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OCR Biology AS Level (10)
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Chapter 10: Classification and EvolutionChapter 10.1: Classification
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.
Several different classification systems exist, these are referred to as taxonomic groups.
The seven groups are:
o kingdom
o phylum
o class
o order
o family
o genus
o species
Kingdoms are the biggest and broadest taxonomic group, with species being the smallest and most specific
classification.
Hierarchical classification systems are often referred to as even Linnaean classification, after the 18th
century Swedish botanist, Carl Linnaeus who was the first to propose such at 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
Why do scientists classify organisms?
To identify species
o By using a dearly defined system of classification, the species an organism belongs to can be easily
identified.
To predict characteristics
o 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
o 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.
How are organisms classified?
The classification system begins by separating organisms into the three domains -Archaea, Bacteria, and
Eukarya
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 can reproduce to produce fertile offspring.
o 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 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(32pairs) whereas donkeys
have 62 chromosomes (31pairs).
Naming organisms
Before classification systems were widely used, many organisms were given names according to certain
physical characteristics, behaviour, or 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.
To ensure scientists the world over is 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:
o 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.
o 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.
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, except for the first letter of the genus name, which should be
uppercase.
Split the name into two parts and you can easily work out which genus and species the organism belongs to.
Chapter 10.2: The five kingdoms
What are the five kingdoms?
Living organisms can be classified into five kingdoms:
o Prokaryote (bacteria)
o Protoctista (the unicellular eukaryotes)
o Fungi (yeasts, moulds, and mushrooms)
o Plantae (the plants)
o Animalia (the animals)
Organisms were originally classified into these kingdoms based on similarities in their observable features, as
described below.
Prokaryote
General features:
o Unicellular
o No nucleus or other membrane-bound organelles - a ring of 'naked'
o DNA - small ribosomes
o No visible feeding mechanism - nutrients are absorbed through the cell wall or produced internally
by photosynthesis.
Examples include the bacteria Escherichia coli, Staphylococcus aureus, and Bacillus anthracis
Protoctista
General features:
o (mainly) unicellular
o a nucleus and other membrane-bound organelles
o some have chloroplasts
o some are sessile, but others move by cilia, flagella, or by amoeboid mechanisms
o nutrients are acquired by photosynthesis, ingestion of other organisms, or both - some are parasitic.
Examples include species belonging to the genera Paramecium and Amoeba.
Fungi
General features:
o unicellular or multicellular
o a nucleus and other membrane-bound organelles and a cell wall mainly composed of chitin
o no chloroplasts or chlorophyll
o no mechanisms for locomotion
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