Modern Synthesis
Modern evolutionary biologists can use the DNA and chromosomes found in more recent roses to determine that thorns have been inherited from previous generations. Genes that are dominant for thorns and recessive for no thorns would over time result in a genotype that was 100% dominant for thorns because all the roses without them would have died off. Due to a 100% dominant genotype for thorns (theoretically speaking) the phenotype would display thorns on the stem. When repopulation occurred, only roses dominant for thorns would be left.
Did You Know....
The Roses from Hildesheim CatherdalThe oldest rose in the world has flourished for over 1,000 years on the wall of Hildesheim Cathedral in Germany.