[1] To determine if natural selection specifically is the cause of parallel speciation, a fourth requirement has been established that includes identifying and testing an adaptive mechanism, which eliminates the possibility of a genetic factor such as polyploidy being the responsible agent.
[1] Parallel evolution is a common phenomenon that occurs when separate yet closely related lineages evolve the same, non-ancestral trait as a result of inhabiting the same environment, and thus, facing the same selection pressures.
It can also be difficult to assess due to populations of the same species that may have a small amount of gene flow occurring between them despite living in different areas, but do not have physical barriers to overcome.
Similarly, this hypothesis is supported by the fact that the annual grasses have been evolved (adapted) to the dry climate of monsoonal Asia.
[28] Thus, the flowering is considered a “magic trait” [29] in plant species that help in adaptation and enables the reproductive isolation required for parallel speciation.
The almost complete isolation in flowering time combined with the difference in mating system is making it a strong premating barrier to gene flow among the species and played a pivotal role in Oryza nivara origin.
Selection is one of the driving forces of genetic diversity among the allopatric populations which gave rise to reproductive isolation as incidental by-product.
[2] But for the reinforcement, the requirement is preexisting reproductive isolation in the form of decreased hybrid fitness and is normally considered a final stride towards the process of speciation.