developed to commemorate Darwin 200. You can now buy the kit from Philip Harris.

(Website accessed October 2011)


References to John Allen’s work:
Allen, JA and Clarke, BC (1968) Evidence for apostatic selection by wild passerines. Nature 220 501-502
Allen, JA (1976) Further evidence for apostatic selection by wild passerine birds – 9:1 experiments. Heredity 36 173-180
Allen, JA, Cooper, JM, Hall, GJ and McHenry, C (1993) ‘Evolving pastry’: a method for simulating microevolution. Journal of Biological Education 27 (4) 274-282.
Cooper, JM, Raymond, DL and Allen, JA (1993) Birds, baits and boards: a method for investigating cryptic colouration. School Science Review 74 (268) 69-73.
Allen, JA, Anderson, KP and Tucker, GM (1987) More than meets the eye – a simulation of natural selection. Journal of Biological Education 21 (4) 301-305.


Page last updated on 29 November 2011