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Practically, biologists define species as populations of organisms that have a high level of genetic similarity.[link widoczny dla zalogowanych] This may reflect an adaptation to the same niche, and the transfer of genetic material from one individual to others, through a variety of possible means. The exact level of similarity used in such a definition is arbitrary, but this is the most common definition used for organisms that reproduce asexually (asexual reproduction), such as some plants and microorganisms.
This lack of any clear species concept in microbiology has led to some authors arguing that the term "species" is not useful when studying bacterial evolution. Instead they see genes as moving freely between even distantly related bacteria, with the entire bacterial domain being a single gene pool. Nevertheless, a kind of rule of thumb has been established, saying that species of Bacteria or Archaea with 16S rRNA gene sequences more similar than 97% to each other need to be checked byDNA-DNA Hybridization if they belong to the same species or not.[11] This concept has been updated recently,[link widoczny dla zalogowanych] saying that the border of 97% was too low and can be raised to 98.7%.[12]
In the study of sexually reproducing organisms, where genetic material is shared through the process of reproduction, the ability of two organisms to interbreed and produce fertile offspring of both sexes is generally accepted as a simple indicator that the organisms share enough genes to be considered members of the same species. Thus a "species" is a group of interbreeding organisms.
This definition can be extended to say that a species is a group of organisms that could potentially interbreed – fish could still be classed as the same species even if they live in different lakes, as long as they could still interbreed were they ever to come into contact with each other. On the other hand, there are many examples of series of three or more distinct populations, where individuals of the population in the middle can interbreed with the populations to either side,[link widoczny dla zalogowanych] but individuals of the populations on either side cannot interbreed. Thus,[link widoczny dla zalogowanych], one could argue that these populations constitute a single species, or two distinct species. This is not a paradox; it is evidence that species are defined by gene frequencies, and thus have fuzzy boundaries.

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