AS AQA Variation

One way of measuring is SAMPLING. Scientists do this because they cannot take just one measurement of one thing, but cannot measure all the same thing in the world either. 

SAMPLING: Involves taking measurements of individuals, selected from the population of organisms which is being investigated. This is a REPRESENTATION OF THE POPULATION

Reasons why they wouldn't be representative: 

SAMPLING BIAS: Selection process may be biased, take measurments in unrepresentative areas

CHANCE: By pure chance, sample collected may not be representative

RANDOM SAMPLING:

• Divide area into a grid of numbered line
• Use random numbers from a number generator or pulling them out of a hat ect. 
• Take samples at the intersections of each pair of coordinates

Minimise chance by having a: LARGE SAMPLE SIZE and ANALYSISING THE DATA COLLECTED

If one species differs from another species, it's called INTERSPECIFIC VARIATION

If an individual in a species differs from another, it's called INTRASPECIFIC VARIATION

GENETIC DIFFERENCES: Due to the different genes that each individual organism possesses

• MUTATIONS: Sudden changes to genes + chromosomes which may/may not be passed down
• MEIOSIS: Special form of nuclear division froms the gametes. Genetic info gets mixed up before being passed into the daughter cells -> ALL ARE DIFFERENT 
• FUSION OF GAMETES: In sexual reproduction, the offspring inherits some characteristics from each parent -> OFFSPRING DIFFERENT TO PARENTS

ENVIRONMENTAL: 

• Environmental influences effect species
  • Climate conditions: Temperature, rainfall, sunlight
  • Soil conditions, pH and food avaliability
• These influences affect how the genes are expressed, so which characteristics are shown

Most cases of variation are due to a mixture of GENES AND ENVIRONMENTAL FACTORS

GENETIC FACTORS:

• Where variation is the result of genetic factors fit into a few distinct forms which environment factors don't normally effect
  • EG. Blood types - A B AB O --> Usually controlled by a single gene
• This variation can be shown in a bar graph or pie chart

ENVIRONMENTAL INFLUENCES:

• Where variations aren't due to one gene but many genes but effected by the environment alot. Some characteristics grade into one another, forming a CONTINUUM. Environmental factors play a major role in determining where on the continuum an organism lies.
  • EG. Humans --> HEIGHT and MASS
  • Effected by parents height but may be taller/shorter due to environmental factors eg. diet 
• This variation is due to genes and environment
• Take a sample, plot on a graph, we obtain a BELL-SHAPED CURVE called NORMAL DISTRIBUTION CURVE

A normal distribution curve always has the bell-shaped curve. It differs in two measurements: MAXIMUM HEIGHT and WIDTH

MEAN:

• The measurement at the MAXIMUM HEIGHT of the curve
• Sample of data provides an average value and is useful information when comparing one sample with another
• DOES NOT provide any info about the range of values within the sample

STANDARD DEVIATION:

• Measurment of the WIDTH of the curve
• Indication of the RANGE of values either side of the mean
• SD is the distance from the mean to the point where the curve changes from being convex to concave
• 68% of all measurements lie within +-1.0 SD
• 95% of all measurements lie within +-2.0 SD (actually +-1.96)