(a how would biologists explain how a living rose species with thorns evolved from an ancestral rose species that lacked thorns? (b how would biologists explain how a living rose species lacking thorns evolved from an ancestral rose species that had thorns?

Over time, due to different environment and climate adaptations, different selection pressures are exerted on the rose species. Phenotypic variations exists within the rose species. Roses with thorns were able to better survive predation (or any other reason) than that of the roses without thorns. Plants producing roses with thorns are selected for and is able to survive to maturity and reproduce thus having higher reproductive fitness to pass on genes coding for favourable phenotypes (with thorns) to offsprings. Over time, the allele frequency of roses with thorns increase in the gene pool and rose species with thorns evolve.

Same explanation for b just that a possible reason will be thorns are not useful in another different environment.

A) it will stay on the table