Sexual reproduction combines the genes of two parents to produce an offspring. Asexual reproduction takes many forms, but in general, the genetic material of only one parent is included in the next generation. Each form of reproduction has strengths and weaknesses, and is suited to different circumstances.
Mechanisms of reproduction
In sexual reproduction, the genetic material of two parents is combined. Before that, the genetic heritage of each parent is mixed and then halved, in a process called meiosis. This way, when genes from each parent are combined, they produce a zygote with the normal amount of genetic material. This is only a general rule, incidentally. Honeybee genetics, for one example, are quite different from the human norm.
In asexual reproduction, the genetic material of only one parent contributes to the offspring. Cloning, in which a plant reproduces itself by sending out stolons or offshoots, for example, is a famous form of asexual reproduction. A clone is a duplicate of the parent organism, whether produced by nature or science. Many one-celled organisms reproduce by cellular fission. They split themselves in half to produce daughter cells of their exact genetic makeup.
Parthenogenesis is a form of asexual reproduction in which the female gives birth without mating. Heterogamy is alternation from generation to generation between sexual and asexual reproduction. Daphnia, water fleas, reproduce this way, so do turkeys in some circumstances, and some sharks.
Benefits and drawbacks of sexual reproduction
Sexual reproduction is more adaptive than sexual reproduction is. Environments change. The climate gets hotter, wetter, or drier. New predators may appear, or new diseases. In order to meet these challenges, a species needs to have as much diversity as possible available within its population. Some members of the population will be suited to certain conditions, others will not. The successful traits will prevail, but currently unsuccessful traits will remain in the population in small numbers or as recessive genes. Their time may come.
Sexual reproduction best encourages diversity. However, sexual reproduction is relatively slow and cumbersome. Potential parents must find suitable mates, court them, and breed. Populations grow relatively slowly because only half the population bears offspring.
Benefits and drawbacks of asexual reproduction
Asexual reproduction is faster. In reproduction by cloning or cellular fission, every member of a population can produce offspring. Obviously, population growth by asexual reproduction can be twice as fast. If the parent is adapted to the environment, and the environment does not change, cloning produces the most adapted offspring. In addition, no energy is wasted in finding mates, courting, or breeding. In situations where potential mates are scarce, parthenogenesis can produce offspring without them.
Asexual reproduction is especially suited to extremely harsh environments. Lichens, for example, symbiotic organisms composed of a fungus and an alga or bacterium, can survive where little else can. They often reproduce by fragmentation, by breaking up. This way, each daughter organism retains a piece of the fungus and of the alga or bacteria that are necessary for continued existence.
On the other hand, when environments undergo radical change, organisms that reproduce asexually have less latent diversity to call upon. They have fewer ways of changing to meet new conditions.
When conditions are changing, sexual reproduction shuffles the genetic deck to produce new organisms more likely to suit the circumstances. However, it is slow and cumbersome. When conditions are stable, or when the environment is unfriendly, asexual reproduction produces new offspring quickly. Certain organisms can use either method of reproduction, to fit the situation.