In the context of biology, species is defined as the basic unit of classification of the organisms. In addition, species can also be defined as the taxonomic rank of an organism and it is one of the most significant factors of the biodiversity. Species is often being considered as the largest group of organisms in which two individuals from different sexes meet each other and fertile offspring by the process of sexual reproduction.

Species in its simplest form is considered as a certain group of organisms, which share same genetic heritage and able to create offspring. There is a reproductive barrier, which differentiate the organisms from each other. It is imperative to note in this context that the reproductive barrier can be of several types such as geographical like mountain range which can separate two of the different population. In addition to that, there is a genetic barrier, which prevents the reproduction from two populations. However, the scientists have changed the definition of species. In the contemporary times, species is being considered as one of the major classifications used by the scientists in order to describe the animals. The system of binomial nomenclature has been used by the scientists for the sake of describing the animals of the common name. There are different concepts of species, which have been executed by the scientists of different times. According to the biological concept, species are defined as two individuals are members of identical species, who can reproduce by meeting with each other. On the other hand, according to nominalist species concept, the species do not exist actually. It is true that it is philosophically interesting topic still it is not acceptable at all as species has been described by the entire globe. The evolutionary species is also a significant matter of consideration in this context. This definition of the fossil has been given by the paleontologists who are researching on fossil through the geological time.

Today, the modern scientists are considering the biological species concept. Still it is also posing some of the major issues. It has been observed that there are wide ranges of species, which can reproduce clonally without having sex. The greatest examples of this type of species are bacteria and amoeba and several other types of microorganisms. However, the bigger animals can also challenge the modern biological concept of species. It has been observed that sometimes, the females can reproduce clones of themselves by sexually reproducing greenfly in which the females can experience a mutation which can lead the females to reproduce.

If the aphid can find the best environment, they can reproduce clones rapidly. It is worth to note here that this is the fundamental reason for which the aphids are considered as one of the greatest pesticides. A list of different species along with the examples is presented below:

The species are being considered through the fact that they are reproductively different from each other. This defines the fact that the organisms belonging from one particular species are capable of reproducing with organisms in another species. This is the most basic category of the taxonomy. This is one of the most important scientific systems, which is the reason for which the organisms are categorized in accordance with their biological characteristics. The species can also categorized on the basis shared evolutionary history and ancestry. The method, followed by the scientists in order to define the species is phylogenetic. This can also be defined as the evolutionary relationship in between the organisms. Speciation is considered as the evolutionary process through which the new species.