The two species evolved as s result of adaptive Speciation.
Adaptive speciation occurs when biological interactions induce disruptive selection and the evolution of assortative mating, thus triggering the splitting
understanding processes of adaptive speciation, in which the splitting of lineages is an adaptation caused by frequency-dependent selection. Adaptive, or sympatric, speciation has been modeled since the 1960s.
In evolutionary biology, adaptive radiation/speciation is a process that makes living organisms diversify and change rapidly from an ancestral species into several new forms, especially when a change or natural event in their environment makes new resources available, thereby creating new challenges and opening new environmental niches.
After reviewing the graph you posted with the question and also reviewing the 2011 reference tables, the estimated number of daylight hours that can occur on January 1 at 40° N latitude is 8.5 hours - 9.5 hours. Either of these will be correct to use.
