Biology Reference
In-Depth Information
(a)
(b)
13
K
11
12
10
11
1.5
C
9
10
8
9
7
1.0
8
A
B
C
D
E
0
10
20
Months
Increasing predation
F
S
I
II
Census
Fig. 4.17
Influence of predation on colour pattern of male guppies. (a) Both number
of colour spots per fish and spot size are smaller in streams with greater predation. The
main predators are other fish and prawns. Data from five streams in Venezuela with
increasing levels of predation from A to E. From Endler (1983). (b) A selection experiment
in the laboratory. F, foundation population of guppies kept with no predators. S, start
of the experiment; predators added to population C but not to population K. Note
the rapid change in population C after predation began. I and II are the dates of two
censuses. From Endler (1980).
some streams the main predator was a prawn, which was red blind, and here the male
guppies were significantly redder.
Finally, Endler showed how colours could change in response to changed selection
pressure. In controlled laboratory populations studied over many generations, males kept
with predators evolved to be duller, while those kept isolated from predators evolved to be
brighter, with both larger and more spots (Fig. 4.17b). Similar effects could be shown
under field conditions: when 200 dull males from a high predation stream were introduced
into a new isolated stream in Trinidad, which had no guppies and no predators, the
population evolved during a two-year period into one with much more colourful males.
Endler's experiments provide a convincing example of how natural selection can change
colour patterns in relation to a shifting balance between different selection pressures.
An evolutionary
experiment in the
field
Cuckoos versus hosts
Brood parasites
trick hosts into
raising the
parasite's
offspring
Some species of birds, fish and insects are brood parasites; they lay their eggs in
the nests of other species (hosts) and so get their young raised for free. As with
predator and prey, one party (the brood parasite) gains a benefit while the other
(the host) suffers a cost. Clearly, then, we would expect selection to favour host
defences. This, in turn, should select for counter-adaptations by the parasite. We
shall focus on one example to show how field experiments can help to uncover the
various stages of this arms race.
