Biotic and abiotic controls on evolutionary rates
Biotic and abiotic factors are
widely believed to influence extinction risk and to have influenced the
diversity dynamics of clades. I use the marine bivalve fossil record to
test models relating such factors to evolutionary rates over a
range of spatial and temporal scales.
In my postdoctoral research at Stanford University I
am investigating (1) the contributions of geographic range and life
history traits to
rates of extinction and origination, and (2) the effects of
contemporary
extinction on future diversification, using marine scallops (Mollusca:
Bivalvia: Pectinidae) as a model system. To do this, I am synthesizing
data for extant and extinct scallops from sources spanning
fossils, molecules, modern biogeography, and commercial aquaculture
studies. Few studies have examined ecological drivers of evolutionary
rates
within a phylogenetic framework that encompasses both extant and
extinct taxa. Yet such integrative analyses are necessary in order to
link
the complex history of extinction and origination preserved in the
fossil record with the rich diversity of life observed globally today.
It is my hope that this research will inform predictions
about the
response of
contemporary marine faunas to future environmental change and lay the
groundwork for addressing other questions
such as the origins
of evolutionary novelties (e.g., swimming function in scallops) and the
interactions between body size evolution and global climate over the
Cenozoic.
Macroecological drivers of extinction risk in the early Cenzoic
For my dissertation research at the University of Chicago
I developed and tested a set of multivariate statistical
models that relate the ecology of bivalve species to their persistence
over the Paleogene (65-28 MYA) in the U.S. Gulf and Atlantic Coastal
Plains. Contrary to
the general assumptions of independence implicit in most models of
extinction
selectivity, these analyses show that
broadly distributed species tend to be abundant and larger bodied.
I have used path analysis to examine simultaneously the direct and
indirect contributions of these factors to species duration in three
major lineages of marine bivalves (the Carditoidea,
Pectinoidea, and Veneroidea). Species-level data for these analyses
were gathered through quantitative field sampling and use of existing
museum
collections and
literature records. These analyses indicate that geographic range is
the
primary explanatory variable in predicting species duration with body
size and abundance having little direct effect (Harnik in revision).
Abundance and extinction rates
To examine the influence
of abundance on extinction rates globally over the post-Paleozoic, Carl
Simpson and I analyzed data for marine bivalve genera using the
Paleobiology Database. These
analyses show that abundance was an
important factor in bivalve extinction rates over the last 250 million
years (Simpson and
Harnik 2009). Yet, surprisingly our results reveal a
persistent non-linear relationship between abundance and extinction
rates which only in part corroborates general predictions. With
increasing abundance extinction rate declines yet the most abundant
taxa
exhibit elevated rates.
Structure of diversity
The distribution of species among genera is markedly uneven, with
most genera species-poor and few species-rich. This structure to
taxonomic diversity may arise through differential rates of speciation
and/or extinction. However, ecological factors such as competition and
geographic range expansion/contraction also likely contribute. In
collaboration with David Jablonski, Andrew Krug, and James
Valentine, I have used a global database of extant bivalves to
characterize the taxonomic structure of marine biomes and provinces and
assess the contributions of taxon age and provincial area to these
large-scale diversity
distributions (Harnik et al. in revision).
In this
research I
have used
ecological models, developed to describe the distribution of
individuals among species, to reveal a general form to the distribution
of species among genera in modern marine faunas, and null models to
show that this spatial variation in taxonomic structure is not
explained solely by variation in species richness. These diversity
distributions in combination with age-of-first-occurrence data suggest
that the taxonomic structure of regional faunas is shaped by
differential speciation potentials among clades, as constrained by
spatial variation in diversity accommodation space.
Rarity and sampling
One of the principal challenges in assessing extinction risk today
and in the geologic past is that rarity is believed to influence
extinction but is also known to affect sampling. Rare taxa are, by
definition, encountered infrequently and their observed occurrences
strongly controlled by sampling effort. To minimize this bias, I have
developed new methods for sampling rare species through the integration
of historical data from museum collections and the published literature
and estimates of species abundance gathered from quantitative field
samples (Harnik 2009).
Combining these two sources of
data can
provide a more comprehensive estimate of abundance and taxonomic
diversity without substantial increase in current sampling effort,
thereby expanding the scale of abundance and the sample size of
species that can be included in paleoecological and evolutionary
analyses. Applying these methods to data I have compiled from the
literature for Paleogene mollusks underscores the magnitude of veiled
diversity in marine fossil assemblages and the potential of existing
sources of data to unveil rare taxa, allowing them to be incorporated
into quantitative diversity studies.
Student-Scientist Partnerships
Engaging students in research is among the most
effective and compelling ways to teach science. Student-Scientist
Partnerships (SSP) using local
paleontological materials solve the pedagogical problem of how to get
students in touch with real science that is interesting to them,
connects to their lives and prior knowledge, but requires little
background or training to make a contribution to data acquisition and
analysis. While at the Paleontological
Research
Institution, my
colleagues and I developed the Devonian Seas Project, an SSP which
engaged upper elementary through high school students and teachers in
paleoecology research using the Devonian marine fossil record
of New York State (Harnik and Ross 2003, 2004). Participants were
involved in
classroom-based research experiences and field-based professional
development workshops. The goals of the project included specific
educational and scientific results and also the creation of
a model by which other institutions might institute similar
partnerships in the geosciences.
