Much recent work, for example on diversity hotspots, has shown that species diversity is not distributed uniformly. Within-species (genetic) diversity is also important - it confers resistance to disease, for example, and is needed for a population to be robust to future environmental changes.
The question we are interested in is: how is the diversity that
exists in a population distributed within
Diversity is the sum of all the ways that members of a population
differ from each other. Those differences arose through mutations in
the evolutionary history of the population. The evolutionary history of
a population can be represented by a genealogical tree, with all the
mutations occurring somewhere on the tree:
But what do trees typically look like? Supposing they typically look
like the above. Any one of the four groups circled doesn't carry more
diversity than the others.
Supposing it looks like this, however:
Here, group A carries more diversity than its share of the population.
We quantify this as the uniqueness of
an individual or group: the number of generations since it diverged
from its closest living relative. In fact, we show below that the
uniqueness is unevenly distributed, and that a minority of the population
carries a large fraction of the population's diversity.