It is a challenging task to find and recognize a potential mate of the same species. Males spend much energy in performing elaborate courtship rituals. Fertilization also comes at a cost with separate sexes. External fertilization in water requires large numbers of gametes. Internal fertilization requires an investment in elaborate reproductive organs, including the penis, to transfer sperm to the female.
Energy is set aside for nourishing some number of offspring.
Those eggs with little yolk must develop larval stages quickly. Others, such as birds, have adequate food reserves for a more lengthy development within the shell. Fertilization begins when a sperm penetrates an egg and is completed when the sperm nucleus fuses with the egg nucleus, resulting in formation of the zygote. Organ formation begins as germ layers subdivide into populations of cells destined to become unique in structure and function. During growth and tissue specialization, organs acquire specialized chemical and physical properties.