Reproduction is the process by which new living things are created. People, animals, plants, and even bacteria, reproduce. There are two methods by which living things reproduce--asexual or sexual. Asexual reproduction allows for a living thing to reproduce without another member of its species, while sexual reproduction requires genetic material from two different members of the species, usually but not always, a male and a female. There are a few rare species that have the ability to reproduce both sexually and asexually.
In sexual reproduction, species can be either anisogamous or isogamous. In anisogamous reproduction, there are two parents, generally male and female. In order to reproduce, both contribute a gamete, or reproductive cell containing a haploid, or single set of chromosomes, that will make up the offspring. Each gamete provides half of the offspring's genetic material. Many animals, including humans, reproduce in this way. Males produce sperm, which fertilize a female's ova, or egg, within her body during the process of sexual intercourse. The point at which the egg is fertilized is called conception. The union of the two gametes results in a new cell, called a zygote. This cell continuously divides, implanting itself into the female uterus, and developing into what becomes known first as an embryo, and later, as a fetus. It is nourished and grows in the uterus throughout what is referred to as a period of gestation. This length of time varies for different animals. When development is complete, the birth process allows the female parent to deliver the baby that has grown within her, so that it can begin to thrive and grow on its own. Humans can also fertilize eggs externally, through a process called in vitro fertilization. Some animals, particularly those that live in the water, fertilize eggs externally, by releasing sperm and egg into the water, where they join together and become fertilized. Birds and reptiles have their eggs fertilized internally, and then lay eggs, in which the new creature develops, and then is hatched. Flowering plants even reproduce sexually, through a process called pollination. However, depending on the plant, it may self-pollinate, or cross-pollinate.
In asexual reproduction, a living thing can reproduce without receiving any cells from another living thing. All chromosomes come from one parent causing the offspring to be similar or identical in genetic structure. Organisms that produce asexually include but are not limited to bacteria, molds, yeast and some plants. Methods of asexual reproduction include budding, in which a new organism is formed directly from a part of an organism, such as with plants. Fragmentation means a living thing is split into fragments, which grow to be clones of the original living thing. Molds and starfish are examples of organisms that reproduce through fragmentation. Binary fission occurs when a cell divides into two parts. These organisms may not be genetically identical. Bacteria reproduce in this way. Spore formation is another form of asexual reproduction. According to the New World Encyclopedia, a spore is a reproductive structure that is adapted for dispersion and survival for extended periods of time in unfavorable conditions. Once conditions are favorable, the spore can develop into a new haploid organism using mitotic division.