Variation, the t-test and natural selection

Last time

Before we start: last time, in The lac operon and transcription factors, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

No two alike

Look around a classroom: no two people are the same height. But blood groups come in just four types. Those two patterns of variation have different causes, and each needs a different statistical test. After that, we see how variation allows natural selection to change a population.

Two kinds of variation

Discontinuous variation has distinct categories, with no intermediates. Examples are blood group and flower colour. It is shown on a bar chart. Continuous variation has a full range of values, with a bell shaped distribution. Examples are height and body mass. It is shown on a histogram or line graph. Discontinuous variation is usually controlled by one or two genes. Continuous variation is controlled by many genes, called polygenes, and the environment as well.

Causes of variation

Genetic causes include mutation, meiosis (crossing over and independent assortment) and random fertilisation. The environment affects continuous features. A plant given more nitrate grows taller; a person with a better diet may be taller. A feature can often be a combination: the genotype sets a range, and the environment decides where in the range the organism ends up.

A past-paper example

May June 2024, Paper 41, question 9. The harlequin ladybird shows discontinuous variation in colour pattern and continuous variation in body size. Compare the two kinds of variation. Discontinuous variation has distinct categories and is controlled by one or a few genes. Continuous variation has a range of values, many genes, and environmental influence. Give the type of graph for each: bar chart and histogram.