Biology Reference
In-Depth Information
Figure 2.1.
Dynamics of dispersal and natural selection on wind-dispersed plants
invading islands such as those in Barkley Sound, British Columbia, Canada. New
island populations tend to be founded by seeds likely to be carried farthest by wind.
Once established on islands, selection favors lower tendency to be carried far by
wind, since this increases the chance that seeds will be blown off the island. The
curved arrows indicate the most likely patterns of dispersal. The dashed arrows
indicate the influence of natural selection on dispersability and the thick arrows
indicate stages in the evolutionary sequence. (Reprinted with permission from M.
L. Cody and J. Overton. 1996. Short-term evolution of reduced dispersal in island
plant populations.
Journal of Ecology
84:53-61. © 1996 British Ecological Society.)
of the latter, hairy cat's ear (
Hypochoeris radicata
), seeds of island plants were
about 25% larger and the volume of the feathery pappus about 34% less
than for seeds of mainland plants. For the third species, wall-lettuce (
Lac-
tuca muralis
), populations varying in age from 1 to more than 10 yr were
available. Newly established populations showed smaller seeds than those
of mainland plants, but seed size increased significantly with population
age. Pappus volume also showed a tendency to decrease over time. Exper-
iments also indicated that wind-carried seeds of both
Hypochoeris
and
long-established
Lactuca
fell to the ground more quickly than seeds of
mainland plants or newly established island plants. The significance of
these changes was twofold.The seeds of
Lactuca
that were able to colonize
remote islands were light and had a large pappus that favored their long-
distance transport. Once established on an island, however, selection
favored heavier seed with a smaller pappus, as these seeds would more
likely remain on the island (fig. 2.1).
