  The aim of this paper is to discuss the role of robustness and diversity in
population dynamics in particular to some properties of the multi-step from
healthy tissue to fully malignant tumours. Recent evidence shows that diversity
within the cell population of a neoplasm, a pre-tumoural lession that can
develop into a fully malignant tumour, is the best predictor for its evolving
into a tumour. By studying the dynamics of a population described by a
multi-type, population-size limited branching process in terms of the
evolutionary formalism, we show some general principles regarding the
probability of a resident population to being invaded by a mutant population in
terms of the number of types present in the population and their resilience. We
show that, although diversity in the mutant population poses a barrier for the
emergence of the initial (benign) lession, under appropiate conditions, namely,
the phenotypes in the mutant population being more resilient than those of the
resident population, a more variable noeplastic population is more likely to be
invaded by a more malignant one. Analysis of a model of gene regulatory
networks suggest possible mechanisms giving rise to mutants with increased
phenotypic diversity and robustness. We then go on to show how these results
may help us to interpret some recent data regarding the evolution of Barrett's
oesophagus into throat cancer.
