Species is a biological concept that refers to a population of organisms that are in some important ways similar. What constitutes an "important similarity" is a matter of debate. In the 18th century Carolus Linnaeus classified organisms according to differences in the form of reproductive apparatus. Although his system of classification sorts organisms according to degrees of similarity, it made no claims about the relationship between similar species. At the time some believed that there is no organic connection between species no matter how similar they appear; every species was created by God, a view today called creationism.
By the 19th century most naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. The question was, how did species change over time. Lamarck suggested that an organism could pass an acquired trait on to its offspring. For example, if an animal stretched its neck in order to reach the tops of trees, its offspring could have a longer neck.
Charles Darwin provided what scientists now consider the most powerful and compelling theory of evolution. Basically, he argued that it is populations that evolve, not individuals. He pointed out that variation occurs naturally among organisms. Following Thomas Malthus, he suggested that population would often exceed the amount of food available, and that some organisms would die. Darwin suggested that those organisms that would die would be those less adapted to their environment, and that those that survived -- and reproduced -- would be those best adapted to their environment.
These survivors would not pass acquired traits on to their offspring; they would pass their inherited traits on to their offspring. But since the environment effectively selected which organisms would live to reproduce, the environment would select which traits would be passed on. This is the theory of evolution by "natural selection." For example, among a group of animals some have longer necks, others have shorter necks. If all the leaves are high up, those with shorter necks will die; those with longer necks will thrive. This process is evident today as resistant strains of bacteria evolve.
The development of the field of genetics (many years after Darwin) has revealed the mechanisms that generate variation as well as those through which traits are passed on from generation to generation.
The theory of the evolution of species through natural selection has two important implications for discussions of species -- consequences that fundamentally challenge the assumptions behind Linnaeus' taxonomy. First, it suggests that species are not just similar, they may actually be related. Some students of Darwin argue that ALL species are descended from a common ancestor. Second, it supposes that "species" are not homogeneous, fixed, permanent things; members of a species are all different and over time species change. This suggests that species do not have any clear boundaries but are rather momentary statistical effects of constantly changing gene-frequencies. One may still use Linnaeus' taxonomy to identify individual plants and animals. But one can no longer think of species as independent and immutable.
That species change over many generations is now a cornerstone of biological thought. Prior to Darwin, species were viewed as being a 'type' of organism, with individual differences simply being deviations from the true type. After Darwin, the idea of a true type was rejected, as individual variation was recognized as the raw material of evolutionary change. Differences between individuals could no longer be dismissed as 'imperfections'.
The rise of a new species from a parental line is called speciation. There is no clear line demarcating the ancestral species from the descendent species.
The modern biological species concept defines a species in terms of reproductive isolation. This definition is not considered adequate by all parties - for example, it does not work so well in describing plant species as animal species. It is also problematic when dealing with extinct species.
Whether species is a concept, or whether it has reality independent of our classification systems, is also subject to debate. Answers to this question may hinge on one's understanding of epistemology as much as of biology.
Compare with race.