Adaptation, natural selection
and the flight times of winged fruits
Technical & Teaching Notes
Introduction
In this resource, students focus on the role of natural selection in evolution, by considering the
extent to which the winged fruits (samaras) from a tree are adapted for successful dispersal.
This activity is designed to promote independent thinking among students, reinforce mathematical
skills, offer opportunities for out-of-classroom learning, and to include a range of potential learning
outcomes, from supporting basic conceptual understanding to making links with recent research.
Aims



encouraging the formation and testing of hypotheses
making pupils aware of the importance of variation and natural selection
developing measurement, data handling and graphing skills
Background information
What do ash trees, bees and bats have in common? Natural selection is the mechanism that
continually scrutinises the variations in an individual's offspring. The best-adapted offspring get to
pass their genetic blueprints on to the next generation. Fruit dispersal provides a tree's offspring
with the chance to colonise a new habitat, as well as reducing the danger of competition for
resources with their parent and siblings, and infection with species-specific pests and pathogens.
A single Common Ash tree (Fraxinus excelsior) produces thousands of fruits (samaras), in large
clusters attached to the tips of its branches, which remain for a large part of the year. The
variation in length of the winged samaras can be investigated, and each fruit can have its fitness
for purpose measured – by measuring the speed of descent when dropped.
A samara is a type of fruit in which a flattened wing develops from the ovary wall. The seed can
be in the centre of the wing (e.g. elms), or to one side (e.g. in ashes).
The wings of these falling fruits, like hovering insects and bats, generate more lift than would be
expected by regarding the wings as aerofoil sections. This was investigated in a recent study,
published in the journal Science (Lentink et al, 2009). The leading edge of the rotating wing has a
high angle of attack, and generates a stable vortex, which joins with the vortex at the tip – this
results in an inverted cone of low pressure above the wing, akin to a mini-tornado, reducing the
fruit's speed of descent. The use of such leading edge vortices by samaras, insects and bats
'represents a convergent aerodynamic solution in the evolution of flight in both animals and
plants'.
Science & Plants for Schools: www.saps.org.uk
Adaptation, natural selection and winged fruits: p. 1
This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2011.
Safety Notes
Care must be taken when pupils are dropping the fruits to the ground.
Students should be reminded not to lean over too far when dropping their fruits, and that, if in the
classroom, they should not stand on chairs, desks etc to get increased height. If doing the
practical in a classroom, steps or appropriate stools (e.g. library stools) should be provided. You
may wish to ask members of staff to drop the fruits, and students to record the data.
In view of the current concerns over the spread of ash dieback disease (Chalara fraxinea),
please avoid using ash keys at present (Nov 2012).
Requirements
Locations for the investigation
Students will measure the flight time for different sizes of winged fruit, and investigate the
relationship between flight time and fruit length. To do this, fruits need to be dropped from more
than 2m up. It is preferable to do this indoors, to reduce the complications of wind factor.
The best place to do this will depend on your own school buildings. Adequate data can be
obtained by dropping them from the ceiling to floor of a classroom. A perfect indoor opportunity is
provided by a balcony in a school hall. A first floor landing or corridor above an open stairwell can
also be used.
Apparatus
Per group



20 winged fruits (samaras)
Ruler
Stopwatch
Notes
Winged fruits can be collected and stored dry for a year or more – so that practical work can be
done at any suitable time. Fruits can usually be gathered from Ash trees (Fraxinus), Sycamore
(Acer species) and Lime trees (Tilia species). In view of the current concerns over the spread of
ash dieback disease (Chalara fraxinea), please avoid using ash keys at present (Nov 2012).
Teaching Notes
Part 1
Science & Plants for Schools: www.saps.org.uk
Adaptation, natural selection and winged fruits: p. 2
This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2011.
1.
2.
3.
4.
Each group of pupils in a class is given 20 fruits from a single tree.
The group measures the fruit. Fruit-lengths may vary in size from 10mm to 45mm.
All the class data is gathered together, using size-categories differing by 3mm.
As a class or individually, the data should then be used to construct a histogram, plotting
frequency against size-category. This should produce a smooth binomial distribution.
5. Ask students to think of hypotheses concerning the distribution of fruit size. Ideas might
include that small fruits mean small seeds, with reduced chances for successful
germination, or that the largest fruits require too much 'investment' per seed. Some
students may suggest that it is something to do with flight-times of the fruits.
6. Ask the students how they expect the size of the fruit to effect the flight-time. Most pupils
will predict that larger fruits will fly for longer. At this point, they should record their
predictions, for comparison with the final results.
7. Ask pupils to suggest how they might obtain accurate and reliable data about flight-times
and fruit length. Students will need to agree whether the stem of the fruit should be
measured or simply the winged portion. Students will also need to discuss whether all the
number of seeds on the fruit should be standardised, and if so, what number of seeds
should be chosen as the standard.
Part 2.
1. The class will now carry out an investigation into the relationship between fruit length and
flight-time. Working in pairs or trios, pupils drop, measure and collect their fruits.
2. One pupil should hold the stopwatch, while their partner drops the fruit from an agreed
height. A third student observes the fruit and calls 'stop' when the fruit lands. The flighttime and the fruit length is then recorded.
Part 3.
1. All the class data on flight-time and fruit length is now gathered together. Students now
calculate mean flight-time for each fruit size-category.
2. Again, working either as a class or individually, the mean flight-time values should be
overlaid onto the size/frequency histograms (using a second y-axis). Usually, the pupils
will find that the most frequent class-size is the one with the longest flight-time. This
comes as a surprise to most pupils, who intuitively think that 'bigger is better'.
3. Ask the pupils to plot average flight-time v. size-class frequency, and probe the
hypothesis that the two are linked.
Sample data
Data gathered using freshly-gathered fruits from lime trees. The entire length of the wing was
measured, including all stalk. No seeds were removed from the fruits, so the number of fruits
varied from 0 to 3.
1
2
3
4
5
6
7
8
9
10
11
Wing Length (mm)
90
75
69
81
77
69
78
73
63
73
71
Flight Time (s)
1.368
1.712
1.272
1.104
1.992
1.968
0.959
1.192
1.818
0.984
1.217
Science & Plants for Schools: www.saps.org.uk
Adaptation, natural selection and winged fruits: p. 3
This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2011.
12
13
14
15
16
17
18
19
20
80
62
64
67
82
71
65
79
77
Size-class
60-63mm
64-67mm
68-71mm
72–75mm
76-79mm
80-83mm
84-87mm
88-91mm
Total
2
3
4
3
4
3
0
1
Size-class
60-63mm
64-67mm
68-71mm
72–75mm
76-79mm
80-83mm
84-87mm
88-91mm
Total
2
3
4
3
4
3
0
1
1.184
1.174
1.046
2.184
1.992
1.216
1.584
2.000
1.544
Mean flight-times
1.496
1.604
1.418
1.296
1.624
1.427
1.368
Further Investigations

Dissecting seeds to investigate the importance of blade area and shape in determining
flight-times, or to investigate the relationship between fruit-size and seed mass.
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Investigating the germination success / seedling growth rate of seeds collected from
different sized fruit.
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Exploring the importance of the shape of fruit blades by comparing model fruits created
using balsa wood or paper (origami seeds)
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Developing fieldwork techniques through quadrat sampling of seed density v. distance
from an isolated tree. Is there a relationship between density and prevailing wind
direction?

Exploring evolution and adaptation by comparing and contrasting seeds from other
individuals of the same species and those from different ecological circumstances

Exploring links between biology and engineering, by watching and discussing
monocopter engineering video clips on YouTube.
Science & Plants for Schools: www.saps.org.uk
Adaptation, natural selection and winged fruits: p. 4
This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2011.

You may wish to begin or end the class with this online game introducing seed dispersal:
http://revolution.caret.cam.ac.uk/flash/seeds.swf
References
Delpech, Roger, Education in Science, February 2-1-, 19
Lentink, D., Dixon, W.B., van Leeuwen, J.L & Dickinson, M.H. (2009), 'Leading Edge Vortices
Elevate Lift of Autorotating Plant Seeds', Science, 324, 438-440
Acknowledgements
This resource was developed by Roger Delpech, Head of Biology at the Haberdashers Askes
Boys School.
Science & Plants for Schools: www.saps.org.uk
Adaptation, natural selection and winged fruits: p. 5
This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2011.