Nature does = environmental stress or changes
Because if we're simultaneously moving/manipulating two different variables it can be very hard to determine why one variable is changing and why the other is changing. While this can be done with more complicated experimental designs, it usually is the case that scientists will only manipulate one variable and not two different ones.
The answer is C (when they cannot interbreed) because when the new species cannot have offspring with the old species, they do not mix traits.
Plant tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue. Epidermis - Cells forming the outer surface of the leaves and of the young plant body. Vascular tissue - The primary components of vascular tissue are the xylem and phloem.
