Reproduction in animals can occur through asexual or sexual means. There are two main types of sexual reproduction: external fertilization, where gametes join outside the body which is common in water-dwelling species like fish and frogs, and internal fertilization, where gametes join inside the body which requires specialized organs and is more common in land animals like birds and mammals. As animals evolved from aquatic to terrestrial environments, their reproductive strategies and development changed accordingly through adaptations like internal fertilization, pregnancy, and structures that support development outside the parent's body.