Street-smart settlers

Diaspores make the diaspora po ALTHOUGH forms of life found on
oceanic islands have their mainland
cousins, they develop distinct identities
for themselves. The uniqueness of these
living beings is attributed to the changes
that they undergo in order to adapt to
the island environment.

Scientists believe that such differences could not be chance occurrences
but could come about only through natural processes. Martin Cody and Jacob
Overton, biologists from the University
of California at Los Angeles (UCLA), US,
observed these 'processes during their
decade-long study, conducted on some
240 islands off the Pacific coast of
Canada. Most of the plants belonged to
the daisy family (Asteraceae), which are
wind-dispersed. According to the UCLA
researchers, for a species to establish
itself on an island, it should first be able
to reach the place (for which it should
be mobile). Once this is accomplished5 it
has to find means by which it could stay
put on the island and remain stationary.
Therefore, the early colonisers from the
mainland are believed to have not only
established themselves on the island, but
even adapted themselves to the new
environment by shedding their dispersive tendencies and adopting a sedentary life. These adaptations which were subsequently passed on to
their progeny, developed
well in the stable and insular environment of the
islands. Cody and Overton
suggest that these two
opposing feats could be
achieved only through
rapid evolution. Moreover, their's is the first
study to report on both
rapid evolutionary changes and the founder
effects of wild plant populations. Founder effect
is the phenomenon by
which new colonies of a
species become instantly
distinct from their parent population.

The structure in these plants
resp@nsible for their dispersal is called
the Oiaspore. The diaspore comprises of
two,@parts: the achene, a tiny covered
seed'and the pappus, a ball of fluff which
surrounds the achene. This parachute-
like appearance helps in the dispersal of
th'e@4e seeds. The mobility of the seed
hinges on the ratio of the volume of the
fluff to that of the seed. The experts
arrived at the ratio by dropping the diaspore from a height of two m in still air
and measuring the time taken by it to
reach the ground. They found that the
diaspore remained aloft for a longer
time when the volume of the pappus
was much larger than that of the achene.
The ratio calculated by the scientists was
obtained by taking a sample of 20 diaspores for each of the 155 plant colonies
studied by them. On comparing island
and mainland species it was found
that the volume ratio of the diaspore
was much smaller in the former case,
indicating the inability to disperse to
far-flung areas.

What led the scientists to think of
the possibility of a rapid evolutionary
process was the fact that many of these
colonies were small, with only a handful
of individuals each, and these were also short-lived. This possibility made the scientists undertake a census of the island's plants {daisies happened to be the most abundant species), for the following eight summers. Their study revealed a high population turnover , with older populations becoming extinct and new ones establishing them- selves within a short span of time. These new populations were new colonisers, and did not resemble their predecessors. But the possibility that the latter may have made a reappearance with the help of old, buried seeds is also not being discounted by the scientists. Indeed some species made a re-entry after having been absent for almost nine years. Therefore, Cody and Overton measured the diaspore volume ratio by taking into consideration the age factor too.

The founder effect was established by the fact that the earliest populations of wild lettuce Lactuca muralis- the most abundant Asteraceae species on the islands- had smaller achenes {smaller by about 15 per cent), when compared to its mainland counterparts. On the other hand, it was noticed that subsequent populations on the islands had a smaller volume ratio, resulting in shorter drop time and indicating low dispersal quality of these plants. But most of these weeds are biennials, which makes it apparent that all evolutionary changes in them take place within the first and fifth generations. Therefore by the end of eight or nine years, when the weed got firmly established, there was little or no difference between the mainland and island populations.