Variation is the second important force in the theory of Darwinian Evolution and it is most obvious to us among our own species. There are dozens of different hair colours and many shades of skin colour. A casual look around your town reveals people of different heights and weights. People have different amounts of muscle and strength, different artistic abilities and different scholastic achievements. Some of these variations are due to environmental influences and are thus acquired characteristics but those acquired characteristics are never inherited. Much of the variation in individuals is due to inherited characteristics.

On average, tall parents have tall offspring.
Note that I said "on average". There's some complex genetics involved in height and environmental influences can also play a part.

This graph shows the number of men of a certain height in a population. (No numbers are given because it isn't important here.) Notice the variation. Most men fall within an average range but some men are very short and others very tall. We say the "variation in height is distributed". A similar distribution of height would be found if the heights of women were graphed. The average height for women would be smaller but there would still be some women who were shorter than most women and some women would be taller than the average woman. Indeed, there would be some women taller than most men.

Variations like these are found throughout nature and at all levels of detail.
A similar graph to the one above (its shape and similar distributions of variation) is found when you tally up strength, intelligence or number of children. This graph could also represent the distribution of different enzyme activities found in a population, ability to fit disease or red blood cell counts.
Almost every feature of an individual could be compared to the same features in other individuals. If you did that comparison you would find a distribution of extremes flanking an average.
Such variation is not restricted to humans. Similar variation is seen in populations of all species - from oak trees to orangutans.

Much of the variation in a population is inherited. That does not mean that offspring will have the exact characteristics as their parents. (They seldom do because of complex genetics which we will not go into here.) Instead, I mean that many of the characteristics of an individual are passed on to the offspring. Those inherited variations can continue being passed on from one generation to the next.

There are rare instances in nature where two individuals are the same - identical (or pretty close).
Think of an example of that before going on to the next page.