Download Darwinian balancing selection: predation counters sexual selection

Darwinian balancing selection:
predation counters sexual
selection in a wild insect
a.k.a. Balancing selection in wild tree crickets
Kyla Ercit and Darryl T. Gwynne
Introduction
Sexual selection models
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Define well known models for the reasoning behind sexual selection and
female choice
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Darwin-Fisher model = sexy sons hypothesis
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State that viability and sexual selection should oppose each other but
do not state why this occurs
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The reference here is Fisher 1915
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Connected to Runaway Selection
Indicator Mechanism model = Zahavi’s handicap
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In this model, viability and sexual selection act in the same direction,
but the rational behind this is not stated
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“If the indicator mechanism process is more common in
nature, it may reflect either that there is strong selection
on females to choose attractive males that are robust
enough to bear attractive traits, or that the cost of
female choice is high for many species”
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Say may reflect but this is the basis for the
indicator model
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Cost for male or female?
Support
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Theoretical and empirical support for both models
but never indicate how much
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Later indicate few studies that report on DarwinFisher model
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Despite this, they really push the Darwin- Fisher
model, blaming the analysis of the studies on
failing to recognize potential differential survival
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“Demonstrations of how sexually selected traits affect
survival in nature use selection analyses, a robust
method of measuring selection in the wild which
determines the strength and direction of sexual and
viability selection on traits”
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This statement is circular and redundant
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Leads into further support for the Darwin-Fisher
model even though other studies dispute it
Male singing rates
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Evidence given for selection of singing rates as an
attractive male trait in three different cricket species
(not including the study species)
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Only study body size in relation to singing rates in
Oecanthus nigricornis, never the individual trait
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Differences between cricket species not explained
Predation
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“The only studies to relate mating success to survival of
gryllids in the wild used lifespan (in part limited by
predation) to estimate viability in populations of G.
campestris.”
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This leads us to believe that there are other
factors to be considered
•
Yet predation is a huge aspect of their
experiment
Formal selection analysis
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Researchers reported that formal selection analysis
did not yield significant viability selection on traits
male size and condition that had been sexually
selected for
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They decided to study these traits despite the nonsignificant results found in other studies
Traits studied
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Tegmen width: sexually selected and dimorphic
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Body size: sexually selected
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Head width: later described as not sexually dimorphic
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Mentioned that it is advantageous in male contests in
related species (intrasexual selection)
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O. nigricornis does not use male-male combat, instead
relying on female choice (intersexual selection)
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Hind jumping leg size: beneficial in viability and sexual selection
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No traits that solely effect predation and viability rates
Female vs. Male Morphology
www. biodiversity.ubc.ca
Prediction example
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“If sexually attractive traits have evolved in our population
of O. nigricornis via the Darwin-Fisher model (where
sexual traits impose fitness costs that balance the mating
advantage), then we predict selection on traits that
improve mating success will be opposed by viability
selection. Opposing selection would also be revealed in
significant viability and sexual selection on multivariate
axes that have similar relative contributions of traits, but
where the sign of selection will be opposite.”
Predictions
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Four predictions, further go on to explain entire
model for Darwin-Fisher and Indicator Model
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Cover all bases to explain both models under all
circumstances
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No possibility for a null hypothesis associated with
each
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Descriptive rather than predictive
Methods
Oecanthus nigricornis
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Discuss female choice in
response to male song and
body size
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Males produce a pricey
nutritious secretion as a gift and
later a spermatophore to mating
females
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This suggests that males
have a high cost (not only
females)
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May suggest room for male
choice
Predator: Isodontia
mexicana
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Only examine predation by wasps, not the
behavioural response of prey
•
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Wasps are diurnal predators, overlapping predation
times later in the day with cricket mating times
•
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Are defence mechanisms present?
Light pollution from the study area (near suburbs)
may push back day-time boundaries
Reared wasps from larvae to control population
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Did not control population of crickets
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May have affected predator behaviour
Study site
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Started with a field of goldenrod at Koffler Scientific
Reserve
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Used a brush cutter to create rows of vegetation, in
order to facilitate observation
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This decreases the edge to area ratio, which can
alter predator behaviour, as well as introduce other
variables
•
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Removed crickets from the meadow, marked adult males (n=22)
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Never say the estimated proportion of marked to unmarked
males
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Later mention that they only found 4 marked prey compared to
hundreds of unmarked prey in wasp nests
“In part to replace adult male crickets that we sampled over the 5-week
period (August 8 to September 13 2012), we periodically released in the
meadow additional marked adult male crickets collected from nearby
goldenrod meadows. In total, 50 adult males were added to the
meadow.”
•
These added males are from another population and could have lower
mating success or be selected for another trait (sexually and viability)
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Where does this number come from? It’s not clear
Sexual selection data
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2 males collected for each sexual trial
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Male that mounted the female, and the nearest male within 2
metres
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These were assigned a value of either 1 or 0
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No suggestion of a mating continuum or multiple matings
Collected pairs of marked/marked or unmarked/unmarked to account
for behavioural differences
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No testing done on marked/unmarked pairs
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Should have done preliminary testing for potential behaviour
differences
Measurement
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This section makes a lot of assumptions which the researchers
base their study predictions and choice of traits upon
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For example: “[…] we assumed that since larger males have more
attractive, lower-frequency song (Brown et al. 1996), we would
capture the variation in song frequency (and therefore attractiveness)
in our pronotum length (representing body size) measurement.”
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This seems more like a correlation, rather than a causation
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The species may be adapting, with different populations
selecting for different traits (19 years is time for a lot of
generations)
Statistical analysis
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They took 3 different leg measurements, but then combined them
into a single leg size data point
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Never explained how they combined sexual and viability statistics to
create a fitness value
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Tested linear and non-linear significance, however they used values
of 0 and 1, so data had to be linear
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Used Phillips and Arnold’s (1989) matrix method to analyze their
data but never explain the purpose of the matrix analysis or what the
eigenvalues generated represent
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It seemed like they were trying to make their data more complicated
than it was
Results
Sexual selection
•
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Created Sm(1-4) values to observe selection direction
based on multiple traits
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Never explicitly state was Sm1 or Sm2 represent
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As well, did not state exact weighting of traits within
Sm3 or Sm4
Repeatedly say “significantly improved the model”
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This makes it seem like they are making up values
to fit their model and prove their hypothesis
Viability selection
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Created Vm(1-4) values to observe selection direction
based on multiple traits
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Vmx ≠ Smx, they do not represent the same
combination of traits
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Do not state what Vm2 represents and it is left out of
the results as is significant
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Data should not be left out because it is non-significant
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Again, manipulated weightings to achieve desired
results
Discussion
Head Size
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Reference Gryllus pennsylvinicus difference in head size between
winners and losers mating success as being driven by combat
between males
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However, O. nigricornis uses female choice rather than
combat, based on song attractiveness
“Larger heads and mandibles may allow for more efficient chewing,
which may allow a cricket to assimilate nutrients faster, and have
access to more energy for sexual displays.”
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Would also have more energy for defence or escape so
viability selection shouldn't have had a negative relationship
Gryllus pennsylvinicus
Oecanthus nigricornis
Balance
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“We did, however, find a fitness cost of head width as sexual
selection for wider male heads was countered by significant viability
selection for narrower heads. The magnitude of directional sexual
and viability selection gradients was remarkably similar, but the
direction of these gradients was opposite.”
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In essence they found the balance point by manipulating the
weighting of the Sm and Vm values
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Created the weightings to fit the model
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“This indicates that, in our system, there is a fitness cost of having
attractive combinations of traits.”
Sexual dimorphism
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Head size is not a sexually dimorphic (yet this is a
studied trait for sexual selection)
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Yet they state that male heads are slightly smaller
than female heads (not significant?)
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Assumed to be related to predation by wasps in specific
habitat but in their study they manipulated both the
habitat and the predator population
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If this trait is not different between the sexes, why was it
chosen?
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Females are found more often as prey in wasp
nests (69-92%)
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This suggests there is a cost to being female
that is not related to head width
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Yet, they state that males with larger heads are
at a higher risk for predation
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With manipulated predation rates and edge
effects this may have been changed in the
current study
Conclusions
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“In conclusion, we have found an unusual effect in a natural
system: viability selection countered sexual selection for a male
trait, possibly constraining the elaboration of a sexually dimorphic
trait that is seen in related species”
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Should have focused more on sexual dimorphism as well as
differing female/ male strategies to maximize fitness
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The results were so manipulated that they were not applicable to
the natural system
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Very focused on achieving a rare result: balancing selection
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Created model beforehand with no room for wrong results
Questions and comments?