However, further research must be undertaken to develop adequate indices of selection that consider genetic and epigenetic effects.
Other studies using phylogenetic contrasts and direct experiments also fail to find a significant relationship between indices of selection, such as sperm competition, with sperm total length [ 18- 23].
More recent publications (e.g., Jin et al., 2015; Rücker et al., 2011; Schwarzer et al., 2010; Stanley & Doucouliagos, 2014; Stanley & Doucouliagos, 2015) have urged the adoption of more complex indices of selection bias: 1.
Data sets which included codon sites with significant evidence of negative (purifying) selection occurring (PP > 0.95 achieved in a category with a low omega value) were regarded as having the power to resolve purifying selection and were therefore deemed suitable to generate meaningful confidence indices of selection.
We used resource selection function models to create an index of habitat selection by males and female elephants.
dN/dS ratios greater than 1 reflect positive selection against virus (i.e., under immune pressure), while dN/dS ratios lower than 1 provide an index of purifying selection: the lower the dN/dS ratio, the stronger the purifying selection.
The optimal threshold was determined using the Youden index for selection of the highest sum of sensitivity and specificity.
Population sex ratio has therefore been a frequent index of sexual selection, with female "choosiness" expected to increase with male-biased sex ratios [ 67- 69].
Competitor number typically individuals of the same sex is another important index of sexual selection, which can operate independently of population sex ratio (e.g., males in a population with an even sex ratio might still experience intense competition for resources because the density of male competitors, reflecting overall population density, is high).
