There are several possible ways that differences between the EGFR and PDGFRA amplified sub-populations can occur in our model: by giving them a selective advantage (changing ρE, ρP, DE or DP); by changing the phylogenetic ordering of mutations (changing tE* or tP*); by changing the location that mutations arise in the evolving tumour (changing xE* or xP*); by changing the strength of interaction felt by each population (changing αEP or αPE); or, finally, by changing any combination of these factors. Here, EGFR is linked to neoplasm.