In the simplest sense, reproduction occurs when parents produce offspring. Becasue of the great diversity of life, organisms reproduce by a variety of methods. Reproduction can be divided into two broad types, asexual reproduction and sexual reproduction, that differ in the genetic mechanisms used and the resulting amount of genetic variation between parents and offspring. Most species reproduce sexually, some reproduce only asexually, whereas some species reproduce both sexually and asexually during their life cycles.
Asexual budding in Hydra viridissma. Photo Wikipedia In asexual reproduction, one parent gives rise to offspring that are genetically identical to the parent as well as to the other offspring. There is no genetic variation produced because asexual reproduction uses mitosis as the genetic process to duplicate the genome. The process of mitosis attempts to create genetically identical genetic copies, but slight genetic variation might be produced by mutations occuring during the process.
Most bacteria and some protists, fungi, plants, and animals reproduce asexually. There are a number of mechanisms of asexual reproduction including binary fission, budding, vegetative reproduction, fragmentation, partenogenesis, agamogenesis and apomixis.
Sexual reproduction results in the production of offspring that are genetically different from their parents and from their siblings. Typcially, sexual reproduction occurs when two parents produce haploid gametes, by the process of meiosis, which then combine to produce a diploid zygote. Genetic variaiton results during the process of meiosis which produces gametes and the random combination of genes that occurs during fertilization.
Generally, two parents are involved in sexual reproduction. The sex of a parent is defined by the size and function of gametes they produce. The female parent produces eggs that contain a haploid genotype and resources to support the growth and development of the embryo. The male parent produces sperm that contain a hapoid genotype. Sperm are generally mobile, so they are able to locate and fertilize the egg to produce a diploid zygote.
However, two sexes are not always required for sexual reproduction! Some species (for example some algae such as Chlamydamonas sp.) are isogamous. In isogamous species, individuals use the process of meiosis to produce haploid isogametes, which are all of the same size and function (they are motile and they contain resources to nourish the zygote). Two isogamtes fuse (syngamy) to produce a zygote.
Because of the great variation in life cycles of different organisms, the exact details of how sexual reproduction takes place varies among different groups of organisms. See the References and Further Reading section for links to brief descriptions of sexual reproduction in different organisms.
Sex in bacteria
Because the production of genetic variation is such an important result of sexual reproduction, mechanisms that produce genetic variation in bacteria (e.g., conjugation, transformation, and transduction) are sometimes referred to as "bacterial sex". Note that genetic variation in bacteria can occur without reproduction.
Importance of Reproduction
Stylized view of the human life cycle. Photo from wikispaces.psu.edu For a number of physical, chemical, and biological reasons there are no immortal organisms. Thus, without reproduction to produce new individuals all genetic lineages would come to an end relatively quickly. Reproduction is therefore a critical component of the life cycle of all species. In the simplest sense, all lifeforms begin existence as "offspring"; the offspring mature over time into adult that are capable of reproduction, and eventually the adults reproduce to produce offpsring, thus completing the cycle.
Reproduction adds new individuals into a population. Reproduction and mortality are the two most important factors influencing patterns of population growth (along with emigration and immigration). When the rate of production of new individuals by reproduction (the birth rate) is greater than the death rate then populations will increase in size. Alternatively, populations will decrease in size when there are more deaths than births.
The process of natural selection requires genetic variation of DNA. Genetic variation can be produced by mutations in any cells in an organism's body. However, somatic mutation, those that occur anywhere in the body except for the germ cells, are not able to be passed on, so they are not able to create variation that can be acted on by selection (except for some cases of vegetative asexual reproduction). Most of the genetic variation in sexually reproducing species is produced during reproduction.
Furthermore, differential reproduction is an important component of natural selection and most organisms have adaptations and stragegies that allow them to maximize reproduction in their environments. Thus, genetic variation created during reproduction and the variation of reproductive success of individuals with different genotypes plays and important role in natural selection and speciation.
Environmental impacts to reproduction
There are a number of human activities which generate significant adverse impacts to reproduction in the natural environment. Some of the chief of these phenomena are:
- Soil and water contamination by pesticides, including herbicides, produce substantial mutagenic and reproductive success interference, particularly for many amphibian, arthropod and other species groups
- Habitat fragmentation can have a major adverse impact, especially upon the mating success of large fauna, whose populations are already low in density such as the Sumatran rhinoceros, Cheetah and Cougar. Habitat fragmentation also produces metapopulations, where there may have been an earlier landscape of a larger intact habitat unit; these smaller isolated metapopulations may be more difficult to sustain, given the realities of stochastic reproductive success in small populations.
- Genetic pollution is a phenomenon in which introduced alien species may interbreed with wild populations of a native species. The outcome is decreased reproductive success of the wild species itself. An example is the widespread human intrusion into natural areas inhabited by the Painted hunting dog in Africa. Humans bringing in domesticated canines are responsible for genetic interference with the Painted hunting dog, Australian dingo and other wild canids.
References and Further Reading
- Asexual Reproduction Biology. About.com
- Asexual Reproduction University of California Museum of Paleontology
- Asexual Reproduction Wikipedia
- Isogamy Wikipedia
- Protists- Reproduction and Life Cycles Britanica Academic Edition
- Reproduction in Fungi MicrobiologyBytes
- Plant Reproduction Wikipedia
- Animal Life- Reproduction The Learning Zone, Oxford University Museum of Natural History