Nicotiana sylvestris flowers only during long days, and N.tabacum flowers only during short days. If raised in the laboratory under different photoperiods, they can be induced to flower at the same time and can be cross fertilized to produce self fertile offspring. What is the best reason for considering N.sylvestris and N.tabacum to be separate species ?

Explanation

(a) : According to biological species concept a species is a natural inbreeding or panmictic species or group of natural populations which have essentially similar morphological traits, they are genetically distinct and reproductively isolated from others. Since under laboratory conditions N. tabacum and N. sylvestris can produce selffertile offspring so they are not reproductively isolated. They are considered as separate species because since their flowering periods are different so cross pollination is not possible between them naturally. This makes them different species.