|My current research focuses on evolutionary biology and ecology, and involves a combination of theoretical ecology and field studies. Specific interests include the speciation process, adaptive radiation and global patterns of species diversity. I am using land and marine gastropoda as a model system. Questions I am addressing are: what ecological and genetic factors cause diversification of species; what determines spatial and temporal patterns of species diversity; what are responsible for evolutionary novelties? And how do we conserve biodiversity when human disturbances have strongly affected ecosystems? I am investigating these questions and related issues, as follows|
Current Research Projects:
|Adaptive radiation in the land snails Mandarina|
The endemic land snail genus Mandarina in the Oasawara Islands in the north-western Pacific is a particularly interesting group, because this genus has undergone extensive radiation within the islands and it has diversified into many species. They separated into arboreal, semi-arboreal, exposed ground and sheltered ground ecotypes. I have estimated phylogenetic relationship among the species of
|Mandarina using mtDNA sequences, and showed that diversifications of ecotypeshave
occurred independently and repeatedly in different times and different
areas. The inferred phylogeny suggests that morphological differences equivalent
to the differences between families were produced between Mandarina and its ancestor during the Pleistocene.
|Sympatric speciation in the land snails Mandarina|
|Recent much focus has been placed on the possibility of sympatric speciation
due to ecological processes. Mandarina is an excellent model system to understand how speciation occurs. I am
currently examining diversification of habitat use between different colour
morph and levels of gene flow between these morphs using microsatellite
DNA to test the hypothesis that these morphs have appeared via sympatric
speciation. In addition, an individual-based model is constructed to see
whether the observed patterns can be explained by this speciation mechanism.
Collaboration: Angus Davison (University of Edinburgh)
|Conservation of the endemic land snails in the Ogasawara Islands.|
|The Ogasawara Islands are located in the north-western Pacific approximately 1000 km south of the mainland of Japan. More than 100 species of land snails have been described in Ogasawara, and approximately 90% of these species are endemic. These native land snails of Ogasawara are now facing serious crisis. The population density and range of the distribution of most of the extant species have markedly decreased in the past 10 years.|
|The most plausible candidate that has caused this decline is predation
by both intentionally and unintentionally introduced carnivorous flatworms.
The effect of the introduced flatworms on native land snail fauna is so
serious that they may rapidly cause the extinction of many of the native
land snails in Ogasawara. A conservation program of the endemic land snails
is urgently needed.
Collaboration: Isamu Okochi (FFPRI), Takashi Obayashi, Angus Davison (University of Edinburgh)
|Interspecific hybridization in land snails|
Euhadra senkenbergiana minoensis
hybridization provides substantial problems for several areas in evolutionary
analysis. It creates disagreement between gene genealogies and genealogies of taxa,
and affects estimation of phylogeny and divergence times. I am examining variationsin mtDNA and nuclear genes of the land snails Euhadra and Mandarina to show that hybridization occur frequently between distantly related species. It is important to show how hybridization events may be an important source
of evolutionary novelties and that the phenomenon of reticulate evolution
can not be ignored.
Collaboration: Angus Davison (University of Edinburgh)
|Dart-shooting behaviour of the land snails Euhadra|
|In several land snail species, a "love dart" is expelled during courtship and is forcefully stabbed through the skin of the mating partner. We are currently working on dart-shooting behaviour of Euhadra to investigate the use and function of a "love dart". This unique behaviour provides an excellent model to address the role of sexual conflict in speciation and the evolution of reproductive isolation.
Collaboration: Joris Koene (Vrije University)
|Pallarel evolution of an unique shell morphology in the land snails Ainohelix|
|An extremely flat shell with a sharp peripheral angle is found in the populations of Ainohelix from some limestone areas in Hokkaido. This unique form has evolved independently from different lineages. Populations of the unique form and normal form are genetically very close. The aim of this research is to explore the genetic and ecological factors that cause the evolution of this unique form.
Collaboration: Masakado Kawata, Jun Yokoyama (Tohoku University)
|Paterns of morphological evolution and species diversity in fossil land snails.|
Fossils provide direct evidence of evolutionary patterns and temporal changes
of species diversity. I have investigated the Pleistocene fossil records
of land snails and found that extinction of a lineage and speciation occurred
during the intervals of the rapid phenotypic shift. I have shown
that major phenotypic changes in phyletic evolution, separation of different
species, and extinction of lineages occur synchronously in many lineages
during an extremely short interval.
|Theory of long-term patterns of species diversity and macroevolution.|
The aim of this research is to build a bridge between the theory of short-term
evolution (e.g. population genetics, population ecology) and that of long
term evolution (palaeontology). I have constructed a mathematical model
to examine the relationships between environmental instability and long-term
macroevolutionary trends. This model suggests that mass extinction can
occur as a result of long-term adaptation to a stable environment following
a minor change of environment without catastrophes. I am currently investigating
how population structure, gene flow and interactions among species affect
temporal patterns of species diversity and phenotypic evolution using an
|Geometrical effect of island shape on the species diversity.|
|This reserch aims to develop a new model to describe spatial patterns of
species diversity on the basis of fractal geometry. We have developed a
model considering immigration of species and constraint on the statistical
self-affinity (anisotropy) of island shapes. This immigration model derives
the famous power law on species-area relation. In addition, this model
shows that the number of species on the anisotropic island is constrained
by not only the size of the island area but also the shape of the island.
Collaboration: Hiroyuki Nagahama (Tohoku University), Kazuhito Yamazaki (Kobe University)
|Impact of sea-level changes on the species diversity.|
|In this research project, we investigate how changes in area of an island
due to sea-level changes cause changes in species diversity. Historical
changes in the coastline of the island are estimated using a numerical
simulation based on a glacio-hydro-isostacy model. The species number in
the past can be theoretically estimated by the area of the island in the
past and a species-area relationship among the modern fauna of the archipelagos.
Temporal changes in the estimated species number of the island during the
last 40 Kyr are compared with temporal changes in species diversity in
fossil organisms of the island. This research enables estimation of species
loss caused by sea-level change due to global warming.
Collaboration: Kazuhito Yamazaki (Kobe University), Junichi Okuno (University of Tokyo),
|Genetic background of invasive species.|
Biological invasions present interesting evolutionary problems. How do
invasive species survive rapid habitat transitions? How do genetic architecture
impact the propensity to invade? To address these questions, levels of
genetic variations and population structure of introduced populations of
intertidal gastropoda are compared with the source populations of the introduced