Species

A species (pl. species) is often defined as the largest group of <a href="/facts/Organism/j8JkL09u">organisms</a> in which any two individuals of the appropriate <a href="/facts/Sex/6VG77YCN">sexes</a> or <a href="/facts/Mating_type/cIw9lAKr">mating types</a> can produce fertile <a href="/facts/Offspring/v7LD4qBl">offspring</a>, typically by <a href="/facts/Sexual_reproduction/hmsXy53s">sexual reproduction</a>. It is the basic unit of <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">classification</a> and a <a href="/facts/Taxonomic_rank/bPFQpe9A">taxonomic rank</a> of an organism, as well as a unit of <a href="/facts/Biodiversity/GruakEzs">biodiversity</a>. Other ways of defining species include their <a href="/facts/Karyotype/UM8j41yA">karyotype</a>, <a href="/facts/DNA/nB89Iauz">DNA</a> sequence, <a href="/facts/Morphology_(biology)/Nl3B0myy">morphology</a>, behaviour, or <a href="/facts/Ecological_niche/YnhNkL2e">ecological niche</a>. In addition, <a href="/facts/Palaeontologist/60n5yldd">palaeontologists</a> use the concept of the <a href="/facts/Chronospecies/vRHe5aHg">chronospecies</a> since <a href="/facts/Fossil/by7NzIA1">fossil</a> reproduction cannot be examined. The most recent rigorous estimate for the total number of species of <a href="/facts/Eukaryote/SkwyPLMA">eukaryotes</a> is between 8 and 8.7 million. About 14% of these had been described by 2011. All species (except <a href="/facts/Virus/mf88Bcbl">viruses</a>) are given a <a href="/facts/Binomial_nomenclature/jgBqAhPv">two-part name</a>, a "binomen". The first part of a binomen is the name of a <a href="/facts/Genus/m8EFjSms">genus</a> to which the species belongs. The second part is called the <a href="/facts/Specific_name_(zoology)/RuH34bxb">specific name</a> or the <a href="/facts/Specific_name_(botany)/31ckrsXv">specific epithet</a> (in <a href="/facts/Botanical_nomenclature/7KmBSwV8">botanical nomenclature</a>, also sometimes in <a href="/facts/Zoological_nomenclature/Pxg8gsqf">zoological nomenclature</a>). For example, <a href="/facts/Boa_constrictor/x2Krn5RT">Boa constrictor</a> is one of the species of the genus <a href="/facts/Boa_(genus)/mXh7FpLq">Boa</a>, with constrictor being the specific name.
While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. For example, the boundaries between closely related species become unclear with <a href="/facts/Hybrid_(biology)/pzVJ54xU">hybridisation</a>, in a <a href="/facts/Species_complex/u6qEMdRQ">species complex</a> of hundreds of similar <a href="/facts/Microspecies/u6qEMdRQ">microspecies</a>, and in a <a href="/facts/Ring_species/fDAfgocV">ring species</a>. Also, among organisms that reproduce only <a href="/facts/Asexual_reproduction/HpovW0VS">asexually</a>, the concept of a reproductive species breaks down, and each clonal lineage is potentially a microspecies. Although none of these are entirely satisfactory definitions, and while the concept of species may not be a perfect model of life, it is still a useful tool to scientists and <a href="/facts/Conservation_biology/S2693mIF">conservationists</a> for studying life on Earth, regardless of the theoretical difficulties. If species were fixed and distinct from one another, there would be no problem, but <a href="/facts/Evolution/qE5amP6a">evolutionary</a> processes cause species to change. This obliges <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">taxonomists</a> to decide, for example, when enough change has occurred to declare that a fossil lineage should be divided into multiple <a href="/facts/Chronospecies/vRHe5aHg">chronospecies</a>, or when populations have diverged to have enough distinct character states to be described as <a href="/facts/Cladistic/JasKVABk">cladistic</a> species.
Species and higher taxa were seen from <a href="/facts/Aristotle/U2bDlBFW">Aristotle</a> until the 18th century as categories that could be arranged in a hierarchy, the <a href="/facts/Great_chain_of_being/rMzBFDtc">great chain of being</a>. In the 19th century, biologists grasped that species could evolve given sufficient time. <a href="/facts/Charles_Darwin/uwDLGItY">Charles Darwin</a>'s 1859 book <a href="/facts/On_the_Origin_of_Species/zTMxkKVY">On the Origin of Species</a> explained how <a href="/facts/Speciation/pmRTQbTK">species could arise</a> by <a href="/facts/Natural_selection/JXTte6en">natural selection</a>. That understanding was <a href="/facts/Modern_synthesis_(20th_century)/anJdPq34">greatly extended in the 20th century</a> through <a href="/facts/Genetics/bu8wne0A">genetics</a> and population <a href="/facts/Ecology/YE8upuJU">ecology</a>. Genetic variability arises from <a href="/facts/Mutation/Ee4NnWbY">mutations</a> and <a href="/facts/Recombination_(biology)/uocNH0m7">recombination</a>, while organisms are mobile, leading to <a href="/facts/Allopatric_speciation/H6K8mSDf">geographical isolation</a> and <a href="/facts/Genetic_drift/lvoEEctX">genetic drift</a> with varying <a href="/facts/Evolutionary_pressure/8zrwXR50">selection pressures</a>. Genes can sometimes be exchanged between species by <a href="/facts/Horizontal_gene_transfer/FPyUIsf2">horizontal gene transfer</a>; new species can arise rapidly through hybridisation and <a href="/facts/Polyploid/L7xyKHdK">polyploidy</a>; and species may become <a href="/facts/Extinction/24p5NP2q">extinct</a> for a variety of reasons. <a href="/facts/Viruses/mf88Bcbl">Viruses</a> are a special case, driven by a <a href="/facts/Mutation%25E2%2580%2593selection_balance/snNB7Acd">balance of mutation and selection</a>, and can be treated as <a href="/facts/Viral_quasispecies/CQNDYgrX">quasispecies</a>.

Species open-in-new

Species