Species
A species is a population of individual organisms that can interbreed in nature, mating and producing fertile offspring in a natural setting. Species are organisms that share the same gene pool, and therefore genetic and morphological similarities.
All organisms are given two names (a binomial name); the first is the genus name and the second is the species name, for example Homo sapiens, the name for humans. The Linnaean classification system places all organisms into a hierarchy of ranked groups. The genus includes one or more related species, while a group of similar genera are placed in the same family. Similar families are grouped into the same order, similar orders in the same class, and similar classes in the same phylum.
Organisms are assigned to the higher ranks of the Linnaean classification scheme largely on the basis of shared similarities (syna pomorphisus). Species are identified on the basis of an organism's ability to interbreed, in addition to its morphological, behavioral, and biochemical characters. Although species are defined as interbreeding populations, taxonomists rarely have information on an organism's breeding behavior and therefore often infer interbreeding groups on the basis of reproductive system morphology, and other shared characters.
In the last 20 years, modern molecular techniques such as DNA hybridization have allowed biologists to gain extensive information on the genetic distance between organisms, which they use to construct hypotheses about the relatedness of organisms. From this information researchers hypothesize as to whether or not the populations are genetically close enough to interbreed.
While the biological species concept has historically been the most widely used definition of a species, more recently the phylogenetic and ecological species concepts have taken the forefront as a more inclusive and useful definition. Whereas the biological species concept defines a species as a group of organisms that are reproductively isolated (able to successfully breed only within the group), the phylogenetic species concept considers tangible (and measurable) differences in characteristics. This idea, also called the cladistic species concept, examines the degree of genetic similarity between groups of related individuals (called clades) as well as their similarities in physical characteristics. For instance, the biological species concept might group coyotes and wolves together as one species because they can successfully breed with one another. In contrast, the phylogenetic concept would definitively split coyotes and wolves into two species based upon the degree of divergence in genetic characters and larger observable traits (like coat color, for instance). In contrast to these, the ecological species concept might classify wolves and coyotes as different species by comparing the differing environmental resources that they exploit, called adaptive zones. Currently, the precise definition of a species is a topic under constant scientific debate and likely will never fully be resolved. Rather, the definition may change with the perspectives and needs of each sub-discipline within biology (ecology versus zoology, for example). A pluralist approach combines some or all of these species concepts to arrive at a more inclusive definition.
Speciation is the process whereby a single species develops over time into two distinct reproductively isolated species. Speciation events are of two types--either allopatric or sympatric. Allopatric speciation results from the division of a population of organisms by a geographical barrier. The isolation of each of the two populations slowly results in differences in the gene pools until the two populations are unable to interbreed either because of changes in mating behavior or because of incompatibility of the DNA from the two populations. The early stages of allopatric speciation are often evident when one examines the same species of fish from different ponds. Fish from the two ponds may not appear to be morphologically different, but there may be slight differences in the gene pools of each population. If the two fish populations remain separated for enough generations, they may eventually become two separate reproductively isolated species.
Sympatric speciation is less frequent than allopatric speciation and occurs when a group of individuals becomes reproductively isolated from the larger population occupying the same range. This type of speciation may result from genetic changes (or mutations) occurring within individuals that inhibits them from interbreeding with others, except those in which the same mutation has occurred. Polyploid plant species, that is, species with more than two copies of each chromosome, are thought to have arisen by sympatric speciation.
More than 1.5 million species have been described, and it is estimated that there are between 10-50 million species currently inhabiting Earth.
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