Final answer:
Sexual dimorphism is the term for the difference in form between male and female elephant seals, arising from sexual selection. Successful males with longer trunks pass on their genes, and this trait is not related to directional selection for smaller size but maintains sexual dimorphism in the species.
Step-by-step explanation:
Sexual Dimorphism in Elephant Seals
The term describing the difference in form between male and female elephant seals is sexual dimorphism. Male elephant seals, which establish territories where one successful male can mate with many females, have trunks that are getting longer over time. This characteristic, where males and females of a species have distinctly different appearances or sizes, is a result of sexual selection pressures. Sexual selection drives the evolution of exaggerated traits like larger body size or, in this case, longer trunks in males due to their success in mating competitions.
Alternative mating strategies, such as the small males that sneak copulations, demonstrate that not only the large alpha males pass on their genes. This is not an example of directional selection towards smaller size, because the larger males still dominate the mating system and pass on their genes. Hence, the observed sexual dimorphism in elephant seals persists.
Sexual dimorphism is widespread across many species, with examples including the peacock's tail or male lions' manes. Traits favored by sexual selection may enhance reproductive success but do not necessarily increase the individual's survival chances. This is known as the handicap principle.