So far in this book, we have looked at populations as if all the individuals comprising them were interchangeable. In reality, this is not so. Walk through an unmown meadow and you will see that different plants of the same species have different sizes, colours and proportions; some may be thornier, have larger leaves, more flowers, or thicker stems than others. Some of these differences are simply attributable to environment: malnourished individuals will usually be relatively small, for example. However, many differences, such as human eye colours, cannot be explained by the environment alone, and instead are influenced by variations in the genetic instructions that regulate development. These instructions tell each cell when and how to construct highly specialised proteins. Genetic instructions are encoded by long strands of deoxyribonucleic acid (DNA). Different sections of a DNA strand are used at different times; each such section is called a gene. Organisms usually have a very large number of genes; for example, the human genome includes around 30,000 genes. Changes in the prevalence of genes are studied by population geneticists. These changes are intimately tied to the ecology of populations at several levels. Evolution, the long-term consequence of changing gene frequencies, generates the diversity of living forms whose interactions are the subject of ecology. Conversely, ecological factors greatly influence evolution. Lastly, the dynamics of genes have much in common at a theoretical level with the ecological dynamics of species.