Thesis (Ph. D.)--University of Rochester. Dept. of Biology, 2014.
Life on Earth may be characterized by many patterns. The species that surround us are
not only numerous, they are often phenotypically and ecologically diverse. The fossil
records shows that these species and their phenotypic diversity arose heterogeneously
throughout history, and further inspection demonstrates species and phenotypes are nonrandomly
distributed across the globe and environments. Ecology and evolutionary
biology attempt to explain how these patterns emerge by identifying underlying
processes. For instance, Charles Darwin and Alfred Russell Wallace recognized that
there were similarities between the species inhabiting adjacent regions and proposed that
organic evolution (common descent and modification) can explain these similarities as
an alternative to special creation. My research explores three patterns that emerge from
the examination of life, and how a single evolutionary process is capable of generating
these patterns. That process is adaptive radiation.
Adaptive radiation occurs as a response to ecological opportunity in a
diversifying lineage. It is an interaction between speciation and adaptation that results in
ecologically distinctive new species. If the ecological opportunities available to a
diversifying lineage are limited, then rates of speciation and adaptation might decline
during the course of adaptive radiation, potentially contributing to differential rates of
diversification seen in both the fossil record and molecular phylogenies. Furthermore, if
adaptive radiation produces the ecological diversity necessary for species to survive in a
variety of climates and habitats, then it might also explain how and why species
distribute themselves across landscapes. Although adaptive radiation has the potential to
explain much about the diversity of life, current studies are limited to a few iconic clades
making it difficult to identify the general elements of adaptive radiation because of vast
historical contingencies. This thesis is a comparative effort that explores how adaptive
radiation contributes to patterns of (1) species richness and ecological diversity, (2)
macroevolutionary diversification rates, and (3) biogeography, by examining clades that
radiated in similar regions, habitats, and times.
In chapter 1 I focus on the pattern of species richness and phenotypic diversity:
why are there groups of related species that differ phenotypically? In particular, I
examine a group of predominately Caribbean geckos (Sphaerodactylus) and address
whether or not the variation seen in the shape of their skulls has an adaptive origin.
Sphaerodactylus geckos are remarkable because they are co-distributed with the wellstudied
adaptive radiation of Anolis lizards and potentially provide an important system
to evaluate the generalities of conclusions made from Anolis. I show that adaptive
radiation probably contributed to variation seen in the shape of their skulls. I also
suggest that Sphaerodactylus and Anolis both possess ecologically distinct habitat
specialists. These findings show that Sphaerodactylus is an excellent clade to study
adaptive radiation by revealing that adaptive radiation may be simultaneous in codistributed
clades and ecological diversity may accrue via different pathways.
Next, I focus on macroevolutionary patterns of diversification rates through time.
Adaptive radiation is hypothesized to result in declining rates of speciation through time
if ecological opportunities are limited. As adaptive radiation produces new species,
ecological opportunities diminish and the rate at which new species form also declines.
Many studies have recovered the signature of declining diversification rates in clades
distributed around the world and with different diversification histories, though they do
not explicitly prove that adaptive radiation produced these patterns. To date, no study
has explored how diversification proceeds in clades that radiated in the same region and
habitats during overlapping periods of history. In chapter 2, I use time-scaled
phylogenies from seven reptile and amphibian clades from the island of Madagascar to
compare diversification dynamics in groups that radiated in same region and through
overlapping periods of history.
Madagascar is an outstanding region to examine diversification dynamics
because it has been isolated and geographically cohesive for the majority of its history,
and its many endemic clades provide replication. Given its stability and isolation
throughout history, processes general to diversification on Madagascar might be general
to the diversification of life elsewhere, demonstrating what happens in the absence of
paleogeography or other historical contingencies. I show that diversification rates have
declined during the history of the seven clades, and that these declines are probably
related to ecological limits to diversity. Although I cannot conclude that adaptive
radiation produced these patterns, I note that there are ancillary observations to suggest
it played a role. Regardless, these results suggest diversification declines are a general
phenomenon on Madagascar and demonstrate the island is an excellent region for further
investigation of this macroevolutionary pattern.
In chapter 3, I explore how adaptive radiation might underlie regional
biogeographic patterns and community assembly. Community assembly is the process
by which species come to co-occur locally. Like others, I show that community
assembly may be viewed as picking species from sets of regionally distributed species
called regional species pools, and indicate that adaptive radiation makes an important
prediction regarding the identity of these species pools and their geographic distribution.
Several recent studies have indicated that adaptive radiation is multidimensional, with
adaptation and ecological diversification occurring along multiple ecological
dimensions. If one dimension confers adaptation to regionally variable environmental
conditions, then we can predict that regional species pools will correspond to these
environmental gradients, and local communities will be assembled from varying
combinations of these species pools. I demonstrate that assembly may be modeled with a
hidden Markov model. With this model, I use species distributions and community
composition data to estimate the minimum number of regional species pools necessary
to explain the patterns of co-occurrence in Hispaniolan Anolis lizards that have been
documented through over a century of herpetological research. Consistent with my
predictions, I find that the regional species pools correspond to a mesic-xeric habitat
gradient and that this pattern is replicated across a paleogeographic boundary.
