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Exam ple (FUND ED Narrat iv e)
12.
Narrative Statement:
The Relative Roles of Reproducti ve and Ecol ogical Traits in Producing
Biodiversity
Goal of Research:
Understanding the production of biodiversity is a fundamental problem in
biology. It is thought that a major component of speciation is the achievement of
population divergence, suggesting that with time divergence will result in speciation.
However, evolutionary biologists have learned that the relationship between
population divergence and speciation is not straightforward (Magurran 1998). In
other words, speciation does not appear to be simply the result of population
divergence. Rather, it appears that speciation may be driven by selection. This and
recent evidence of rapid speciation suggests that the study of biodiversity may
benefit from focusing on the forces driving speciation.
Recently, two types of driving forces for speciation have received much
attention: sexual conflict (i.e. antagonistic intergenomic coevolution) and ecological
speciation (i.e. divergent natural selection in distinct environments). Theoretical
models suggest that each of these driving forces may result in rapid speciation (Rice
1984; Gavrilets 1998). In addition, initial evidence suggests that these mechanisms
may be common (Arnqvist 2000; Schluter 1998).
The evolution of trait differences that isolate two lineages is viewed as an
especially important aspect of speciation - the event that separates two lineages and
assures their future independence. Moreover, the nature of these trait differences
provides insight into the forces driving speciation. For example, each hypothesized
speciation mechanism (sexual conflict or ecological speciation) makes unique
predictions regarding trait differences between sister species.
I propose to test these mechanisms for reproductive isolation in the ground
crickets of the genus Allonemobius. Of the ten species in this genus, there are two
pairs of closely related sister species. One sister pair has been the subject of intense
study. This sister pair, Allonemobius fasciatus and A. socius, has been developed into
a model system for studying speciation. They have been the subject of several
genetic (allozyme, AFLP, RAPD, and QTL), behavioral and ecological studies by Dan
Howard and his colleagues. These studies show that this sister pair is isolated by
reproductive traits (Howard and Gregory 1993) and probably diverged by sexual
conflict as a result of sexually antagonistic coevolution between males and females in
geographically separated populations. However, several lines of evidence suggest that
ecological factors may have played a role in the speciation of other members of the
genus. For example, the other pair of closely related sister species exhibits variation
in both ecological (habitat) and reproductive (male calling song) traits (Fulton 1931).
By using a multi-disciplinary approach (phylogenetic, behavioral and ecological), I will
attempt to dissect the relative roles of sexual conflict and ecology in driving
speciation in this group.
Rationale:
Present patterns of biodiversity (i.e., species richness) are the result of two
processes: speciation and extinction. Traditionally, scientists interested in preserving
biodiversity have focused on understanding the process of extinction. Undeniably,
current extinction rates appear to be high and increasing at an alarming rate (Wilson
1989; Heywood 1995). Rightly so, conservation biologists have focused on
understanding the driving forces behind these high extinction rates and on the best
methods to reduce these rates. This focus ranges from global extinction patterns to
local threats to biodiversity.
While it may be possible to slow the high rate of species extinction, the
eventual fate of any species is extinction. Therefore, it seems that scientists
interested in biodiversity have focused on only half the equation. A true
understanding of biodiversity will come only with an additional focus on the processes
leading to the production of biodiversity.
Perhaps the reason conservation biologists have neglected these processes is
that the study of speciation is viewed as particularly difficult; it is a process that
occurs over a long time span. However, phylogenetic techniques and the comparative
method can result in powerful analyses of the factors driving the production of new
species. Moreover, speciation may not always be a slow and gradual process as
previously believed.
One of the most exciting discoveries in evolutionary biology in the last few
years has been the finding that adaptive evolution can occur rapidly enough to be
studied directly in natural populations (Thompson 1998; Hendry and Kinnison 1999).
Moreover, recent results indicate that speciation may occur with extraordinary
rapidity, ranging from speciation in historical time (Feder 1998) to the evolution of
reproductive isolation in just 13 generations (Hendry 2000). Such extraordinary
speciation rates indicate that selection (sexual or natural) is driving speciation. Data
suggesting selection drives speciation, in conjunction with recent theoretical models
(Kondrashov and Shpak 1998; Diekman and Doebeli 1999) introduces an important
consideration for conservation biology: that sympatric speciation (i.e. speciation
without geographic isolation) contributes to biodiversity. Understanding the relative
importance of sympatric versus allopatric speciation will affect nature preserve design
and genetic conservation programs.
With the acceptance that reproductive isolation is the keystone of speciation,
many evolutionary biologists have developed a process-oriented approach to the
study of speciation. These scientists believe that understanding the processes
leading to reproductive isolation will allow the identification of factors driving
speciation. It is not until we understand how speciation occurs and its’ driving forces,
that we can bring this knowledge together with our knowledge of extinction to reach a
complete understanding of biodiversity.
Approach:
Species of the ground cricket genus Allonemobius are particularly well suited for
studying the effects of ecological and reproductive traits on speciation. These ground
crickets span a range of relatedness (from very closely related to relatively distantly
related), most species are sympatric across a broad geographic range, and most
inhabit unique habitats. With regard to reproductive traits, the sister species
Allonemobius fasciatus and A. socius was one of the first groups in which conspecific
sperm precedence was discovered (Howard and Gregory 1993). This isolating
mechanism is inherently sexual in nature and likely to have evolved through sexual
conflict. However, whether conspecific sperm precedence is common in this genus is
unknown. Another conspicuous reproductive trait is male calling song. Male calling
song is thought to be important in mate recognition and therefore, in isolating many
cricket species (Walker 1957), because females phonotax to species-specific songs.
However, differences in male calling song may not be important for reproductive
isolation in Allonemobius. In particular, females may not be sensitive, or may not
respond, to variation in male calling song. For example, the sister species A. fasciatus
and A. socius exhibit species specific male calling songs but females demonstrate no
preference for conspecific songs (Doherty and Howard 1996). Moreover, it appears
that calling song differences in A. fasciatus and A. socius have evolved by drift (Roff
et al. 1999).
Phylogenetic Analysis:
The first step in determining the relative roles of sexual and natural selection in
speciation is obtaining a complete phylogeny. To date, I have compiled an allozyme
data set for eight of the ten species (two species remain to be collected along with
addition populations for most species). However, allozymes represent a relatively
coarse measure of genetic divergence. Therefore, I plan to use more rapidly evolving
molecules (i.e., DNA) to produce a species level phylogeny. Because phylogenetic
analysis of DNA sequence data does not necessarily estimate the true phylogeny of a
group (rather it estimates a gene tree for that gene), I will use multiple independent
markers. The first gene I plan to sequence, cytochrome b, is a mitochondrial gene and
as such, evolves at a rapid rate and is inherited maternally. To test for congruence
and to have a higher probability of estimating the species phylogeny rather than the
gene tree, I will sequence a nuclear marker as well. The internal transcribed spacer, or
ITS, occurs between two highly conserved regions of the nuclear genome: the 16S and
12S RNA genes. The ITS region is transcribed but then excised prior to use, and
therefore free to evolve at a rapid rate. Congruence between the mitochondrial and
nuclear-based phylogenies will improve the confidence that the gene trees estimate
the correct species phylogeny.
Phylogenetic Independent Comparisons:
After producing a robust phylogeny, I will map reproductive and ecological traits
onto the tree. Initially, reproductive traits will be limited to male calling song
characters. To date, I have recorded calling songs for at least one individual for five
species. I need to record songs for the other five species and increase my sample size
to at least ten individuals per species. I have begun collecting ecological data as well.
Thus far, I have rough measures of habitat for eight species. These measures are
based on soil characteristics such as soil moisture and texture. Additional habitat
measures will include percent ground cover and dominant vegetative species at three
heights: ground, one meter and greater than 5 meters. Height of ground vegetation
(i.e., grass) will also be measured. These traits will be mapped onto the phylogeny
and used in several analyses.
One way to evaluate the role of ecology in speciation is to determine the mode
of speciation and identify factors that may have played a role in reproductive
isolation. For example, a sympatric speciation event with a corresponding habitat shift
would implicate a role for ecology in speciation. This analysis hinges on the
identification of speciation modes. To this end, Berlocher (1998) has demonstrated
that geographic distribution data in conjunction with phylogenetic relationships is
useful in determining the mode of speciation for closely related organisms. In his
analysis, Berlocher (1998) showed that the relationships between range overlap and
genetic distance between sister species was unique for sympatric and allopatric
speciation events. Sympatric speciation events showed a negative relationship
between percent geographic overlap and genetic distance (i.e., the percent overlap
can only decrease through time from the initial condition of complete overlap),
whereas allopatric speciation events showed a positive relationship (i.e., the percent
overlap can only increase through time from the initial condition of no overlap).
I have acquired geographic range maps for all members of this genus and
additional collections will add to these maps. This information in conjunction with the
phylogeny will allow the identification of the modes of speciation in Allonemobius. I will
then ask whether sympatric speciation events correspond to shifts in habitat. This
correspondence between sympatric speciation events and mapped habitat shifts
would be an indication that habitat shifts have played a role in speciation.
Another approach to test the role of ecology and sexual traits in speciation is to
use phylogenetic independent comparisons to factor out the influence of phylogeny
on the evolution of these traits. To factor out phylogenetic influence, the traits of
interest will be weighted based on the degree of relatedness, thereby extracting
phylogenetic signal (Harvey and Pagel 1991). By regressing the weighted ecological
and reproductive divergence measures against genetic divergence I will identify those
traits which have played an important role in the evolution of this genus. For example,
if ecological divergence plays a large role in speciation there should be a negative
relationship between ecological divergence (weighted for phylogenetic independence)
and genetic distance between nodes (i.e., speciation events). That is, if ecology has
been important in speciation, close relatives should be more divergent in ecological
traits (following phylogenetic weighting) than distant relatives.
In addition, by using phylogenetic independent comparisons I will be able to test
the exciting idea that divergence in ecological traits can spur divergence in sexual
traits and together lead to reproductive isolation. This intriguing idea originated with
Paterson (1980, 1981) and is based on the premise that sexual signals are perceived
differently in different environments. Therefore, adaptation to a new habitat may
drive the evolution of new mating signals which itself may be a step towards
reproductive isolation and speciation. Initial evidence for this hypothesis would be a
positive correlation between habitat and calling song.
A Test Case for the Importance of Habitat Shifts:
The sister species Allonemobius allardi and A. tinnulus provide an ideal test case
for hypotheses posed by the phylogenetic analysis. These species are closely related,
exhibit broadly sympatric geographic distributions and differ in both song and habitat
characteristics. Simple laboratory experiments can support or refute hypotheses
posed by the phylogenetic analysis. For example, female preference experiments
designed to measure the strength of preference (by manipulating song
characteristics) afford some measure of isolation. In addition, if ecology appears to
drive speciation through habitat selection I can remove habitat as a factor in the
laboratory (song will still vary) and measure the degree of reproductive isolation.
Weak female preference for song or weak isolation in the absence of habitat choices
would implicate ecology as a reproductive isolating mechanism.
A Test Case for the Importance of Sexual Conflict:
Regardless of whether sexual selection is implicated in speciation by the
phylogenetic analysis, it may drive speciation through a cryptic mechanism: sexual
conflict. Sexual conflict is a recent, exciting hypothesis which states that unequal
interests between males and females can result in rapid evolution of traits related to
reproduction. This hypothesis suggests that an inherent conflict ensues between the
sexes when females mate with multiple males. For example, a male benefits when
females use only his sperm; however, females frequently choose which sperm will
fertilize her eggs – even after copulation. This conflict of interest can have sweeping
effects on all aspects of reproduction through antagonistic coevolution of the sexes,
with males adapting to changes in female behavior and/or reproduction and viceversa. I propose to test the role of sexual conflict in speciation using the sister
species Allonemobius fasciatus and A. socius. These species are known to mate
multiply and are reproductively isolated by conspecific sperm precedence.
Although sexual conflict is difficult to demonstrate, a prediction of the
hypothesis is that sexual and reproduction related (SRR) genes should evolve at a
much higher rate than non-SRR genes, and that this evolution is driven by selection.
These predictions have been tested and supported in two Drosophila species, using a
cDNA (complementary DNA) library (Swanson, unpublished manuscript). cDNA is
produced by reverse transcription from RNA. Therefore, by isolating mRNA from
specific tissues (i.e. reproductive tissues such as the male accessory gland) the DNA
coding for reproductive proteins can be easily obtained. To test sexual conflict in
Allonemobius, I will produce a cDNA library and sequence SRR and non-SRR genes. By
comparing DNA sequence divergence between A. fasciatus and A. socius, I will test
whether SRR genes evolve at a faster rate than non-SRR genes. In addition, by
comparing the ratio of non-synonomous (amino acid altering) to synonymous (silent)
nucleotide substitutions I will be able to ask whether selection has driven the
divergence in these genes (as evidenced by a relatively high rate of amino acid
altering substitutions). Moreover, once the initial analysis is complete the technique
may be applied to the rest of the genus relatively easily, if warranted.
Rarity:
Although species richness is the most commonly discussed measure of
biodiversity in conservation, the geographic distribution and abundance of species are
important aspects of biodiversity, as well. Although these two aspects of biodiversity
are at the heart of ecology, we still do not have a good understanding of the factors
controlling them. Moreover, variation in abundance patterns are thought to be
important in conservation. A wide variety of approaches have been used to
understand abundance patterns, however, to my knowledge no study has taken an
explicitly evolutionary approach. I suggest that an explicitly evolutionary approach is
just what is needed.
Species, by their very nature, differ from one another in biologically relevant
characteristics. It may be these species-level differences that confound our attempts
to understand the factors controlling abundance patterns. However, closely related
organisms, by definition, share most traits. Therefore, any differences in abundance
may be more easily assigned to a causative factor in closely related organisms.
By using a group of closely related organisms (Allonemobius) that exhibit very
different abundance patterns (from rare to very common), I will analyze the
contribution of phylogenetic constraints to abundance patterns. Moreover, I will
measure ecologically relevant traits (such as, competitive strength in the laboratory,
parasitism, fecundity, and mortality rates). I will test for relationships between these
traits and abundance patterns, using phylogenetic independent comparisons.
Expected results:
This project will produce valuable results on several fronts. The production of a
robust phylogeny for this genus will be of value to evolutionary biologists. The genus
Allonemobius is viewed as an emerging model system in evolutionary biology and
therefore information on patterns of relatedness are of utmost importance. In
addition, phylogenetic analyses can lead to the discovery of cryptic species. Thus far,
at least one new Allonemobius species has been discovered while collecting allozyme
data for phylogenetic analysis (Braswell, unpublished data). Given the large number of
undescribed species, studies that identify new species are useful and integral to
conservation strategies (Heywood 1995).
Phylogenetic analysis is integral to analyze patterns of divergence among
species. In conjunction with ecological and sexual traits, these patterns of divergence
are critical to test hypotheses about the role of sexual and natural selection in
speciation. Tests of these hypotheses are viewed as important by evolutionary
biologists. More importantly, each hypothesis has unique implications for the
production of biodiversity, and therefore, is of interest to conservationists.
Finally, besides species richness, abundance patterns also contribute to
biodiversity. By measuring basic ecological characteristics, extracting phylogenetic
signal using phylogenetic independent contrasts and then correlating these to
patterns of species abundance, I will search for explanations of variation in abundance.
Understanding the importance of ecological factors, such as habitat variability,
in producing and maintaining biodiversity is crucial for making conservation decisions.
For example, one should understand the importance of habitat variability before
making decisions regarding nature preserve design. My work will analyze biodiversity
at multiple levels, from variation in species abundance patterns to the process of
speciation itself. Moreover, by combining a pattern-oriented and process-oriented
approach my work will lend important insights into the origins of diversity. Studies of
this type are needed to assess the relative role of ecological factors in producing
biodiversity, particularly given the rapid rate of habitat destruction and environmental
change.
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