Final answer:
After a drought, sperm-depleted males preferentially select larger, more fecund females to maximize reproductive success. Sexual dimorphism may be furthered by selective pressures such as food scarcity, and mate choice can significantly shift the physical characteristics of a population under both sexual selection and natural selection.
Step-by-step explanation:
Sexual selection after a drought can result in changes to the reproductive behaviors and physical characteristics of a species. Particularly, the example involving sperm-depleted males indicates a preference for larger, more fecund females. Studies show that sperm-depleted males who have a limited opportunity to mate because of their depleted sperm supplies are more likely to select larger females who can produce more offspring. This behavior is an adaptive strategy to maximize the reproductive potential with the limited resources the male has available. Furthermore, sexual dimorphisms may be accentuated during periods of drought as the available resources also become scarce, leading to the females selecting mates based on traits that may increase their chances of reproductive success, such as the apparent health and vigor of smaller males over larger ones. In the context of a bird population during a drought, with the prevalence of large, hard seeds, it is predictable under natural selection that birds with larger and broader beaks would have an advantage in accessing food resources, likely leading to a population shift favoring these traits. This mirrors sexual selection pressures that also influence reproductive success through mate choice and can lead to sexual dimorphism in species.