In asexual reproduction the traits of only one parent appear in offspring, without any genetic mixing. Though this reduces diversity, it does have advantages. For example, large quantities of offspring can be produced quickly, to fill up a niche so thickly that competitors cannot intrude.
The commonest form of reproduction is cloning; it produces offspring that are genetically identical to the parent and to one another. Since cloned offspring are essentially all alike, they are less equipped to deal with changed circumstances. Additionally, an adverse trait from one gene cannot be balanced by a favorable gene from the other parent.
Some creatures use asexual reproduction in some circumstances and sexual reproduction in others; they take advantage of the strengths of each mode. However, many species are stuck in their ways, destined to keep breeding or cloning whether or not it will give their descendants a survival advantage.
Asexual reproduction advantages
Rapid reproduction is an advantage because it shuts out competitors. Redwood trees often clone when something happens to the parent tree. They form fairy circles of new trees around the circumference of a dead or dying parent tree, fascinating sensitive children, and quickly shutting out sunlight that might encourage competing trees. Little competition thrives in the unbroken understory of an old growth redwood grove.
Diatoms, tiny oceanic creatures, also reproduce asexually in certain circumstances. Each hard-shelled diatom splits in half, and then each half completes itself by secreting a new half shell, which is only slightly smaller than the one it started from. This enables diatoms to colonize empty waters fast, but has the drawback that diatoms shrink with each generation. Before they vanish though, they switch to sexual reproduction, and breed strapping large diatoms again.
For some plants cloning works when no pollinator is available. African violets, for example, can replicate themselves when a torn leaf touches dirt and takes root, when a rosette of leaves is accidentally split by an accident or when dirt covers leaves. In the right circumstances, African violets reproduce sexually and set seed as well.
Some animals clone themselves too. Parthenogenesis is the term for the way some lizards, snakes and sharks breed without a partner, possibly in compensation for the scarcity of mates in the loneliness of the desert or the ocean.
Some populations of species that reproduce with parthenogenesis form pair bonds between same sex individuals, and appear to alternate male and female roles. Their reproduction is twice as efficient as sexual reproduction because every animal involved eventually bears young.
Asexual reproduction disadvantages
A single harmful mutation in cloned offspring can cause an entire generation to fail because it is not countered by a gene from the other parent, and has no chance to be discarded when half of each parent's genes are eliminated in sexual reproduction.
Also, clones are identical copies of the parent, which inherit the parent's precise traits and responses to the environment. There is no chance for individuals to appear with a superior response to particular conditions. That is fine if conditions do not change, but sometimes they do.
While asexual reproduction is obviously more efficient, it does not provide for change and adaptation in new generations. As individuals, organisms produced by sexual reproduction have a better chance of thriving in a changing world.