Biology assistant professor Lin Jiang displays a microscopic image of a protist species he used to support Darwin’s hypothesis that the struggle for existence is stronger between more closely related species. Photo: Gary Meek
A new study provides support for Darwin’s
hypothesis that the struggle for existence is stronger between more closely
related species than those distantly related. While ecologists generally accept
the premise, this new study contains the strongest direct experimental evidence
yet to support its validity.
“We found that species extinction occurred more frequently and more
rapidly between species of microorganisms that were more closely related,
providing strong support for Darwin’s theory, which we call the phylogenetic
limiting similarity hypothesis,” said Lin Jiang, an assistant professor in
the School of Biology at Georgia Tech.
The study was published online in Ecology Letters. The work was
supported by the National Science Foundation.
Jiang and his team—Cyrille Violle, formerly a postdoctoral fellow at Georgia
Tech currently at the Centre d’Ecologie Fonctionnelle et Evolutive in
Montpellier, France, and Georgia Tech biology graduate student Zhichao Pu—conducted
experiments with 10 common ciliated protist species in artificial, simplified
ecosystems called microcosms. Diana Nemergut, an assistant professor in the
Institute of Arctic and Alpine Research and the Environmental Studies Program
at the Univ. of Colorado at Boulder, helped the team generate a family tree of
the 10 microorganisms to determine how closely related the species were.
“We selected bacterivorous ciliated protist microorganisms for this
study because they rapidly reproduce, allowing us to examine species
co-existence over multiple generations in a closed system during a period of a
few weeks, which wouldn’t be possible if we were testing the hypothesis with plants
or animals,” said Jiang.
The researchers set up 165 microcosms that contained either an individual
protist species or a pairing of two species, along with three types of bacteria
for the organisms to eat. They collected weekly samples from each microcosm and
examined them under a microscope, recording the presence or absence of species.
After 10 weeks, the researchers estimated the density of the protist species in
The study results showed that all species survived until the end of the
experiment when alone in a microcosm. However, in more than half of the
experiments in which protists were paired together, one of the two species
dominated, leading to the extinction of the other species.
Lin Jiang, an assistant professor in the Georgia Tech School of Biology, recently found that extinction occurred more frequently and more rapidly between protist species that were more closely related. Photo: Gary Meek
The researchers found that the frequency and speed of this extinction
process—called competitive exclusion—was significantly greater between species
that were more closely related. In addition, in microcosms where both
competitors coexisted for the duration of the experiment, the abundance of the
inferior competitor was reduced more as the phylogenetic relatedness between
the two competitors increased.
The study also showed that the frequency of competitive exclusion was
significantly greater between species that had similar mouth sizes.
“We documented the mouth size of each species because there is some
evidence that this morphological trait affects the selectivity and uptake rate
of prey particles, and we thought that similarity in mouth size might translate
into the exploitation of similar bacterial resources and result in competitive
exclusion,” said Jiang.
While they found that phylogenetic relatedness predicted the likelihood of
coexistence better than mouth size, the results suggest that other traits
involved in resource uptake may also be important predictors of the outcomes of
competitive interactions in ecological communities.
“This study is one step toward a better understanding of how
phylogenetic relatedness influences species interactions,” said Jiang.
“We hope our experimental validation of the phylogenetic limiting
similarity hypothesis in microorganisms will encourage other ecologists to
conduct additional studies with other types of organisms to further validate Darwin’s hypothesis.”
The phylogenetic limiting similarity hypothesis is just one of the many
published in his 1859 book called “The Origin of Species.” In this
introduced his scientific theory that populations evolve over the course of
generations through a process of natural selection. The book presented a body
of evidence that the diversity of life arose by common descent through a
branching pattern of evolution.