Try these parameter values:
No Differential Mortality
Some Differential Mortality
Maximum Differential Mortality
Observe the above for each graph type.
The strength of natural selection is directly related to the degree of differential mortality in the population. If the probability of survival for an individual is a function of the value of a particular trait, then that trait is subject to selection pressures (assuming, of course, that the trait is heritable). Selection will not occur if the probability of mortality is the same for all individuals.
The relationship between mortality and a trait's value need not be a linear one for selection to occur. The probability of mortality may increase with the trait value, decrease with the trait value, increase with extreme trait values, increase for intermediate trait values or density dependent; an infinite array of possibilities exist.
Adjust the amount of differential mortality to the right:
In this case the value chosen is the probability of mortality for individuals with a color value of 255. The probability of mortality is 0.0 for individuals with a color value of 0. Probability of mortality for individuals with intermediate color values is a linear function of the color value. This is an example of directional selection.