Variation

Variation means differences between living things. There is variation between species (a robin and a blackbird) and variation within a species (two robins). A species is a group of living things that can breed together to produce fertile offspring.

	Continuous variation	Discontinuous variation
What it means	Can take any value within a range	Falls into a few distinct groups, nothing in between
Examples	Height, mass, hand span, foot length, leaf length	Blood group, eye colour, whether you can roll your tongue, a pea seed being wrinkled or smooth
How to record it	Measure carefully (to the nearest mm), then group the results into equal ranges	Count how many are in each group with a tally
Chart	A bar chart of the groups with the bars touching (a histogram). Lots of people gives a bell-shaped curve	A bar chart with a separate bar for each group

Reading the results. The mode is the most common value or group: the tallest bar. The range is the biggest value minus the smallest, and it tells you how spread out the results are. A sample is the smaller group you measure to learn about a bigger one; a bigger sample gives a fairer picture. For a fair investigation, measure everyone the same way: same tape, shoes off, standing straight.

What causes variation? Inherited differences, the environment, or both. Two seeds from the same plant grown in the sun and in a cupboard end up different heights: that difference is environmental.

Stretch Grouping continuous data does not make it discontinuous: foot length stays continuous even when it is sorted into shoe sizes, and the groups are just a way of drawing it. In 1846 Adolphe Quetelet measured 5,738 soldiers’ chests and found a bell curve, one of the first times the pattern was spotted in living things.