Fig. 1.

Darwin's model of sexual selection. (A) First, males and females are variable with respect to phenotypic quality. (B) Males, which have the ornaments, move to the breeding area (large circle) before the females. (C) The highest-quality females are ready to mate sooner, so they pair with the highest-quality males. (D) Finally, the lower-quality females pair with the lower-quality males. Sexual selection occurs because the higher-quality pairs produce more offspring than the lower-quality pairs.