Chapter 5. Variance

I refer to the small and large differences we see in the natural world: differences among species, differences among populations or among individuals within populations, or even differences among cells within individuals.  You may not have thought about it this way, but many biologists are driven by a desire to understand this variation.  For example, a developmental biologist might wish to know how cells shift from one type to another, while an evolutionary geneticist might want to understand why some genes have more variable DNA sequences than other genes do.

Variability is also the reason that the field of statistics exists.  Imagine we lived in a world where we knew that all individuals in a population would respond identically to an experimental treatment.  In that world we could conduct an experiment in which we manipulate a single individual (instead of many individuals) and we would not need statistics (as they exist in the real world) to make conclusions.

This chapter introduces ‘standard deviation’ and ‘variance’ as two related measures that quantify variability.  We also discuss why variation arises in the natural world and in experiments.