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(a)
(b)
horned
(a)
hornless
(b)
Body size
Body size
Fig. 5.16 Douglas Emlen's (1996) artificial selection experiment with Onthophagus dung beetles. Left-hand
graph: in one line, he bred from males with larger horns (a) and in the other line from males with smaller horns
(b) than expected for their body size. Right-hand graph: after just seven generations the threshold switch to
horns differed between the two selection lines.
(a)
(b)
0.7
0.6
8.0
A
0.5
7.0
0.4
6.0
5.0
0.3
4.0
0.2
3.0
0.1
-3
-2
-1
0
1
2
3
0.0
0
10
20
30
40
50
60
70
80
Population density (number caught per trap)
Standardized pronotum width
Fig. 5.17 A threshold morphological switch for male forceps length in European earwigs. From Tomkins and
Brown (2004). Reprinted with permission from the Nature Publishing Group. (a) The sigmoidal relationship
between male forceps length and body size (measured as pronotum width) for two island populations in the
North Sea, UK (Bass Rock, open circles; Knoxes Reef, Farne Islands, solid circles). The x axis is the standardized
pronotum width for each population (mean zero ± 1, 2, 3 standard deviations). The switch to long forceps
occurs at a relatively (and absolutely) smaller body size on the Bass Rock, so a greater proportion of the males
here has large forceps (proportion with large forceps indicated by length of the yellow bars in the lines below
the x axis). (b) Relationship between population density of earwigs and proportion of macrolabic (long forceps)
males in the population for 22 islands in the North Sea.
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