  A tumor can be thought of as an ecosystem, which critically means that we
cannot just consider it as a collection of mutated cells but more as a complex
system of many interacting cellular and microenvironmental elements. At its
simplest, a growing tumor with increased proliferation capacity must compete
for space as a limited resource. Hypercellularity leads to a contact-inhibited
core with a competitive proliferating rim. Evolution and selection occurs, and
an individual cell's capacity to survive and propagate is determined by its
combination of traits and interaction with the environment. With heterogeneity
in phenotypes, the clone that will dominate is not always obvious as there are
both local interactions and global pressures. Several combinations of
phenotypes can coexist, changing the fitness of the whole.
  To understand some aspects of heterogeneity in a growing tumor we build an
off-lattice agent based model consisting of individual cells with assigned
trait values for proliferation and migration rates. We represent heterogeneity
in these traits with frequency distributions and combinations of traits with
density maps. How the distributions change over time is dependent on how traits
are passed on to progeny cells, which is our main inquiry. We bypass the
translation of genetics to behavior by focussing on the functional end result
of inheritance of the phenotype combined with the environmental influence of
limited space.
