Variation

There are two types of variation.
IntERspecific: The variation that occurs between different species. E.g,The horse varies from the human.

IntRAspecific: The variation between members of the same species. E.g The coats on the horses are different. 

No two individuals within a species are exactly the same. This can be down to genetic and environmental factors.

Genetic variation: Members of the same species have the same genes, but they have different versions of the genes (alleles) which make up the organisms genotype.
The genotype is to do with the genes which results in the variation of the phenotype which is the characteristics shown by the organism.
For example:
   The genotype for blue eyes could be Bb, but the phenotype would be Blue eyes.
Genetic variation is inherited from the parents.

Sometimes the phenotype of an organism can be effected by the environment.

For example, twins are genetically identical, but if one grew up in scotland, they would probably have a scottish accent, whilst if the other lived in England, they would probably speak English. So the environmental change here would be accent.

Some animals fur colour is affected by temperature, such as the Himalayan rabbit. The colder parts below 25 degrees C (ears, feet and tail) have black fur, whilst the rest is mainly white in the warmer areas.

Plant growth can be effected by the mineral concentration in the soil, like nitrates and phosphates.

Environmental factors can affect the influence of the genes someone has.

A person might have the genotype to be very tall, but they might not get to be tall if they are in poor health or have a bad diet.

Another person could have the genotype to be very tanned (caused by the amount of a pigment called melanin) but this would be influenced by how much sun they were exposed to. If they lived in Florida, they could be very tanned, but if they lived in Russia, they could be quite pale.

Variation is studied by taking a sample of a population rather than the whole thing. The sample is used as a model of the whole population as it would be very time consuming or virtually impossible to get all the individuals within a population.

The sample has to be random, so it is an accurate representation of the overall population. It has to be random because bias may influence the outcome of the investigation, so random number generators or grids are often used.

Any variation observed is measured by statistical tests (chi-squared, spearmans rank, 95%) to make sure that the outcome of the investigation isn't down to chance. 

• Standard Deviation tells you about the variation within a sample, and how much the values within that sample vary, by telling you the spread of data about the mean. A large standard deviation means the values within the sample vary a lot. A small standard deviation tells you most of the data is around the mean, so there is little variation.
• Mean tells you the average of the values in a sample, this can be used to tell if there is variation between different samples.
• A normal distribution usually produces a bell shape on a graph.