B4.2.2: Classification and Evolution - OCR A Biology A level A* student notes
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Course
B4.2.2: Classification and Evolution
Institution
OCR
Book
A Level Biology A for OCR Student Book
These notes use information from two textbooks, the internet and extra information from my lessons, and cover each specification point for this topic. They are fully comprehensive and include diagrams etc. Helped me achieve an A* in Biology A level.
Biological classification: process by which living + extinct organisms are organised into
systematic groups based on similarities + differences between species
Carl Linnaeus (C18 botanist) proposed the first hierarchical classification system (‘Linnaean
classification’), in which large groups were continually subdivided down to the level of species
Taxonomy: study and practise of naming + classifying species + groups of species within the
hierarchical classification scheme
Hierarchical classification: arrangement of organisms into taxonomic groups of different ranks
Taxonomic rank: any level w/in the hierarchical classification system
Taxon: a group of organisms at any rank in the hierarchical classification scheme
- Most widely used until recently = hierarchy of 7 taxonomic groups
- Similar / related groups at one hierarchical level are combined into more inclusive
groups at the next, higher, level
- [__ move down hierarchy → more groups at each level but fewer organisms in each group
- share more of the same characteristics
Kingdom
Phylum (pl ‘phyla’)
Class
Order
Family
,Genus (pl ‘genera’)
Species
(King Phillip Cried Out For Good Soup)
1. K - largest group of organisms sharing common features
2. P - major subdivision of a kingdom
3. C - group of related orders, subdivision of a phylum
4. O - group of related families, subdivision of a class
5. F - group of closely related genera, subdivision of order
6. G - group of related species, subdivision of family
7. S - breed freely with one another
(can have a subspecies - a subgroup within a species that has different traits)
Eg. humans - Homo sapiens:
[D = Eukarya]
K = Animalia
P = Chordata
C = Mammalia
O = Primates
F = Hominidae
G = Homo
S = sapiens (have also had eg. homo erectus)
The higher the taxa, the more diversity is shown between individuals within a group (there is
more variation between organisms in the kingdom animalae than between organisms in the
class mammalia)
Species
- Smallest basic taxonomic unit used to define living organisms - each group cont only 1
type of organism
- Defined as group of organisms that can reproduce to produce fertile offspring
, - Eg. donkeys + horses are classified as belonging to different species because when bred
together, the offspring prod (hinny / mule) is infertile as their cells cont an odd no of
chromosomes (63) → meiosis / gamete production can’t take place correctly as all the
chromosomes must pair up
- Definition doesn’t take into account simple organisms that reproduce asexually!
● Current system may change again as new scientific discoveries are made eg. through
genome sequencing
Reasons why scientists classify organisms
➔ To create a world wide record of all living organisms - eg. All Species Foundation
➔ Identify species - easy to identify species a (new or old) organism belongs to
➔ Predict characteristics - likely a species in a group will share same specific characteristic
as other members
➔ Find evolutionary links - species in same group probably share characteristics because
they have evolved from a common ancestor
➔ Universal - scientists can share research worldwide → see links between different
organisms, even if they live on different continents
➔ [__ helps communication
➔ Convenience - make studying living organisms more manageable
★ Classification systems created to order observed organisms - NOT defined by nature
The binomial system of naming species and the advantage of such a system
- Linnaeus also founded the system of binomial nomenclature
- Convention of writing scientific names
- All species given a scientific name consisting of 2 parts
1. Genus - capitalised
2. species - lowercase
- If handwriting underline, if typing use italics
- No 2 species could have the same generic + specific name (could have same specific
name but their genus would be different)
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