Evolution__3rd_Edition

Frequency
Frequency
Frequency
Frequency
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
Clone 12
1992
Clone 22
1992
Clone 19
1992
Clone 63
1992
1993
1993
1993
1993
1994
1994
1994
1994
1995
1995
1995
1995
1996
1996
1996
1996
0.3
0.2
0.1
0
0.3
0.2
0.1
0
0.3
0.2
0.1
0
0.3
0.2
0.1
0
Infection rate
Infection rate
Infection rate
Infection rate
Figure 12.8
Genetic cycles in snails, possibly driven by parasite–host
coevolution. Data are shown for the four commonest clones of
the snail Potamopyrgus antipodarum in a lake in New Zealand.
For each, the histogram at the bottom of each part shows the
frequency of the clones over time. Note that all four undergo
frequency cycles, but not synchronously: clone 12, for instance,
peaks in 1992 and clone 63 in 1995. The line graphs at the top
of each part shows the infection rate of each clone: that is, the
fraction of snails of that clone that are parasitized. The total
infection rate, by all parasites, is the green line. The infection rate
due to Microphallus is the black line. Note that the infection rate
tends to peak 1 year after the clone’s frequency peak. Clone 22,
for example, peaked in 1993 and its peak of infection was in
1994. Clones 19 and 63 show the same pattern, but clone 12 is
exceptional. Asterisks indicate that the clone is significantly
more infected by Microphallus than would be expected by chance
(*, P < 0.05; ***, P < 0.0001). From Dybdahl & Lively (1998).
..