Populations have several kinds of structure.
The genetic structure describes the patchiness
of gene frequency and genotypes,
and the spatial structure describes the variation
in density within a population.
Populations also have an age structure that
describes the relative numbers of young and old individuals,
and a size structure describing the relative
numbers of large and small individuals.
Populatiuon biology attempts to explain the origin of these
different kinds of structure, to understand how they influence
each other, and how and why they change with time.
``Introduction to Plant Population Biology'',
p.1-2.