All organisms continue their species through reproduction. Sexual reproduction involves the fusion of reproductive cells, called gametes, in a process called fertilization. Organisms reproduce asexually when they produce offspring without the fusion of gametes. This type of reproduction is primarily found among plants, microorganisms and lower animals such as insects and reptiles.
Biologists recognize several forms of asexual reproduction. Budding occurs when an organism produces small buds, or outgrowths, that break away from the parent. Fragmentation occurs when an organism breaks into pieces and each piece grows into a new individual. Fission occurs when single cell organisms divide into two or more similar daughter cells. Parthenogenesis occurs when offspring develop from an unfertilized egg. Vegetative propagation occurs when new plants grow from specialized parts such as tubers or bulbs that break off from the adult plant. A spore is a reproductive cell that develops into a new individual without combining with another cell. Spores develop into either a small version of the parent or another phase in the organisms reproductive cycle.
Microorganisms and Animals
A wide variety of microorganisms reproduce asexually. Protozoans, bacteria and a group of algae called diatoms reproduce through fission. The simple microscopic animals known as cnidaria and the annelids, also called ringworms, reproduce through fragmentation. Biologists have discovered nearly 70 species of vertebrates that can reproduce parthogenetically. Examples include frogs, chickens, turkeys, Komodo dragons and hammerhead sharks.
Biologists call asexual reproduction among plants apomixis, which means "without mixing." Biologists theorize that plants developed asexual reproduction as a way of colonizing a large area in harsh environments such as arctic and alpine environments. Strawberries reproduce through horizontal stems called runners. Dandelions and blackberries reproduce through seeds that form asexually. Ferns and mosses reproduce through spores. Some trees, such as seedless navel oranges, can only reproduce with the aid of humans who cut off part of the tree and plant it.
Asexual and Sexual Reproduction
Some species reproduce both sexually and asexually. For example aphids reproduce through parthenogenesis in the spring and summer when environmental conditions such as the food supply can support rapid population growth. They reproduce sexually in the fall and winter when resources are limited. Among some species of ants, wasps and bees, the type of reproduction determines the sex of the babies. For example, unfertilized bee eggs produce males while fertilized eggs produce females. Tiny aquatic creatures called rotifers reproduce parthenogenetically in the spring and summer. However, their eggs only produce females. In the fall they produce tiny offspring that lack digestive tracts but produce sperm. These creatures fertilize eggs and hatch a new generation of females in the spring.