Galápagos warbler, Dendroica petechia
aureola, Santiago island, Galápagos. Birds - with the exception of
Darwin's finches -- are still diversifying on the Galápagos islands.
Credit: Luis Valente
Islands are seen as natural laboratories for the study of evolution.
They form isolated ecosystems with barriers to migration. Classical
Island Theory predicts that a dynamic equilibrium will occur between
immigration and extinction of species. Recent theory adds that as
species diversity increases, ever more ecological niches become
occupied, which has a negative effect on immigration (new immigrants
from outside of the Galápagos cannot settle) and diversification
(radiation into new species is blocked).
Evolutionary dynamics
'However, this has never been tested in detail, for lack of data and
the right analytical tools', explains Rampal Etienne, Associate
Professor of Theoretical and Evolutionary Community Ecology at the
University of Groningen, the Netherlands. Together with Luis Valente
(University of Potsdam, Germany) and Albert Phillimore (University of
Edinburgh, UK), he developed DAISIE, a mathematical model that uses
phylogenetic data on living species to reconstruct evolutionary
dynamics. DAISIE stands for Dynamic Assembly of Islands by Speciation,
Immigration and Extinction, and was named after famous radiations of
daisy-like plants on Hawaii.
DAISIE was fed with the phylogenetic trees of existing bird species
on the Galápagos Islands. These were constructed with genomic data that
has become available in recent years. DAISIE then estimates diversity
limits and rates of immigration, speciation and extinction per lineage.
'The analysis shows that for the finches, diversity does indeed have a
negative effect. There is no more room for new species, unless one of
the existing species becomes extinct, so the islands are saturated
regarding finch-type species', Etienne explains. This does not mean the
radiation is static. 'We found that the rates of both evolution and
extinction are very high for Darwin's finches. That is probably why
these birds have reached an equilibrium.'
Isolated ecosystems
Other species like mockingbirds have not yet reached equilibrium,
which contrasts sharply with the current view that oceanic islands are
at equilibrium. 'Our data shows that they are evolving more slowly and
are still diversifying.' In a million years or so, more mockingbird
species may have appeared -- granted that conditions on the islands
remain the same.
The study shows that the DAISIE model -- which the authors have made
available as a library* in the free and widely used R software
environment -- is a valuable tool for the study of evolutionary dynamics
on islands. It includes speciation and thus extends existing island
theory, which is based on immigration and extinction. Etienne: 'And of
course, it works for all isolated ecosystems, not just islands but also
lakes or mountain tops.'
Apart from explaining evolutionary history, DAISIE also predicts
future diversity. 'This could be interesting from a conservation point
of view: we are not just conserving existing species, but also future
diversity.'
*Link to the DAISIE programme:
http://cran.r-project.org/web/packages/DAISIE/index.html
Story Source:
The above post is reprinted from
materials provided by
University of Groningen.
Note: Materials may be edited for content and length.
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