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# Organisms ## A3.1 Diversity of organisms ### 1. Variation between organisms An organism is an individual plant, animal, bacterium, or any other living thing. Variation is one of the key features of living organisms. The patterns of variation are complex and are the basis for naming and classify...

# Organisms ## A3.1 Diversity of organisms ### 1. Variation between organisms An organism is an individual plant, animal, bacterium, or any other living thing. Variation is one of the key features of living organisms. The patterns of variation are complex and are the basis for naming and classifying organisms. * There is variation between members of a single species. * No two individuals are identical in all their traits. * There is more variation between members of different species. If the traits of two closely related species are compared, relatively few differences are found. If two distantly related species are compared, major differences are observed. ### 2. Defining a species From the 17th century onwards, biologists used the term "species" for a group of organisms with shared traits. Biologists have been naming and classifying species ever since. Carl Linnaeus was a pioneer of this research in the 18th century. Linnaeus described the morphology of species—the outward form and inner structure. The idea of a species as a group of organisms with a characteristic outward form and inner structure is the morphological concept of a species. For example, *Apodemus sylvaticus* is a species of mouse with characteristic morphology that occurs across much of Europe from Ukraine to France. Mice with similar traits live on many islands including Iceland. They are considered to be part of the same species, despite being unable to breed with populations in mainland Europe. ### 3. Binomial naming system When species are discovered, they are given scientific names using the binomial system. This system is universal among biologists and has been agreed and developed at a series of international congresses. It avoids the confusion that would result from using the many different local names that can exist for a species. The binomial system is a very good example of cooperation and collaboration between groups of scientists. The binomial system has these features: * The first name is the genus name. A genus is a group of closely related species. * The genus name is given an upper case first letter. * The second name is the species name. * The species name is given a lower case first letter. * Italics are used when a binomial appears in a printed or typed document. **Examples of binomials:** * **Animals:** *Homo sapiens* - humans * **Plants:** *Scrophularia landroveri* - a plant discovered by an expedition that travelled in a Land Rover * **Fungi:** *Candida auris* - a yeast that can cause severe infections in humans * **Bacteria:** *Yersinia pestis* - the microorganism that causes bubonic plague (Black Death). * **Viruses:** Viruses are not given binomials as they are not considered to be living organisms. ### 4. Defining the biological species concept The morphological species concept was based on an expectation of clear differences in structure between all species. The concept works well in some groups of organisms, but not in other groups. An alternative species concept was developed, based on the understanding that interbreeding continuing generation after generation prevents members of a species from diverging. This is the biological species concept: a species is a group of organisms that can successfully interbreed and produce fertile offspring. By interbreeding, members of a species share the genes in a gene pool, so share many traits. By not interbreeding, different species can diverge and develop more and more differences. The biological species concept works well with some groups of organisms. For example, the genus *Allium* contains hundreds of species, including onion and garlic, but few interspecific hybrids have been reported in natural habitats and these hybrids are usually sterile. The biological species concept is very difficult to apply in other groups of plants and animals. Fertile offspring may be produced by hybridization between individuals that are clearly members of different species in terms of morphology, such as lions and tigers. Other challenges to the biological species concept are explained in Section A3.1.12. Many other species definitions have been proposed. This is a part of biology where consensus has not yet been achieved.