Figure 1.

Comparison of species (color) richness in the population and in a sample. A For a homogeneous population, a small sample is sufficient to observe a sample richness that is close to the true richness. B The sample richness of a population with many rare species underestimates the true richness. C Rarefaction curve. Increasing the sample size leads to an increase in the observed richness, converging to the true richness. For a heterogeneous population with many rare species, the convergence is slow. D T-cell diversity. Somatic recombination of V, D, J gene segments during T-cell maturation gives rise to numerous distinct T-cell receptors that form an immune system that is able to recognize almost all potential pathogens. However, analysing T-cell receptor diversity based on a small blood sample is challenging, because the sample only contains a minute portion of all T-cells from a person.