Simple Mendelian characters, like blood groups or the mimetic variation of Papilio, often have discrete variation: but many of the characters of species, like beak size in Darwin's finches, vary continuously: every individual in the population differs slightly from every other individual. There are no discrete categories of beak size in Geospiza fortis (pictured opposite) or in most other species of birds.

Two reasons for continuous variation:

• Mendel had noticed in his original paper in 1865 that multifactorial inheritance (i.e. the character is influenced by many genes) can generate a continuous variation. Characters influenced by many genes are called polygenic characters.

• Differences in external environmental conditions also produce continuous variation. In nature, each character will be influenced by many environmental variables, some tending to increase it, others to decrease it.

Thus differences between individuals are said to be caused by nature - differences in the genes of the individuals - and by nurture - differences in the environments they were raised in.