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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 • Define well known models for the reasoning behind sexual selection and female choice • Darwin-Fisher model = sexy sons hypothesis • • State that viability and sexual selection should oppose each other but do not state why this occurs • The reference here is Fisher 1915 • Connected to Runaway Selection Indicator Mechanism model = Zahavi’s handicap • In this model, viability and sexual selection act in the same direction, but the rational behind this is not stated • “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” • Say may reflect but this is the basis for the indicator model • Cost for male or female? Support • Theoretical and empirical support for both models but never indicate how much • Later indicate few studies that report on DarwinFisher model • Despite this, they really push the Darwin- Fisher model, blaming the analysis of the studies on failing to recognize potential differential survival • “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” • This statement is circular and redundant • Leads into further support for the Darwin-Fisher model even though other studies dispute it Male singing rates • Evidence given for selection of singing rates as an attractive male trait in three different cricket species (not including the study species) • Only study body size in relation to singing rates in Oecanthus nigricornis, never the individual trait • Differences between cricket species not explained Predation • “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.” • 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 • Researchers reported that formal selection analysis did not yield significant viability selection on traits male size and condition that had been sexually selected for • They decided to study these traits despite the nonsignificant results found in other studies Traits studied • Tegmen width: sexually selected and dimorphic • Body size: sexually selected • Head width: later described as not sexually dimorphic • Mentioned that it is advantageous in male contests in related species (intrasexual selection) • O. nigricornis does not use male-male combat, instead relying on female choice (intersexual selection) • Hind jumping leg size: beneficial in viability and sexual selection • No traits that solely effect predation and viability rates Female vs. Male Morphology www. biodiversity.ubc.ca Prediction example • “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 • Four predictions, further go on to explain entire model for Darwin-Fisher and Indicator Model • Cover all bases to explain both models under all circumstances • No possibility for a null hypothesis associated with each • Descriptive rather than predictive Methods Oecanthus nigricornis • Discuss female choice in response to male song and body size • Males produce a pricey nutritious secretion as a gift and later a spermatophore to mating females • This suggests that males have a high cost (not only females) • May suggest room for male choice Predator: Isodontia mexicana • Only examine predation by wasps, not the behavioural response of prey • • Wasps are diurnal predators, overlapping predation times later in the day with cricket mating times • • 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 • Did not control population of crickets • May have affected predator behaviour Study site • Started with a field of goldenrod at Koffler Scientific Reserve • Used a brush cutter to create rows of vegetation, in order to facilitate observation • This decreases the edge to area ratio, which can alter predator behaviour, as well as introduce other variables • • Removed crickets from the meadow, marked adult males (n=22) • Never say the estimated proportion of marked to unmarked males • 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) • Where does this number come from? It’s not clear Sexual selection data • • 2 males collected for each sexual trial • Male that mounted the female, and the nearest male within 2 metres • These were assigned a value of either 1 or 0 • No suggestion of a mating continuum or multiple matings Collected pairs of marked/marked or unmarked/unmarked to account for behavioural differences • No testing done on marked/unmarked pairs • Should have done preliminary testing for potential behaviour differences Measurement • This section makes a lot of assumptions which the researchers base their study predictions and choice of traits upon • 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.” • This seems more like a correlation, rather than a causation • The species may be adapting, with different populations selecting for different traits (19 years is time for a lot of generations) Statistical analysis • They took 3 different leg measurements, but then combined them into a single leg size data point • Never explained how they combined sexual and viability statistics to create a fitness value • Tested linear and non-linear significance, however they used values of 0 and 1, so data had to be linear • 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 • It seemed like they were trying to make their data more complicated than it was Results Sexual selection • • Created Sm(1-4) values to observe selection direction based on multiple traits • Never explicitly state was Sm1 or Sm2 represent • As well, did not state exact weighting of traits within Sm3 or Sm4 Repeatedly say “significantly improved the model” • This makes it seem like they are making up values to fit their model and prove their hypothesis Viability selection • Created Vm(1-4) values to observe selection direction based on multiple traits • Vmx ≠ Smx, they do not represent the same combination of traits • Do not state what Vm2 represents and it is left out of the results as is significant • Data should not be left out because it is non-significant • Again, manipulated weightings to achieve desired results Discussion Head Size • Reference Gryllus pennsylvinicus difference in head size between winners and losers mating success as being driven by combat between males • • 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.” • Would also have more energy for defence or escape so viability selection shouldn't have had a negative relationship Gryllus pennsylvinicus Oecanthus nigricornis Balance • “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.” • In essence they found the balance point by manipulating the weighting of the Sm and Vm values • Created the weightings to fit the model • “This indicates that, in our system, there is a fitness cost of having attractive combinations of traits.” Sexual dimorphism • Head size is not a sexually dimorphic (yet this is a studied trait for sexual selection) • Yet they state that male heads are slightly smaller than female heads (not significant?) • Assumed to be related to predation by wasps in specific habitat but in their study they manipulated both the habitat and the predator population • If this trait is not different between the sexes, why was it chosen? • Females are found more often as prey in wasp nests (69-92%) • This suggests there is a cost to being female that is not related to head width • Yet, they state that males with larger heads are at a higher risk for predation • With manipulated predation rates and edge effects this may have been changed in the current study Conclusions • “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” • Should have focused more on sexual dimorphism as well as differing female/ male strategies to maximize fitness • The results were so manipulated that they were not applicable to the natural system • Very focused on achieving a rare result: balancing selection • Created model beforehand with no room for wrong results Questions and comments?