Continuous And Discontinuous Variation

Tip a bag of garden pea seeds onto a bench and sort them by shape. You finish with two heaps, round and wrinkled, and nothing at all in the space between them: no seed is halfway. Now put those same seeds on a balance, one at a time. This time hardly any two readings match, and every value between the lightest seed and the heaviest is filled in by something in the bag. One handful of seeds, one species, two completely different shapes of difference.

That contrast is the whole of this lesson. You will learn what puts a characteristic into distinct groups and what spreads it smoothly across a range, why some differences are written into an organism at fertilisation while others are picked up during its life, and how to handle the numbers: building a frequency table, choosing between a bar chart and a histogram, estimating a mean from grouped data, picking a sensible sample and spotting the moment a conclusion runs further than the evidence behind it.