Lacing our climbing boots even more securely we prepare to hammer in another piton. Remember that selection by females is pulling male tails in one direction, while 'utilitarian' selection is pulling them in the other ('pulling' in the evolutionary sense, of course) the actual average tail length being a compromise between the two pulls. Let us now recognize a quantity called the 'choice discrepancy'. This is the difference between the actual average tail length of males in the population, and the 'ideal' tail length that the average female in the population would really prefer. The units in which the choice discrepancy is measured are arbitrary, just as the Fahrenheit and Centigrade scales of temperature are arbitrary. Just as the Centigrade scale finds it convenient to fix its zero point at the freezing point of water, we shall find it convenient to fix our zero at the point where the pull of sexual selection exactly balances the opposite pull of utilitarian selection. In other words, a choice discrepancy of zero means that evolutionary change comes to a halt because the two opposite kinds of selection exactly cancel each other out.
Obviously, the larger the choice discrepancy, the stronger the evolutionary 'pull' exerted by females against the counteracting pull of utilitarian natural selection. What we are interested in is not the absolute value of the choice discrepancy at any particular time, but how the choice discrepancy changes in successive generations. As a result of a given choice discrepancy, tails get longer, and at the same time (remember that genes for choosing long tails are being selected in concert with genes for having long tails) the females' ideal preferred tail gets longer too. After a generation of this dual selection, both average tail length and average preferred tail length have become longer, but which has increased the most? This is another way of asking what will happen to the choice discrepancy.
