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Biological diversity does not vary in a smooth, continuous way. People have long recognized groups of similar organisms that mate with one another, and they have noticed that there are usually distinct morphological breaks between these groups. Groups of organisms that mate with one another are commonly called species (note that this is both the plural and singular form of the word). Species are the result of the process of speciation: the divergence of biological lineages and the emergence of reproductive isolation between lineages.
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Biologists approach the concept of species differently, depending on the focus of their research.
The various species concepts are connected by the importance of reproductive isolation in understanding the origin of species.
Although “species” is a useful and common term, its usage varies among biologists who are interested in different aspects of speciation. Different biologists think about species differently because they ask different questions: How can we recognize and identify species? How do new species arise? How do different species remain separate? Why do rates of speciation differ among groups of organisms? In answering these questions, biologists focus on different attributes of species, leading to several different ways of thinking about what species are and how they form. Most of the various species concepts proposed by biologists are simply different ways of approaching the question “What are species?” Let’s compare three major classes of species concepts to contrast the way that biologists think about species.