In biology, a species (abbreviated sp., with the plural form species abbreviated spp.) is one of the basic units of biological classification and a taxonomic rank. A species is often defined as the largest group of organisms in which two individuals are capable of reproducing fertile offspring, typically using sexual reproduction. While in many cases this definition is adequate, the difficulty of defining species is known as the species problem. For example, a species complexis a group of closely related species that are very similar in appearance to the point that the boundaries between them are often unclear. Differentiating measures include similarity of DNA, morphology, or ecological niche. Presence of specific locally adapted traits may further subdivide species into "infraspecific taxa" such as subspecies (and in botany other taxa are used, such as varieties, subvarieties, and formae).
Species hypothesized to have the same ancestors are placed in one genus, based on similarities. The similarity of species is judged based on comparison of physical attributes, and where available, their DNA sequences. All species are given a two-part name, a "binomial name", or just "binomial". The first part of a binomial is the generic name, the genus to which the species belongs. The second part is either called the specific name (a term used only in zoology) or the specific epithet (the term used in botany, which can also be used in zoology). For example, Boa constrictor is one of four species of the Boa genus. While the genus gets capitalized, the specific epithet does not. The binomial is written in italics when printed and underlined when handwritten.
A usable definition of the word "species" and reliable methods of identifying particular species are essential for stating and testing biological theories and for measuring biodiversity, though other taxonomic levels such as families may be considered in broad-scale studies. Extinct species known only from fossils are generally difficult to assign precise taxonomic rankings, which is why higher taxonomic levels such as families are often used for fossil-based studies.
The total number of non-bacterial and non-archaeal species in the world has been estimated at 8.7 million, with previous estimates ranging from two million to 100 million.
What is a species? At first glance, the answer to this question may appear to be straightforward: a species is a distinct "kind" of animal, plant, fungus, or other organism. But where does one species end and another begin? How do we define or recognize the boundary between two species? Biologists have long debated this question on both philosophical grounds and practical grounds. How should we define species ideally? Given the limits of our information about the biology and evolutionary history of most organisms, what practical criteria can we use to consistently define species?
Perhaps the most widely accepted species concept is known as the Biological Species Concept (BSC). According to this definition, proposed by the evolutionary biologist Ernst Mayr in the mid-20th century, species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups. Within this definition, a species represents a set of individuals connected by gene exchange ("gene flow") that is genetically isolated from all other such sets of individuals. There is gene flow among individuals within a species, but not between different species. This lack of gene exchange means that different species can evolve independently.
Other approaches to defining and thinking about species have focused on lineages in branching evolutionary trees, with ancestral species splitting into two different lineages and the tips of the branches representing the species that are with us today. The focus on reproductive isolation (biological species concept) versus the relationships among branches in evolutionary trees (lineage-based species concepts) represents two perspectives on the speciation process (the origin of new species) that are not mutually exclusive.
Today, most biologists are more interested in understanding the process of speciation than in trying to find a strict species definition that is always applicable. Speciation is usually a gradual process, so it is not unusual to encounter populations that are only partly reproductively isolated. This means that individuals from diverged lineages may still exchange genes to a limited degree, perhaps even to the extent that they will merge again. These situations are challenging for both the biological species concepts and lineage species concepts. Although some people may wish for a black-and-white criterion for defining species, this is unrealistic. By analogy, imagine a population of maturing humans. Most individuals will be easily recognized as children or adults, but some will be difficult to categorize and these difficult individuals might be tagged differently by different people using different criteria (e.g., different physical characteristics, different measures of emotional maturity, etc.). Similarly, most individual birds or snails or mushrooms can be readily categorized as belonging to one species or another, but exceptions are not rare.
In practice, direct information on reproductive isolation or evolutionary relationships between groups of individuals is often unavailable. Because reproductively isolated lineages are genetically isolated, they evolve independently and tendand tend to accumulate genetic differences (and therefore morphological and other differences). Scientists can use these differences to infer reproductive isolation without direct evidence on gene exchange. Historically, and still today, careful analysis of morphology (physical characteristics) has been used to infer which groups are probably reproductively isolated and represent independently evolving lineages. In recent decades, newly available DNA sequence data has provided a rich new source of data for both assessing evidence of gene exchange in recent generations (relevant to biological species concept status) and estimating the evolutionary relationships (relevant to lineage species concepts) among living organisms.
The question of what constitutes a species can clearly be a very complex one, and in many cases it may not be obvious where a species boundary should be drawn. Nevertheless, more often than not the organisms we encounter can reasonably be placed in distinct "bins". This is why field guides work well most of the time.
Inferring Evolutionary Relationships Among Organisms
For a variety of reasons, figuring out the relationships among species is much more complex than simply grouping by similarity in appearance. For example, two organisms that are not closely related (such as a cactus and a euphorb) may resemble each other because they have evolved similar adaptations for similar environmental conditions (this phenomenon is known as "convergent evolution"). Alternatively, two individuals that appear morphologically identical may nevertheless belong to distinct reproductively isolated species--with reproductive isolation based, for example, on different sex pheromones or mating behaviors (distinct species that are morphologically indistinguishable, or nearly so, to human scientists are known as "cryptic species").
While morphological data are clearly extremely useful for investigating the evolutionary relationships among organisms, morphology can be limited or misleading. In recent years, DNA sequence data have become central to most studies of evolutionary relationships. Although the quantity of available data that can be obtained from comparing DNA sequences is very large, inferring evolutionary relatedness based on simple similarity in DNA sequence is similarly problematic. DNA consists of long strings of chemical units, or nucleotides, of just four types (abbreviated as G, A, C, and T). As DNA is copied across generations the base at each position may mutate among these four nucleotides, making the history challenging to reconstruct when we look at the result of this history by sequencing the DNA of current-day species. These challenges have spurred the development of very sophisticated analytical methods, beyond the scope of this overview, for reconstructing the evolution of DNA and other characters from hypothetical common ancestors to the organisms alive today. By applying these methods, with ever increasing amounts of computer power and quantities of genetic data as technology advances, scientists' understanding of the relationships among organisms continues to improve at a rapid rate.
Defining a species
A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.
For example, these happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator.
That definition of a species might seem cut and dried, but it is not — in nature, there are lots of places where it is difficult to apply this definition. For example, many bacteria reproduce mainly asexually. The bacterium shown at right is reproducing asexually, by binary fission. The definition of a species as a group of interbreeding individuals cannot be easily applied to organisms that reproduce only or mainly asexually.
Also, many plants, and some animals, form hybrids in nature. Hooded crows and carrion crows look different, and largely mate within their own groups — but in some areas, they hybridize. Should they be considered the same species or separate species?
If two lineages of oak look quite different, but occasionally form hybrids with each other, should we count them as different species? There are lots of other places where the boundary of a species is blurred. It's not so surprising that these blurry places exist — after all, the idea of a species is something that we humans invented for our own convenience!
a class of individuals having some common characteristics or qualities;distinct sort or kind.
Biology. the major subdivision of a genus or subgenus, regarded as thebasic category of biological classification, composed of relatedindividuals that resemble one another, are able to breed amongthemselves, but are not able to breed with members of anotherspecies.