Why in many species have males and females evolved to be sexually dimorphic?

Why in many species have males and females evolved to be sexually dimorphic?

Intro: define terms

Sexual dimorphism is where there are distinct differences in the appearance of the sexes within a species aside from the sexual organs themselves. 

Evolution occurs when alleles that induce specific traits become widespread in a population through natural selection. For this to occur, the allele therefore has to have a beneficial affect on the reproductve fitness of individuals that have them or at least have a neutral impact. Therefore any differences between the sexes in species today, most likely benefit the propogation of the species in some way.

Paragraph 1: Intrasexual selection

Selection between members of the same sex can occur through male-male rivalry for female mates whch can include combat and infanticide of previous dominant males to ensure their offspring have less competition. If physical fighting is involved, males are often much larger than the females or have combat adaptations.

Example: Elephant seal - weigh up to 4 tonnes whereas females may weigh 700kg. Males also have enlarged canine teeth for fighting . Mating in the species works where one dominant male controls areas of beaches where females nurse their pups. Males are known as "beach masters" and can control areas with up to 100 females known as a "harem". They fight off potential rivals to gain exclusive rights to mate with the females and therefore increase their reproductive fitness. Fighting is fierce however, so there is strong selective pressure for size and strengh in males. 

Paragraph 2: Intersexual selection

Reproduction often has high…