Nature / nurture
Differences between individuals are said to be caused by
• nature - differences in the genes of the individuals
• nurture - differences in the environments they were raised in.
Usually, both genetic and environmental factors play a role in determining differences between individuals. The value of a character, like the beak size of this finch Geospiza fortis, will usually be influenced by the environment in which the individual grows up - all characters to do with body size are influenced by the amount of food an organism happens to find during its life.
Many characters are polygenic - influenced by genes at many loci - and display continuous variation. For such characters it becomes particularly difficult to determine whether the main cause is nature or nurture. The theory of quantitative genetics is concerned with teasing apart the many different factors that cause, for example, some birds to have larger beaks than others.
Biologist Richard Lewontin points out that both genetic and environmental factors play a part: even if we knew the complete genome of an organism, it would not tell us what it was like.