Final answer:
Sexual reproduction involves the fertilization of an egg by a sperm, forming a zygote. This process varies across species and can occur internally, as in mammals through intercourse, or externally in water, as in many aquatic animals and amphibians.
Step-by-step explanation:
During sexual reproduction, the process of fertilization involves the union of male and female gametes. The male gamete, or sperm, is transferred to the female reproductive tract through a variety of mechanisms across different species. In mammals, this typically occurs during sexual intercourse, where sperm are released into the vagina and travel through the uterus to reach the fallopian tube. Here, if an egg (oocyte) is present, fertilization can occur, resulting in a fertilized egg, known as a zygote. This zygote will contain genetic material from both parents and is the first stage in the development of a new individual.
In other species, such as some aquatic animals and amphibians, external fertilization occurs when the male releases sperm into the water near where the female has laid her eggs. For example, during the mating of frogs, the male clasps the female and releases sperm as she lays her eggs in water. In birds, on the other hand, a process known as cloacal joining occurs. Regardless of the method, the ultimate goal is for sperm to meet with an egg to result in fertilization, which can lead to implantation and development of an embryo, culminating in the birth of offspring.