Variation with a Species

Look around a classroom of people. Everyone is a human being — the same species — yet no two people are identical. Some are taller, some have brown eyes and some blue, some have curly hair and some straight. These differences between individuals of the same species are called variation.

Variation is everywhere in living things. Even kittens from the same litter, or seeds from the same plant, show small differences from one another.

The specification says variation within a species can come from three sources:

• genetic causes (the genes you inherit from your parents),
• environmental causes (the surroundings you live in and the things that happen to you), or
• a combination of both working together.

So when you see a difference between two individuals, the key question is always: is this caused by their genes, by their environment, or by both?

Exam tip. A reliable one-mark answer for "what is meant by variation?" is the differences between individuals of the same species. Adding "of the same species" is what makes the definition fully correct.

Genetic variation is variation caused by the genes an organism inherits from its parents. Because you receive a mixture of genes from your mother and father, you end up with your own unique combination — which is why brothers and sisters are similar but not identical.

Features that are caused only (or almost only) by your genes do not change with the environment. The classic Double Award examples are:

Feature	Why it is genetic
Blood group (A, B, AB, O)	Fixed by the genes you inherit — your surroundings cannot change it.
Eye colour	Determined by inherited genes; not changed by where you live.

Other examples you can use include natural hair colour, the ability (or not) to roll your tongue, and whether your earlobes are attached or free.

The important idea is that genetic variation is inherited — it is passed from parents to offspring through genes — and it is present from birth because it was set when the parents' gametes joined.

Exam tip. If a question asks for a feature showing only genetic variation, blood group is the safest answer because nothing in the environment can change it.

Environmental variation is variation caused by the environment — the surroundings an organism lives in and the things that happen to it during its life. This kind of variation is not inherited; it is acquired after birth.

The classic Double Award examples are:

Feature	Why it is environmental
Scars	Caused by injuries or accidents during life — not present in your genes.
Language spoken	Learned from the people around you, not inherited from parents.

Other good examples include a suntan (caused by sunlight), a footballer's muscle development from training, and the scars left by an injury.

A neat way to remember this: a person born in one country but raised in another will speak the language of the country they grew up in, not the language of their genetic parents — proof that language is environmental, not genetic.

Because environmental variation is not passed on through genes, a feature like a scar on a parent will not appear on their children.

Exam tip. If a question asks for a feature caused only by the environment, a scar or the language a person speaks are the safest answers — neither can be inherited.

Many features are not purely genetic or purely environmental — they are a combination of both. The genes set a potential range, and the environment then decides where in that range the individual ends up.

The classic Double Award examples are height and mass (body weight):

• Height — you inherit genes that influence how tall you could be, but your actual height also depends on your diet (especially in childhood). Good nutrition lets you reach your genetic potential; poor nutrition can keep you shorter.
• Mass / body weight — partly set by inherited genes, but strongly affected by how much you eat and how much exercise you do.

This is why two people with similar "tall genes" can end up at different heights if one had a much better diet growing up.

A simple way to picture it:

Combined variation = genes set the potential, the environment fills it in.

Exam tip. Height is the examiner's favourite example of combined (genetic and environmental) variation. If asked for one feature affected by both, write height and mention diet/nutrition as the environmental part.

Variation can also be sorted by the pattern it makes when you measure or count a feature across many individuals. There are two patterns: continuous and discontinuous.

Continuous variation gives a smooth range of values with no gaps between them. You can have any value in between two others. These features are usually measured (with a number that can be any value).

• Examples: height, mass (body weight), hand span, foot length.
• Usually controlled by many genes, and often the environment too.
• Plotted as a histogram; the bars usually form a bell-shaped (normal) curve — most people are in the middle, fewer at the extremes.

Discontinuous variation falls into separate, distinct categories with no in-between values. You belong to one group or another. These features are usually counted or named, not measured.

• Examples: blood group (A, B, AB, O), tongue rolling (can / cannot), eye colour, sex (male / female).
• Usually controlled by one or only a few genes, and not affected by the environment.
• Plotted as a bar chart with separate bars for each group.

Here is the difference shown as graphs:

Exam tip. Remember the matching pairs: continuous → measured → histogram → smooth curve and discontinuous → grouped → bar chart → separate bars. Use height for continuous and blood group for discontinuous — they are the safest examples.