of Pennsylvania computational biologists are claiming that
species on Earth are accumulating
much more slowly now than they did in the
past. Hélène Morlon
and Joshua Plotkin, both of the Department of Biology in the School
of Arts and Sciences at the University of Pennsylvania, conducted
this study using the family trees of today's species and found
rates have declined over time.
researchers were curious as to whether the diversity of species on
Earth is in equilibrium or continually expanding. In addition, they
wondered if Earth has "an invisible stop sign" that would
limit species' diversity.
answer these questions, Morlon and Plotkin used an updated
computational approach to indicate the dynamics of species
diversification. Nine patterns of diversification were used as
alternative models where a total of 289 phylogenies (evolutionary
trees) were examined, representing arthropods, mammals, flowering
mollusks and amphibians. The conclusion was that "diversity is
generally not at equilibrium," but speciation rates have fallen
over time nonetheless. This might mean that the diversification of
species are somewhat restrained, and may eventually reach
we see is diversification rates that are declining but not yet to
zero," said Plotkin. "We are not yet in equilibrium. Either
there is a limit to the total species number and we haven't reached
it yet, or there is no such limit. But the rates of diversification
are typically falling; when we will hit zero is not yet obvious."
is obvious is that there have been recent losses of certain
species because of human impact, but Morlon and Plotkin's
study involves geologic time scales that are much longer, which helps
them to better understand today's species.
these recent losses, researchers were surprised at the lack of
extinction they observed in the evolutionary trees of species because
fossil records show that several species have become extinct "over
geologic time." Morlon and Plotkin found this when looking
specifically at the diversity of whales. The diversity of whales has
declined during the last 12 million years, but in the analyses of
evolutionary trees, extinction was not obvious.
taking advantage of existing data from the flood of genomic research,
we hope to combine efforts with paleontologists gathering fossil
data," said Plotkin.
study was published in PLoS
One this month.