Sexual reproduction is the creation of new individuals by the joining of the separate sex cells of two parents. Each offspring produced by sexual reproduction has a unique collection of genes and are well-equipped to adapt to change. Most animals reproduce sexually.
Sexual reproduction describes the process in which the gametes (sex cells) of the two parents come together and form a fertilized egg cell called a zygote. The zygote then develops into a new individual. Since each parent contributes genetic information to the new and unique individual created, sexual reproduction is a far more complex phenomenon than asexual reproduction. (Asexual reproduction is when just one parent copies its genetic material creating a new, but identical, individual.) The gametes of each parent must have specific characteristics in order for sexual reproduction to work properly. Specifically, each set of gametes must have only half the total number of chromosomes that nonsex cells of that species contain. This ensures that when the nuclei (singular, nucleus; the cell's control center) of the male sperm and female egg join together, instead of having twice the normal number of chromosomes, the fertilized egg will have exactly the right number. In the case of humans, that number is forty-six chromosomes (with twenty-three obtained from each parent).
Sexual reproduction also applies to many plants—pollen is the equivalent of male sperm and it fertilizes the female sex cell of the flower, called an ovum. However, sexual reproduction in the animal world is the focus here. Organisms that reproduce sexually are usually fairly complex, and the individuals involved are usually either male or female, with the sex of each being separate. Males and females have different types of gonads or sex organs in which the gametes develop. The males produce sperm in gonads called testes, and the females produce egg, or ova, in gonads called the ovaries. Sperm are generally smaller than the egg and must swim through a liquid to get to the egg in order to fertilize it.
EXTERNAL AND INTERNAL FERTILIZATION
Although there are some instances where one individual produces both sperm and eggs, these are the exception. Fertilization or the union of a male sex cell and a female sex cell can occur inside the female (internal fertilization) or outside (external fertilization). Both methods have advantages and disadvantages. Aquatic organisms (those that live in water) more commonly use external fertilization. The sperm and eggs are released into the water where they meet, and fertilization occurs. (Actually, the sex cells are scattered into the environment and the rest is left to chance.) To increase the odds for fertilization, gametes are released in large numbers. For example, oysters release millions of sperm and eggs into the water. Although there may be great waste in this process, there is little cost to the parents who do not have to care for the offspring produced. Animals that use external fertilization, like fish and frogs, have generally short lives and do not reproduce many times.
Internal fertilization is practiced by most land animals, including humans. Since the sperm and egg are united inside the body of the female, both are protected, and there is a better chance the gametes will meet. Although there are still large numbers of sperm, fewer eggs are needed. The female's body also provides the proper environmental conditions for the survival of the gametes (unlike the harsh conditions of external fertilization in which many eggs are often devoured by predators).
For most animals, internal fertilization must take place within a certain time period. For example, in humans, there is only one twenty-four-hour period every twenty-eight days that the egg is able to be fertilized. Internal fertilization also requires special adaptations such as gamete delivery. This means that specialized sex organs (such as the male penis and female vagina) are used to place the sperm as close as possible to the egg inside the female's body. After internal fertilization occurs, the zygote (fertilized egg cell) is either released from the body within some sort of protective covering, like a shell, or it remains within the body where it develops fully. Development inside a shell or body means that the zygote goes through a series of cell divisions. These begin to specialize and form individual body parts.
ADVANTAGES OF SEXUAL REPRODUCTION
Probably the biggest advantage of sexual reproduction over asexual reproduction is that there is great genetic diversity since the new individual inherits genetic material from two different individuals. Because of this, populations (or the many individuals that make up a single species) are better able to survive any environmental changes that may occur. While those less-adapted might die, other better-adapted individuals would survive and therefore reproduce passing on their better-adapted genes.