classification Flashcards
classification systems
Classification is the name given to the process by which living organisms are sorted into groups. The organisms within each group share similar features.
A number of different classification systems exist. Until recently the most widely used system contained seven groups ordered in a hierarchy - these are referred to as taxonomic groups. The seven groups are: kingdom, phylum (plural phyla), class, order, family, genus (plural genera) and species (Figure 1). Kingdoms are the biggest and broadest taxonomic group, with species being the smallest and most specific classification. Similar or related groups at one hierarchical level are combined into more inclusive groups at the next higher level.
Hierarchical classification systems are often referred to as Linnaean classification, after the 18th century Swedish botanist,
Carl Linnaeus who was the first to propose such a system.
Based on recent studies of genetic material many scientists now add a further level of classification into the hierarchy. It is known as a domain (you will find out more about the three domain system of classification in Topic 10.2, The five kingdoms). This level of classification is placed at the top of the hierarchy. As new scientific discoveries are made (for example, through genome sequencing), the current system of classification may change again.
why do scientists classify organisms
-To identity species - by using a clearly defined system of classification, the species an organism belongs to can be easily identified.
• To predict characteristics - if several members in a group have a specific characteristic, it is likely that another species in the group will have the same characteristic.
-To find evolutionary links - species in the same group probably share characteristics because they have evolved from a common ancestor.
By using a single classification system, scientists worldwide can share their research. Links between different organisms can be seen, even if they live on different continents. Remember, though, that classification systems have been created to order observed organisms.
This form of hierarchical organisation is not defined by ‘nature’.
how are organisms classified
The classification system begins by separating organisms into the three domains - Archaea, Bacteria, and Eukarya (discussed further in Topic 10.2, The five kingdoms). These are the broadest groups. As you move down the hierarchy there are more groups at each level, but fewer organisms in each group. The organisms in each group become more similar and share more of the same characteristics.
The system ends with organisms being classified as individual species.
These are the smallest units of classification - each group contains only one type of organism. A species is defined as a group of organisms that are able to reproduce to produce fertile offspring. For example, donkeys can reproduce with other donkeys, the offspring of which can subsequently breed. Likewise, horses can breed with other horses to produce fertile offspring. However, when a horse is bred with a donkey, the offspring produced (a mule or a hinny) is infertile. Therefore, donkeys and horses are classified as belonging to different species.
Mules or hinnies are not a species.
Mules and hinnies are infertile because their cells contain an odd number of chromosomes (63). This means that meiosis and gamete production cannot take place correctly as all chromosomes must pair up. This chromosome number is created because horses have 64 chromosomes (32 pairs) whereas donkeys have 62 chromosomes (31 pairs).
naming organisms
Before classification systems were widely used, many organisms were given names according to certain physical characteristics, behaviour or habitat. Examples are ‘blackbirds’ for their colour, ‘song thrushes’ for their song and ‘fieldfares’ for their habitat. These are called their
‘common names’.
This was not a very useful system for scientists working internationally, as organisms may have more than one common name, and different names in different languages. Another problem is that common names do not provide information about relationships between organisms.
For example, the blackbird, song thrush, and fieldtare all belong to the genus Turdus, meaning that they have all evolved from a common ancestor, but you wouldn’t know this from their common names, nor necessarily from their observable characteristics.
To ensure scientists the world over are discussing the same organism we now use a system developed in the 18th century, also by Carl Linnaeus, a Swedish botanist. This system is known as binomial nomenclature.
All species are given a scientific name consisting of two parts:
• The first word indicates the organism’s genus. It is called the generic name; you can think of this as being equivalent to your surname or family name, as it is shared by close relatives.
-The second word indicates the organism’s species. It is called the specific name.
-Unlike people, no two species have the same generic and specific name. Two different species could have the same specific name, however their genus would be different. An example of this is Anolis cuvieri (a lizard) and Oplurus cuvieri (a bird). The only link between them is that they are both named after the famous French naturalist and zoologist Georges Cuvier (1769-1832). Many of these scientific names derive from Latin.
When naming an organism using its scientific name the word should be presented in italics. As it is difficult to handwrite in italics, the standard procedure in handwritten documents is to underline the name. The name should be written in lowercase, with the exception of the first letter of the genus name, which should be uppercase.
Some examples of scientific names are included in Table 3. Split the name into two parts and you can easily work out which genus and species the organism belongs to.
