Biology GCSE- Topic 1, variation

The scientific classification of living organisms.

Variation and characteristics within species.

Natural selection and evolution.

Genetic diseases.

GCSE level. 

  • Created by: Dottie
  • Created on: 11-04-12 10:44


Classification (also known as scientific taxonomy) is the process by which living organisms are ordered and categorised according to their characteristics, habitats and evolutionary adaptations and origins.

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There are 7 different kingdoms which organisms are first put into:

Animalia- multicellular, no cell walls, no chlorophyll, nucleus, autotrophs

Plantae- chlorophyll, cell walls, multicellular, nucleus, heterotrophs

Fungi- multicellular, saprophytes, nucleus, cell walls, no chlorophyll

Protoctista- unicellular, nucleus

Prokaryotes- unicellular, no nucleus

Kingdoms are sub-divided into smaller groups of organsims with common features

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Animals are sub-divided into vertebrates (with a backbone) and invertebrates (without a backbone). Vertebrates have a supporting rod like structure (also known as the phylum Chordata.

Vertebrates are divided based on: 

  • How they absorb oxygen
  • How they reproduce
  • How they regulate internal body temperature

Fish- Gills, external fertilisation, poikilotherm, oviparous

Mammals- Lungs, internal fertilisation, homeotherm, viviparous

Birds-  Lungs, internal fertilisation, homeotherm, oviparous

Reptiles- Lungs, internal fertilisation, oviparous, poikilotherm

Amphibians- Some lungs, some gills, external fertilisation, oviparous, poikilotherm

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Binomial naming system

The binomial naming system gives all living organisms scientific names according to their genus and their species. For example, homo sapiens.

Organisms are classified to:

  • Conserve species- clearly identify different species that look alike
  • Study species- naming species means that scientists can share information about them
  • Identify species- avoid language confusion of species names
  • Target conservation efforts- protect areas that have a great variety of different species
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Accurate classification can be difficult because..

  • Contradicting the definition of a species (organisms that can interbreed to produce fertile young), some different species can interbreed to also produce fertile young. 
  • A ring species is a group of related populations that live in neighbouring areas. The populations that live closer together can produce fertile young but those that live further apart cannot.
  • There can be variation within a species. For example, there are many breeds of dogs but they all fall under the same species. 
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Organisms in the same species have differences. These are called variations within a species. The two main causes are: genes and the environment.


Some characteristics are inherited from out parents. 

Most animals get some genes from their mother and some from their father. The combining of the genes causes genetic variation and means that other than identical twins, no two individuals are exactly the same. 

Genetic variation can also occur due to mutation (changes in an organisms genes)

For example: eye colour, blood group and inherited diseases

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The environment/habitat where an organism lives and grows can affect the individuals characteristics.

Environmental variation covers a wide range of differences

For example: piercings, tattoos, skills


For example: body weight, height, skin colour, eye colour, conditions of teeth

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Continuous and discontinuous varitaion

Continuous variation: when individuals in a population vary within a wide range which has no distinct categories e.g. height

Discontinuous variation: when there are two or more distinct categories that individuals in a population fall into e.g. blood group

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Extreme environments

Organisms must adapt to their surroundings in order to survive. Extreme environments such as hydrothermal vents and polar regions are home to organisms who have changed over millions of years in the process of evolution.

For example: 

  • Organisms living deep in the sea have adapted to be able to survive in the dark cold waters - Some are able to emit light (attracting prey) they often have huge mouths, huge eyes and long feelers to locate prey
  • Organisms living in polar regions have also adapted - Polar bears have a thick layer of blubber for insulation, a hairy coat to trap a layer of warm heat, greasy fur which prevents cooling, big feet (which spread their weight and stops them from sinking into the snow or breaking thin ice) and white fur to camouflage into their surroundings
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Natural selection and evolution

Natural selection is when species adapt to their surroundings in order to survive to reproduce. This is a slow process and take thousands, sometimes millions, of years. Evolution/natural selection works like this:

  • Individuals have to compete with each other for resources such as food or shelter. 
  • The individuals with characteristics better adapted to their environment will have a better chance of getting these resources and surviving. 
  • The stronger individuals will then reproduce to produce more organisms with the 'stronger' characteristics. 
  • The 'weak' individuals will not survive to reproduce.
  • Over time, there will be a higher amount of organisms with the new gene and eventually the poorly adapted characteristics will be lost. 
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Validating evidence

The scientific community validate new evidence in three ways: 

  • Publish their work in scientific journals (e.g. scientific magazines) - if other scientists carry out the same experiments and get the same results the evidence is pretty reliable
  • Peer review - other scientists read and review the work to make sure the experiments have been carried out to the highest possible standard
  • Hold scientific conferences - these are meetings where scientists discuss and present their work. 
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This another form of development of a new species.

  • Populations in the same species may become separated and isolated (e.g. floods and earthquakes) 
  • Conditions in the new area may be different and the separated population may have to adapt to the new surroundings (natural selection)
  • Eventually the two populations will be so different that they will no longer be able to breed together to produce fertile young
  • A new species has been created
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Inside every human body cell is a nucleus which controls the rest of the cell. Inside the nucleus are 46 chromosomes (23 pairs). Chromosomes carry DNA (Deoxyribonucleic acid) and genes are found on this DNA. 

There can be different versions of the same gene. These are called alleles. As there are two copies of each chromosome, there are two copies of the allele. These alleles are might be the same or different.

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Genetic diseases

There are many diseases which can be passed on from the parents to the child. For example, Cystic Fibrosis and Sickle Cell Anaemia 

Cystic Fibrosis is caused by two recessive alleles:

This means that two recessive alleles are needed for the offspring to acquire the disease. If both parents are carriers of the disease, there is a 25% chance that the offspring will have Cystic Fibrosis.

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Sickle Cell is another genetic disease caused by recessive alleles. 

Again, if two people who carry the disease have children, there is a 25% chance that the offspring will have the disease. 

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Symptoms of the diseases

Cystic fibrosis: Breathing difficulties, thick sticky mucus in the gut, pancreas and air passageways, lung infections, malnutrition and fertility problems

Sickle Cell: Painful joints and muscles, tiredness, fever, anaemia

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Key words

Classification: Organising living organisms into groups

Binomial naming system: Scientific two-part name for all living organisms

Variation: differences in characteristics within a species

Genes: different genes control the development of different characteristics

Continuous variation: when individuals vary within a range

Discontinuous variation: when there are two of more distinct categories

Acquired characteristics: characteristics from the envrioment

Genetic characteristics: characteristics inherited from parents

Natural selection/evolution: the process by which organisms adapt and change to suit there environment in order to survive to reproduce

Speciation- when organisms from the same species become so different that they can no longer breed to produce fertile offsrping

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